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Investigation of ternary ConCN−1/0 (n = 1–5) clusters by density functional calculations Jinyun Yuan,a Shuaiwei Wang,a Yubing Si,a Baocheng Yang*a and Houyang Chen*b The neutral and anionic ConCN (n = 1–5) clusters were investigated using density functional calculations. The most stable structures of neutral and anionic ConCN (n = 1–5) clusters have been identified. In these structures, the CN radical retains its integrity as a structural unit. For anionic ConCN− (n = 1–5) and neutral ConCN (n = 1–2) clusters, the CN prefers to be adsorbed on the top sites of Con clusters via the C terminal, forming a linear or quasi-linear N–C–Co structure. For neutral ConCN (n = 3–5) clusters, the CN is adsorbed on the bridge sites of Con (n = 3–5) by both C and N atoms. Compared to the free CN radical
Received 30th September 2013, Accepted 22nd January 2014 DOI: 10.1039/c3dt52728j www.rsc.org/dalton
1.
calculated at the same level, the C–N stretching frequencies of neutral ConCN (n = 3–5) are red-shifted while they are blue-shifted for neutral ConCN (n = 1–2). The adiabatic and vertical detachment energies of anionic ConCN− (n = 1–5) clusters are calculated based on density functional calculations. In addition, the most favored dissociation channels of neutral and anionic ConCN (n = 1–5) clusters are determined by calculating the dissociation energies of various possible dissociation pathways.
Introduction
Metal cyanides have potential applications in sensors,1 batteries,2 catalysts,3 photochemistry,4 magnetic devices,5 color imaging,6 and astrophysics.7 A large number of metal cyanides have been investigated experimentally and theoretically, including transition metal cyanides, alkali metal cyanides, and alkaline earth metal cyanides. Ground state structures of sodium and potassium cyanides were found to be T-shaped configurations.8,9 Ab initio calculations and gas-phase rotational spectra have shown that lithium and alkaline earth metal monocyanides exhibit isocyanide MNC equilibrium structures.10–14 Vera et al. confirmed that isocyanide AlNC is significantly more abundant than cyanide AlCN in the interstellar medium.15 In the case of transition metal cyanides, experimental measurements and quantum chemical calculations have shown that the late 3d transition metals (from Co to Zn) prefer a cyanide configuration, whereas the early 3d transition metals (from Sc to Fe) except Cr favor the isocyanide isomer.16–24 The equilibrium geometries and ro-vibrational spectra of the linear coinage-metal cyanides were predicted by theoretical calculations.25 The endohedral clusterfullerene of a triangular YCN cluster entrapped inside a C82 cage has been synthesized.26 A photoelectron spectroscopic study of the palladium cyanide anion suggested that it was a cyanide PdCN
a
Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou, Henan 450006, China. E-mail: [email protected] b Department of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, USA. E-mail: hchen23@buffalo.edu
5516 | Dalton Trans., 2014, 43, 5516–5525
structure and its adiabatic detachment energy (ADE) is 2.54 eV.27 Recently, two kinds of cobalt complexes with mixed cyanide–carbonyl ligands were determined using single crystal X-ray diffraction.28 By employing pure rotational spectroscopy experiments22 and ab initio calculations,23 cobalt cyanide was determined to be a linear CoCN structure. Although monomeric metal cyanides have been widely studied, few investigations have been carried out for multiple metal cyanides. To the best of our knowledge, only SnnCN (n = 1–4) clusters were investigated using photoelectron spectroscopy and theoretical calculations.29 The investigation of cobalt cyanides has also been limited to a single Co atom. Moreover, one characteristic of organocobalt compounds in organic synthesis is that cobalt has a high affinity to carbon–nitrogen π-bonds.30 In this work, by employing density functional calculations, we examined the properties of neutral and anionic ConCN (n = 1–5) clusters, including geometric structures, molecular orbitals, IR spectra, adiabatic detachment energies (ADEs), vertical detachment energies (VDEs) and dissociation energies of various dissociation channels. To understand the chemical bonding in these clusters in detail, natural population analysis (NPA) and Wiberg bond orders analysis were performed. Our study can provide some insights into the interaction between Con clusters and the CN radical and predict the ADEs and VDEs of anionic ConCN− (n = 1–5) clusters.
2. Computational details The calculations were performed using density functional theory (DFT) with the hybrid B3LYP exchange–correlation
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functional.31–34 The structures of ConCN (n = 1–5) were calculated using the 6-311+G(d) basis set for C and N atoms and the LANL2DZ basis set for the Co atom. The 3d74s2 (4F9/2) electronic configuration of cobalt leads to many possibilities of ConCN clusters in low- and high-spin electronic configurations. To determine the most stable adsorption structures of CN on Con (n = 1–5) clusters, we have considered all possible spin multiplicities and various initial structures, including the structures of inserting Co atoms between C and N atoms, bonding Co atoms directly to N atoms and/or C atoms and making CN adsorb on the surfaces of Con clusters horizontally and perpendicularly. Geometry optimizations were conducted without any symmetry constraint. Harmonic vibrational frequencies were calculated to make sure that the optimized structures correspond to real local minima. The zero-point vibrational energies (ZPEs) and spin contamination were considered for all stable structures. The natural population analysis (NPA) of ConCN (n = 1–5) clusters was performed with the Natural Bond Orbital (NBO) version 3.1 program.35 All theoretical calculations were carried out with the Gaussian09 program package.36 The reliability of our computational methods was established in our previous study on ConC2H clusters.37 In order to further validate the accuracy of our computational methods for the ConCN clusters, we optimized the geometries of CoCN cluster anions and neutrals by different functionals (such as B3LYP, WB97XD and M06-2X) and basis sets (such as Def2QZVPD and the mixed basis set of 6-311+G(d)/LANL2DZ). The Co–C and C–N bond lengths of the optimized neutral CoCN ground state geometry are displayed in Table 1. The relative energies (RE), ADEs and VDEs of isomers 1A and 1B for anionic CoCN− clusters by the B3LYP functional with the mixed basis set (6-311+G(d) for C and N and LANL2DZ for Co) or the B3LYP functional with Def2-QZVPD basis set38 are given in Table 2. Table 1 shows that the Co–C and C–N bond lengths from the B3LYP functional match much better with experimental (Co–C)22 and highly accurate theoretical (C–N)39 values than those of WB97XD and M06-2X. Meanwhile, by comparison with the values of ref. 22 and 39, the bond lengths from the B3LYP functional with the mixed basis set are a little better than those from the Def2-QZVPD basis set. Besides, Table 2 shows that isomer 1A with a doublet state and isomer 1B with a quartet state are the first two lowest-energy structures of the anionic CoCN− cluster from both B3LYP/Def2-QZVPD and B3LYP/6-311+G(d)/LANL2DZ methods. Isomer 1B with a quartet
Table 1
state is 0.12 eV higher in energy than isomer 1A with a doublet state, either from the mixed or Def2-QZVPD basis set on the basis of the B3LYP functional. The ADEs and VDEs obtained from the mixed and Def2-QZVPD basis sets have minor differences for isomers 1A and 1B, but the Def2-QZVPD basis set is much more expensive than the mixed basis for late transition metal Con clusters. Therefore, the B3LYP functional in combination with the 6-311+G(d) basis set for C and N and the LANL2DZ basis set for Co was chosen for all the calculations of the ConCN−/0 (n = 1–5) system. The calculated energies of the optimized clusters are useful for probing their photoelectron spectroscopic properties. In our calculations, ADEs of clusters were calculated based on the energy differences between the neutral clusters and anionic clusters. Since photoelectrons detachment is a vertical process, after the electron is detached, the resulting one-electron less neutral structure will relax to the nearest local minimum; we optimized neutral clusters based on the anionic structures when we calculated the ADE. The total energy difference between the neutrals and anions both at the geometries of the anionic species gives the VDE for each low-lying isomer. The ADE of the anionic cluster is equal to the electron affinity (EA) of the neutral cluster if the ground state geometries of neutral and anionic clusters are similar. Otherwise EA is the energy difference between the neutral and anionic clusters both at their respective ground states.40 For the anionic clusters with multiplicity M, the neutral species with multiplicities M − 1 and M + 1 were considered in the VDE and ADE calculations. Based on the theoretically generalized Koopman theorem (GKT),41 the photoelectron spectral features can be viewed as photodetachment involving removal of an electron from the occupied molecular orbitals of the anionic cluster. Hence, we simulated the photoelectron spectra of the lowest-lying isomers of ConCN− (n = 1–5) clusters. In the simulation, we first set the transition related to the highest occupied molecular orbital (HOMO) of the anionic cluster to the position of
Table 2 The relative energies (RE), ADEs and VDEs (eV) of the anionic CoCN− cluster by the B3LYP functional
Mixed
Def2-QZVPD
Unit/eV
RE
ADE
VDE
1A 1B
0 0.12
1.55 1.42
1.55 1.42
1A 1B
RE
ADE
VDE
0 0.12
1.55 1.43
1.56 1.49
The Co–C and C–N bond lengths of neutral CoCN ground state geometry
B3LYP
WB97XD
M06-2X
Unit/Å
Mixed
Def2-QZVPD
Mixed
Def2-QZVPD
Mixed
Def2-QZVPD
Ref.
Co–C C–N
1.8879 1.1631
1.8819 1.1585
1.9008 1.1597
1.8881 1.1544
1.9599 1.1573
1.9565 1.1531
1.8827a 1.1677b
a The experimental value.22 b The highly accurate theoretical value.39 Mixed is the method of Co using LANL2DZ, C and N using the 6-311+G(d) basis set.
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the first VDE, and shifted the transitions of the deeper orbitals according to the HOMO transition. The relative energies of the orbitals (ΔEm) were calculated by the equation: ΔEm = E(HOMO) − E(HOMO−m), where E(HOMO) and E(HOMO−m) are the energies of the HOMO and (HOMO−m) orbitals from theoretical calculations. The higher VDEs were calculated by adding the ΔEm to the first VDE. The simulations were conducted by fitting the distribution of the transitions with unit-area Gaussian functions of 0.04 eV width. This method has been shown to be successful by comparison of the experimental photoelectron spectra with the simulated ones.42–45
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3. Results and discussion 3.1.
Geometries and dissociation energies
The optimized geometries of a few low-lying isomers of anionic and neutral ConCN (n = 1–5) clusters obtained with DFT calculations are displayed in Fig. 1 and 2, respectively. The calculated relative energies as well as ADEs and the first VDEs of the low-lying isomers of ConCN− (n = 1–5) clusters are listed in Table 3. A. CoCN− and CoCN. For the anionic CoCN− cluster, the first two lowest-energy isomers 1A with a doublet state and 1B with a quartet state (Fig. 1) are close in energy. Isomer 1B with a quartet state is only 0.12 eV higher in energy than isomer 1A with a doublet state. They are both linear structures with a Co atom interacting with CN via the C terminal. The C–N bond length of the linear isomer 1A with a doublet state is 1.17 Å and the Co–C bond length is 1.92 Å, longer than the covalent bond length (1.83 Å) of Co–C.46 It is interesting to compare the Co–CN with Co–Cl bond since CN radical is a pseudohalogen and its EA (3.86 eV)47 is close to that of Cl (3.6 eV).48 It is found that the calculated Co–CN bond length of anionic CoCN− is shorter than the Co–Cl average bond length in anionic [CoCl3(NCMe)]− (2.24 Å).49 Besides, we calculated bond dissociation energies of Co–CN and Co–Cl in anionic CoCN− and CoCl− by fragments of Co, CN− and Co, Cl−. The results show that the Co–CN dissociation energy (1.68 eV) is higher than that (1.35 eV) of the Co–Cl bond. These facts reveal that the Co–C bond of anionic CoCN− (isomer 1A) is stronger than the Co–Cl bond of the CoCl− cluster. The calculated VDEs of isomer 1A with a doublet state and 1B with a quartet state are 1.55 and 1.42 eV, respectively. Isomers 1C with a quartet state and 1D with a doublet state are 0.36 and 0.57 eV higher in energy than isomer 1A with a doublet state (Fig. 1). They are both composed of a Co atom interacting with CN via the N terminal. From the relative energies of isomers 1A–1D, we suggest that the ground state structure of the anionic CoCN− cluster is a linear cyanide configuration. Our calculations show that the ground state geometry of neutral CoCN (isomer 1a with a triplet state in Fig. 2) is also a linear cyanide structure, in agreement with the experimental results of the direct absorption techniques.22 The Co–C bond length of isomer 1a with a triplet state is 1.89 Å, in good agreement with the experimental measurement (1.8827 Å)22 and
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Fig. 1 Optimized geometries of the low-lying isomers of anionic ConCN− (n = 1–5) clusters. Bond lengths are given in angstrom.
ab initio molecular orbital studies (1.8540 Å).23,39 The C–N bond length (1.1677 Å) of the ground state structure CoCN from ab initio calculations23 differs from that (1.1313 Å) of the pure rotational spectroscopy experiment.22 Hirano et al.23
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Fig. 2 Optimized geometries of the low-lying isomers of neutral ConCN (n = 1–5) clusters. Bond lengths are given in angstrom and S represents spin multiplicities.
Table 3 Relative energies (RE, eV), spin multiplicities (S) of the lowlying structures of ConCN− (n = 1–5) as well as their ADEs and the first VDEs (eV) obtained by DFT calculations
Cluster CoCN−
Co2CN−
Co3CN−
Co4CN−
Co5CN−
1A 1B 1C 1D 2A 2B 2C 2D 3A 3B 3C 3D 4A 4B 4C 4D 5A 5B 5C 5D 5E 5F 5G 5H
RE
Sym
S
VDE
ADE
0 0.12 0.36 0.57 0 0.23 0.24 0.47 0 0.23 0.25 0.40 0 0.08 0.21 0.23 0 0.15 0.18 0.19 0.22 0.26 0.27 0.27
C∞v C∞v C∞v C∞v C∞v C∞v C∞v C∞v C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1
2 4 4 2 5 3 5 3 8 8 6 6 9 9 9 9 12 12 8 10 10 10 12 12
1.55 1.42 1.60 1.39 1.91 2.86 1.83 2.89 2.07 2.04 2.54 2.55 2.49 2.53 2.08 2.73 2.63 2.73 3.29 1.50 1.93 2.21 2.39 2.54
1.55 1.42 1.60 1.39 1.91 2.86 1.83 2.89 1.96 1.95 2.44 2.19 1.96 2.14 1.97 2.19 2.34 1.87 2.89 1.43 1.31 1.30 1.35 2.24
suggested that this discrepancy is caused by neglecting the large-amplitude bending motion of CoCN when the C–N bond length was calculated from the rotational constant of the pure rotational spectroscopy. Our calculated C–N bond length is 1.16 Å, in good accord with the ab initio molecular orbital study (1.1677 Å) of a high-quality method.39 The calculated dipole moment of isomer 1a with a triplet state is 7.78 D, in accord with the value (7.48 D) of ab initio calculations.30
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Isomer 1b with a triplet state (Fig. 2) is 0.42 eV higher in energy than isomer 1a. It is an isocyanide CoNC structure. Isomer 1c with a quintet state (Fig. 2) is much higher (1.90 eV) in energy than isomer 1a with a triplet state. Therefore, the ground state geometry of neutral CoCN is a linear cyanide configuration. The ground state structure of the neutral CoCN is nearly identical to its anionic counterpart with slight changes in the bond lengths, and thus the ADE of the anionic CoCN− cluster equals the EA of the neutral CoCN cluster. B. Co2CN− and Co2CN. The first two stable structures (isomers 2A with a quintet state and 2B with a triplet state in Fig. 1) of the anionic Co2CN− cluster are both linear cyanide configurations. Isomer 2A with a quintet state is 0.23 eV lower in energy than isomer 2B with a triplet state. They can be described as a Co atom attaching to isomer 1A via a Co–Co bond. The Co–Co bond length of isomer 2A is about 2.42 Å, which is close to the Co–Co bond of the pure Co2 cluster (2.36–2.44 Å).50 The calculated VDE of isomer 2A is 1.91 eV. Isomers 2C with a quintet state and 2D with a triplet state (Fig. 1) are both isocyanide structures. They are 0.24 and 0.47 eV higher in energy than isomer 2A. Therefore, the most stable isomer (2A) of Co2CN− is a linear structure with Co2 attaching to the C terminal of CN. For the neutral Co2CN cluster, the most stable isomer 2a (Fig. 2) is composed of Co2 attaching to the C terminal of CN. The Co–Co, Co–C and C–N bond lengths of isomer 2a are 2.28, 1.96 and 1.16 Å, respectively, a little shorter than those (2.42, 1.97, 1.17 Å) of the anionic Co2CN− cluster. It implies that Co– Co, Co–C and C–N interactions of the neutral Co2CN cluster are stronger than those of the corresponding anionic cluster. The dipole moment of isomer 2a is 8.68 D, close to that of the typical ionic compound NaCl (9.0 D). In addition, we calculated bond dissociation energies of Co2CN and NaCl by fragments of Co2, CN and Na, Cl. The Co2–CN bond dissociation is 5.57 eV, much higher than that (3.99 eV) of NaCl. These facts show that the Co–CN bond of isomer 2a is strong and partly ionic. Isomer 2b with an isocyanide structure (Fig. 2) is 0.16 eV higher in energy than isomer 2a. Isomer 2c with a quartet state (Fig. 2) is much higher in energy (1.17 eV) than isomer 2a with a sextet state. Therefore, the most stable structure of the neutral Co2CN cluster is a linear cyanide configuration. The ground state geometry of neutral Co2CN is nearly identical to the corresponding anionic cluster except for minor differences in bond lengths, revealing that the ADE of the anionic Co2CN− cluster equals the EA of the neutral Co2CN cluster. C. Co3CN− and Co3CN. Our calculated Co3− is a quasi-linear structure, in agreement with the result from photoelectron spectroscopy and theoretical calculations.51 However, in the first two lowest-energy isomers (3A and 3B in Fig. 1) of the Co3CN− cluster, the three Co atoms form a triangle due to attaching a CN to Co3−. Isomer 3A is composed of the vertex Co atom of the triangular Co3 cluster attaching to the C terminal of CN. Isomer 3B is 0.23 eV in energy less stable than isomer 3A. It is composed of the vertex Co atom of the triangular Co3 cluster attaching to the N terminal of CN. The theoretical VDEs of isomers 3A and 3B are 2.07 and 2.04 eV,
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respectively. Isomer 3C with a sextet state (Fig. 1) is 0.25 eV higher in energy than isomer 3A with an octet state. It is formed by the triangular Co3 attaching the CN through the C terminal on its top site. Isomer 3D with a sextet state (Fig. 1) is 0.40 eV higher in energy than isomer 3A with an octet state. From relative energies of isomers 3A–3D, we suggest that isomer 3A is the most stable structure of the anionic Co3CN− cluster. The most stable neutral Co3 cluster is calculated to be an angle-like structure of about 144°. By attaching a CN to a Co3 cluster, the ground state structure (isomer 3a in Fig. 2) of the neutral Co3CN cluster is a quasi-planar five-member ring from the interaction of an acute triangle Co3 with C and N atoms of CN. Isomer 3b (Fig. 2) is 0.25 eV higher in energy than isomer 3a. It is formed by the vertex of the triangle Co3 cluster adsorbing CN via the C terminal. Isomer 3c (Fig. 2) is much higher in energy (0.47 eV) than isomer 3a. It is composed of the vertex of the triangle Co3 cluster adsorbing CN via the N terminal. Therefore, the lowest-energy structure of neutral Co3CN is a five-member ring configuration. The ground state structure of neutral Co3CN is vastly different from the anionic species. This suggests that the ADE (1.96 eV) of the anionic Co3CN− cluster is not equal to the EA (1.71 eV) of neutral Co3CN. D. Co4CN− and Co4CN. We optimized the structures of the anionic Co4CN− cluster by considering all possible geometries of the Co4 cluster. The results show that the first four stable isomers of Co4CN− differ only by 0.08, 0.21 and 0.23 eV in energy. They all have a nonet spin state. Isomer 4A (Fig. 1) is a quasi-planar structure of the CN adsorbed on the top site of the distorted rhombus Co4 cluster through the C terminal. Isomer 4B (Fig. 1) is a structure composed of a Co–Co bond of the distorted rhombus Co4 cluster bridged by the C terminal of CN. Isomer 4C (Fig. 1) is a six-member ring composed of a four-member ring Co4 and the CN. The structure of isomer 4D (Fig. 1) is similar to isomer 4A, but the CN attaches to the Co4 cluster through the N terminal. The calculated VDEs of isomers 4A–4D are 2.49, 2.53 2.08, 2.73 eV, respectively. Because the relative energies of isomers 4A and 4B are very close, we predict either isomer 4A or 4B to be the ground state structure of the anionic Co4CN− cluster. The most stable structure (isomer 4a in Fig. 2) of neutral Co4CN is composed of the distorted rhombus Co4 adsorbing the C terminal of CN on its bridge site. The two Co–C bond lengths are 1.98 and 2.17 Å, respectively. Isomers 4b with a dectet state and 4c with an octet state (Fig. 2) are 0.02 and 0.22 eV higher in energy than isomer 4a with a dectet state, respectively. They are both composed of the distorted rhombus Co4 adsorbing the CN on its bridge site via C and N atoms. Since isomer 4b is only 0.02 eV less stable than isomer 4a, we suggest that either isomer 4a or 4b is the ground state structure of neutral Co4CN. We optimized the most stable structures of bare Co4 neutral and anionic clusters to be planar four-member configurations; therefore, attaching a CN to neutral and anionic Co4 makes little changes to their structures. E. Co5CN− and Co5CN. The most stable structure of the anionic Co5CN− cluster is isomer 5A (Fig. 1), composed of CN
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adsorbed on the top site of the distorted bipyramid Co5 cluster via the C terminal. The calculated VDE of isomer 5A is 2.63 eV. Isomer 5B (Fig. 1) is 0.15 eV higher in energy than isomer 5A. It is composed of CN adsorbed on the top site of the W-like Co5 cluster via the C terminal. The calculated VDE of isomer 5B is 2.73 eV. Isomers 5C with an octet state and 5F with a dectet state (Fig. 1) have similar structures with isomer 5A with a 12-et state. They are 0.18 and 0.26 eV higher in energy than isomer 5A. Isomer 5D with a dectet state (Fig. 1) is 0.19 eV higher energetically than isomer 5A with a 12-et state. It is a structure composed of CN adsorbed on the bridge site of the W-like Co5 cluster via both C and N atoms. Isomer 5E with a dectet state, 5G with a 12-et state and 5H with a 12-et state are 0.22, 0.27, and 0.27 eV higher in energy than isomer 5A with a 12-et state, respectively. As a result, the lowest-energy structure of Co5CN− is composed of the distorted bipyramid Co5 adsorbing the C terminal of CN on its top site. We calculated the most stable Co5 anionic cluster to be a W-like structure, thus attaching a CN makes the structure of the Co5CN anionic cluster very different from that of bare Co5−. The most stable Co5 neutral structure calculated by us is a W-like configuration, in agreement with the result of Sebetci et al.52 For neutral Co5CN, the ground state structure (isomer 5a in Fig. 2) is composed of CN adsorbed on the bridge site of the W-like Co5 cluster via both C and N atoms, which indicates that the attached CN has a slight influence on the structure of the bare Co5 cluster. Isomer 5b (Fig. 2) is 0.33 eV higher in energy than isomer 5a. It is a structure of CN adsorbed on the bridge site of the W-like Co5 cluster via the N terminal. Isomer 5c (Fig. 2) is 0.48 eV higher in energy than isomer 5a. The ground state structure of neutral Co5CN is very different from that of the anionic Co5CN−. The large geometric difference between neutral and anionic Co5CN results in the ADE (2.34 eV) of anionic Co5CN− is not equal to the EA (1.54 eV) of neutral Co5CN. In summary, the most stable structures of neutral and anionic ConCN (n = 1–5) clusters are all planar or quasi-planar structures except the anionic Co5CN− cluster which is a three dimensional structure. In the most stable structures of anionic ConCN− (n = 1–5) clusters, the CN interacts with only one Co atom via its C terminal, and the C–N bond lengths are all 1.17 Å. In the most stable structures of neutral ConCN (n = 1–5) clusters, the CN interacts with one Co atom for n = 1–2, whereas the CN interacts with two Co atoms for n = 3–5. The C–N bond lengths (1.18 Å) of neutral ConCN (n = 3–5) are a little longer than those (1.16 Å) of neutral ConCN (n = 1–2), and they are longer than that (1.15 Å) of HCN. Thus Con (n = 3–5) clusters have greater effect on the C–N triple bond than Con (n = 1–2). F. Dissociation energies. To further investigate the stability of neutral and anionic ConCN (n = 1–5) clusters, we have calculated the dissociation energies of these clusters. Due to the complexity of dissociation channels for ConCN (n = 1–5) clusters, we only considered the channels of losing one Co or CN. For anionic clusters, due to the existence of extra electron, it has more dissociation pathways than the neutral. The stability
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of neutral and anionic ConCN (n = 1–5) clusters was determined via the energies of the following dissociation pathways. Anionic clusters Con CN ¼ Con þ CN
ΔEanion 1
ð1Þ
Con CN ¼ Con þ CN
ΔEanion 2
ð2Þ
Con CN ¼ Co þ ½Coðn1Þ CN
ΔEanion 3
Con CN ¼ Co þ Coðn1Þ CN
ð3Þ
ΔEanion 4
ð4Þ
Neutral clusters
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Con CN ¼ Con þ CN
ΔEneutral 5
Con CN ¼ Co þ Coðn1Þ CN
ð5Þ
ΔEneutral 6
ð6Þ
The dissociation energies of these six channels for neutral and anionic ConCN (n = 1–5) clusters are given in Table 4. The dissociation energies of neutral and anionic ConCN (n = 1–5) clusters in Table 4 reveal that these clusters are stable against fragmentation. From Table 4, we note that the dissociation channel of the anionic CoCN− cluster into Co and CN− is favorable to the dissociation channel into Co− and CN. This preferred channel is consistent with the fact that the EA (3.86 eV) of CN is much larger than that of Co (0.66 eV).53 For anionic ConCN (n = 2–5) clusters, channel (1) is favorable to channel (2), since the EA (3.86 eV) of CN is much larger than those of Con (n = 2–5) clusters (1.11 eV, 1.6 eV, 1.91 eV, 1.56 eV).53 Because of the EAs of ConCN (n = 1–5) clusters larger than that of Co, channel (3) is preferred to channel (4). The most favored dissociation pathway of anionic ConCN (n = 2–5) clusters is channel (3) since ΔEanion is the lowest dissociation energy. 3 This fact occurs due to the bond dissociation of Co–C54 being larger than the Co–Co bond.55 As seen in Table 4, the dissociation energies of Co atom loss channels exhibit a clear odd– even alternation in stability for neutral ConCN (n = 1–5) clusters, with odd n being more stable than the adjacent even n − 1 and n + 1 ones. A slight odd–even alternation in stability for anionic ConCN− (n = 1–5) clusters can also be observed from channel (2), with even n being more stable than the adjacent odd n ones. The odd–even alternation in stability for anionic ConCN− (n = 1–5) clusters is opposite for channel (4) with odd n being slightly more stable than the even n. These odd–even alternations may be due to electron parity effect of parent and fragment molecules or ions, which were also found for 3d transition metal carbides (from ScCn to CoCn clusters).56 The dissociation channel (6)
with the loss of a Co is preferred to channel (5) with the loss of a CN for neutral ConCN (n = 2–5) clusters.
3.2.
The simulated photoelectron spectra
Fig. 3 displays the simulated photoelectron spectra of ConCN− (n = 1–5) clusters at 193 nm photons. From Fig. 3, we can see that the first VDEs of ConCN− (n = 1–5) become larger with the increase of Co atoms. For CoCN− (isomer 1A), we can observe that the VDEs are centered at about 1.56, 2.59, 2.68, 3.63, 3.76, 4.63, 5.89, 6.11 and 6.24 eV, respectively. The peaks centered at 1.97, 3.15, 3.33, 3.55, 3.7, 3.93, 4.58, 4.88, 5.26, 5.46, 5.87 eV can be observed in the simulated photoelectron spectrum of Co2CN− (isomer 2A). For larger size ConCN− (n = 3–5), the simulated photoelectron spectra are very congested. This might be due to a high density of states from 3d electrons of Con clusters and might be also the results of the presence of several isomers. EA is a significant factor for reactivity. Thus, we compared the EAs of bare Con and ConCN (n = 1–5). The calculated EAs of ConCN (n = 1–2) clusters are 1.55 eV and 1.91 eV, much higher than those (0.66 eV and 1.11 eV) of Con (n = 1–2)53 and much lower than that (3.86 eV) of CN, indicating that the interaction between Con (n = 1–2) and CN is strong and they both play a significant role in the photoelectron spectral features of ConCN− (n = 1–2) clusters. The calculated EAs of ConCN (n = 3–5) clusters are 1.71 eV, 1.96 eV and 1.54 eV, respectively, close to those (1.6 eV, 1.91 eV, and 1.56 eV) of Con (n = 3–5).53 It implies that Con (n = 3–5) clusters play an important role in the photoelectron spectral features of ConCN− (n = 3–5).
Table 4 Dissociation energies (eV) of neutral and anionic ConCN (n = 1–5) clusters
n
ΔEanion 1
ΔEanion 2
ΔEanion 3
ΔEanion 4
ΔEneutral 5
ΔEneutral 6
1 2 3 4 5
1.68 3.41 2.48 2.47 2.44
5.14 5.97 5.14 5.19 5.05
1.68 1.73 2.10 2.03 1.92
5.14 2.67 3.41 3.13 3.28
4.20 5.57 4.84 4.58 4.92
4.20 1.37 2.30 1.77 2.35
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Fig. 3 The simulated photoelectron spectra of the ground state structures (isomers 1A–5A) of anionic ConCN− (n = 1–5) clusters.
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3.3. The frontier molecular orbital analysis of anionic ConCN− (n = 1–5) clusters To investigate the interaction of Con clusters with CN, we examined the electron densities of their frontier molecular orbitals. The electron distributions of the first six highest occupied molecular orbitals (HOMO) of the lowest-energy structures of anionic ConCN− (n = 1–5) clusters are presented in Fig. 4. Fig. 4 demonstrates that HOMO of anionic CoCN− is composed of the 3dxz orbital of Co and the π orbital of CN. Similarly, HOMO−1 is contributed by the 3dyz orbital of Co and the π orbital of CN. HOMO−2 is only from the contribution of the 3dxy orbital of Co. HOMO−3 is a σ-orbital, primarily s–dz2 characteristic of C and Co. HOMO−4 is only contributed by the 3dx2−y2 orbital of Co. HOMO−5 is a π-orbital, primarily from the (p–d)π Co–CN bonding orbitals with the interaction of the 3dyz orbital of Co and the π orbital of CN. This indicates that the interaction between Co and CN is strong for CoCN−. For Co2CN−, HOMO and HOMO−1 are contributed by 3dx2−y2 and 3dxy orbitals of Co, respectively. HOMO−2 and HOMO−3 are due to the hybridization of 4s, 3dz2 between two Co atoms, revealing strong interaction between them. HOMO−4 is contributed by 3dxz orbitals of the two Co atoms and the π
Fig. 4
orbital of CN. HOMO−5 is contributed by 3dyz orbitals of the two Co atoms and the π orbital of CN. For ConCN− (n = 3–5), the HOMOs are mainly contributed by 3d orbitals of Con (n = 3–5) clusters. This indicates that the photoelectron spectra features of anionic ConCN− (n = 3–5) are mostly the contributions from 3d electrons of Con (n = 3–5). Perhaps it induces the photoelectron spectra of ConCN− (n = 3–5) to be very congested. 3.4. The calculated NPA charge, Wiberg bond order, and the simulated IR spectra To understand the chemical bonding in anionic and neutral ConCN (n = 1–5) clusters, we carried out natural population analysis (NPA) and Wiberg bond orders analysis. The NPA charge of Con and CN for the most stable structures of neutral and anionic ConCN (n = 1–5) clusters as well as the Co–C and C–N bond orders are displayed in Table 5. The NPA charge distribution of anionic and neutral ConCN (n = 1–5) clusters shows that the negative charge is mainly distributed on CN, indicating that the electrons always transfer from Con to CN, thus Co–C bonds are partly ionic. This is in agreement with the fact that the EA (3.86 eV) of CN is much larger than those (0.66, 1.11, 1.6, 1.91, 1.71 eV) of Con (n = 1–5) clusters.53 For
The frontier molecular orbitals of anionic ConCN− (n = 1–5) clusters (isomers 1A–5A).
5522 | Dalton Trans., 2014, 43, 5516–5525
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Table 5 Theoretical charge populations and bond order analysis for the most stable structures of anionic and neutral ConCN (n = 1–5) clusters (isomers 1A–5A and 1a–5a)
Bond order Anion cluster −
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CoCN Co2CN− Co3CN− Co4CN− Co5CN−
Charge
Bond order
Charge
Co–C
C–N
Con
CN
Neutral cluster
Co–C
C–N
Con
CN
0.57 0.55 0.58 0.61 0.62
2.83 2.84 2.86 2.86 2.87
−0.239 −0.256 −0.328 −0.361 −0.396
−0.761 −0.744 −0.672 −0.639 −0.604
CoCN Co2CN Co3CN Co4CN Co5CN
0.62 0.55 0.41 0.48 0.50
2.92 2.90 2.67 2.73 2.64
0.687 0.625 0.699 0.638 0.631
−0.687 −0.625 −0.699 −0.638 −0.631
anionic ConCN (n = 1–5) clusters, negative charges reside on both Con and CN, resulting in a repulsive electrostatic interaction between them. The bond order (Table 5) of Co–C in anionic ConCN (n = 1–5) clusters shows that the covalent interaction predominates the repulsive electrostatic interaction which makes these anionic clusters stable. Table 5 shows that the C–N bond orders of neutral ConCN (n = 3–5) clusters are smaller than those of the corresponding anions which reveals that neutral Con (n = 3–5) clusters have better activation effect on C–N triple bonds than those of anions. The C–N bond orders of ConCN (n = 3–5) are smaller than those of ConCN (n = 1–2) in neutral ConCN clusters. It implies that larger Con (n = 3–5) clusters have better C–N activation effect than Con (n = 1–2). The bond orders in combination with charge
analysis (Table 5) show the Co–C bonds of neutral and anionic ConCN (n = 1–5) clusters to be a mixture of ionic and covalent characters. The simulated IR spectra of CN radical and the ground state structures of neutral and anionic ConCN (n = 1–5) are presented in Fig. 5. The calculated Co–C stretching mode of neutral CoCN is at 441 cm−1, which is in reasonable agreement with that (478 cm−1) from the pure rotational spectrum of CoCN.22 The C–N stretching mode of CoCN is at 2222 cm−1, in good agreement with the experimental value of 2191 cm−1.22 For neutral Co2CN, the Co–Co, Co–C and C–N stretching frequencies are 237 cm−1, 425 cm−1 and 2228 cm−1, respectively. Fig. 5 shows that C–N stretching frequencies of neutral ConCN (n = 1–2) are larger than that of CN radical (2147 cm−1) at the
Fig. 5 The simulated IR spectra of CN radical and the ground state structures of anionic and neutral ConCN (n = 1–5) clusters (isomers 1A–5A and 1a–5a).
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same level. However, C–N stretching frequencies of neutral ConCN (n = 3–5) are smaller than that of CN radical. For anionic ConCN (n = 1–5), the calculated C–N stretching frequencies are all larger than that of CN radical. Therefore, compared to the stretching frequency of CN radical, the C–N stretching frequencies of neutral ConCN (n = 1–2) and anionic ConCN (n = 1–5) are blue-shifted while they are red-shifted for neutral ConCN (n = 3–5). This fact shows that the neutral Con (n = 3–5) clusters have better C–N bond activation effect than neutral Con (n = 1–2) and anionic Con− (n = 3–5) clusters. The cyanide radical (CN) is isoelectronic to the ethynyl radical (C2H), and they are both very reactive species in organic reactions. Hence, it is interesting to compare ConC2H and ConCN (n = 1–5) clusters. We investigated neutral and anionic ConC2H (n = 1–5) clusters by DFT calculations and photoelectron spectroscopy experiments previously.37 It was found that the ground state VDEs of ConCN− (n = 1–5) are all larger than those of ConC2H− (n = 1–5). Perhaps it is because the EA of CN (3.86 eV) is larger than that of C2H (2.97 eV). The structures of neutral ConCN and ConC2H (n = 1–5) clusters are similar and planar or quasi-planar structures. Anionic ConCN (n = 1–2) and ConC2H (n = 1–2) clusters are also similar and linear structures. However, the structures of anionic ConCN (n = 3–5) and ConC2H (n = 3–5) clusters are different with CN adsorbed on the top sites of Con and C2H adsorbed on the bridge sites of Con via the C terminal. The calculated magnetic moments of the most stable isomers of neutral ConCN (n = 1–5) clusters are 2, 5, 6, 9, and 10μB, respectively. These values are the same as those of ConC2H clusters which shows that adsorption of CN and C2H both have little effect on the magnetic moments of Con clusters.
4.
Conclusions
The anionic and neutral ConCN (n = 1–5) clusters were studied using density functional calculations. The calculations have shown that the CN prefers to be adsorbed on the top sites of Con clusters via the C terminal in the anionic ConCN− (n = 1–5) clusters. In neutral ConCN (n = 1–5) clusters, the CN is mainly adsorbed on the top sites of the Con (n = 1–2) clusters via the C terminal whereas it is adsorbed on the bridge sites of Con (n = 3–5) via both C and N atoms. The calculated C–N stretching frequencies of neutral ConCN (n = 3–5) are red-shifted while they are blue-shifted for neutral ConCN (n = 1–2), which is in agreement with the C–N bond lengthening more in neutral ConCN (n = 3–5) clusters than in ConCN (n = 1–2). The first vertical detachment energies (VDEs) of anionic ConCN− (n = 1–5) clusters were predicted to be 1.55, 1.91, 2.07, 2.49 and 2.63 eV, respectively.
Acknowledgements This work was supported by the Natural Science Foundation of the Education Department of Henan Province (no.
5524 | Dalton Trans., 2014, 43, 5516–5525
13B150987), the Natural Science Foundation of Zhengzhou City (no. 20120324), and the Natural Science Foundation of China (no. 21206049).
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Investigation of ternary ConCN−1/0 (n = 1–5) clusters by density functional calculations Jinyun Yuan,a Shuaiwei Wang,a Yubing Si,a Baocheng Yang*a and Houyang Chen*b The neutral and anionic ConCN (n = 1–5) clusters were investigated using density functional calculations. The most stable structures of neutral and anionic ConCN (n = 1–5) clusters have been identified. In these structures, the CN radical retains its integrity as a structural unit. For anionic ConCN− (n = 1–5) and neutral ConCN (n = 1–2) clusters, the CN prefers to be adsorbed on the top sites of Con clusters via the C terminal, forming a linear or quasi-linear N–C–Co structure. For neutral ConCN (n = 3–5) clusters, the CN is adsorbed on the bridge sites of Con (n = 3–5) by both C and N atoms. Compared to the free CN radical
Received 30th September 2013, Accepted 22nd January 2014 DOI: 10.1039/c3dt52728j www.rsc.org/dalton
1.
calculated at the same level, the C–N stretching frequencies of neutral ConCN (n = 3–5) are red-shifted while they are blue-shifted for neutral ConCN (n = 1–2). The adiabatic and vertical detachment energies of anionic ConCN− (n = 1–5) clusters are calculated based on density functional calculations. In addition, the most favored dissociation channels of neutral and anionic ConCN (n = 1–5) clusters are determined by calculating the dissociation energies of various possible dissociation pathways.
Introduction
Metal cyanides have potential applications in sensors,1 batteries,2 catalysts,3 photochemistry,4 magnetic devices,5 color imaging,6 and astrophysics.7 A large number of metal cyanides have been investigated experimentally and theoretically, including transition metal cyanides, alkali metal cyanides, and alkaline earth metal cyanides. Ground state structures of sodium and potassium cyanides were found to be T-shaped configurations.8,9 Ab initio calculations and gas-phase rotational spectra have shown that lithium and alkaline earth metal monocyanides exhibit isocyanide MNC equilibrium structures.10–14 Vera et al. confirmed that isocyanide AlNC is significantly more abundant than cyanide AlCN in the interstellar medium.15 In the case of transition metal cyanides, experimental measurements and quantum chemical calculations have shown that the late 3d transition metals (from Co to Zn) prefer a cyanide configuration, whereas the early 3d transition metals (from Sc to Fe) except Cr favor the isocyanide isomer.16–24 The equilibrium geometries and ro-vibrational spectra of the linear coinage-metal cyanides were predicted by theoretical calculations.25 The endohedral clusterfullerene of a triangular YCN cluster entrapped inside a C82 cage has been synthesized.26 A photoelectron spectroscopic study of the palladium cyanide anion suggested that it was a cyanide PdCN
a
Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou, Henan 450006, China. E-mail: [email protected] b Department of Chemical and Biological Engineering, State University of New York at Buffalo, Buffalo, New York 14260-4200, USA. E-mail: hchen23@buffalo.edu
5516 | Dalton Trans., 2014, 43, 5516–5525
structure and its adiabatic detachment energy (ADE) is 2.54 eV.27 Recently, two kinds of cobalt complexes with mixed cyanide–carbonyl ligands were determined using single crystal X-ray diffraction.28 By employing pure rotational spectroscopy experiments22 and ab initio calculations,23 cobalt cyanide was determined to be a linear CoCN structure. Although monomeric metal cyanides have been widely studied, few investigations have been carried out for multiple metal cyanides. To the best of our knowledge, only SnnCN (n = 1–4) clusters were investigated using photoelectron spectroscopy and theoretical calculations.29 The investigation of cobalt cyanides has also been limited to a single Co atom. Moreover, one characteristic of organocobalt compounds in organic synthesis is that cobalt has a high affinity to carbon–nitrogen π-bonds.30 In this work, by employing density functional calculations, we examined the properties of neutral and anionic ConCN (n = 1–5) clusters, including geometric structures, molecular orbitals, IR spectra, adiabatic detachment energies (ADEs), vertical detachment energies (VDEs) and dissociation energies of various dissociation channels. To understand the chemical bonding in these clusters in detail, natural population analysis (NPA) and Wiberg bond orders analysis were performed. Our study can provide some insights into the interaction between Con clusters and the CN radical and predict the ADEs and VDEs of anionic ConCN− (n = 1–5) clusters.
2. Computational details The calculations were performed using density functional theory (DFT) with the hybrid B3LYP exchange–correlation
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functional.31–34 The structures of ConCN (n = 1–5) were calculated using the 6-311+G(d) basis set for C and N atoms and the LANL2DZ basis set for the Co atom. The 3d74s2 (4F9/2) electronic configuration of cobalt leads to many possibilities of ConCN clusters in low- and high-spin electronic configurations. To determine the most stable adsorption structures of CN on Con (n = 1–5) clusters, we have considered all possible spin multiplicities and various initial structures, including the structures of inserting Co atoms between C and N atoms, bonding Co atoms directly to N atoms and/or C atoms and making CN adsorb on the surfaces of Con clusters horizontally and perpendicularly. Geometry optimizations were conducted without any symmetry constraint. Harmonic vibrational frequencies were calculated to make sure that the optimized structures correspond to real local minima. The zero-point vibrational energies (ZPEs) and spin contamination were considered for all stable structures. The natural population analysis (NPA) of ConCN (n = 1–5) clusters was performed with the Natural Bond Orbital (NBO) version 3.1 program.35 All theoretical calculations were carried out with the Gaussian09 program package.36 The reliability of our computational methods was established in our previous study on ConC2H clusters.37 In order to further validate the accuracy of our computational methods for the ConCN clusters, we optimized the geometries of CoCN cluster anions and neutrals by different functionals (such as B3LYP, WB97XD and M06-2X) and basis sets (such as Def2QZVPD and the mixed basis set of 6-311+G(d)/LANL2DZ). The Co–C and C–N bond lengths of the optimized neutral CoCN ground state geometry are displayed in Table 1. The relative energies (RE), ADEs and VDEs of isomers 1A and 1B for anionic CoCN− clusters by the B3LYP functional with the mixed basis set (6-311+G(d) for C and N and LANL2DZ for Co) or the B3LYP functional with Def2-QZVPD basis set38 are given in Table 2. Table 1 shows that the Co–C and C–N bond lengths from the B3LYP functional match much better with experimental (Co–C)22 and highly accurate theoretical (C–N)39 values than those of WB97XD and M06-2X. Meanwhile, by comparison with the values of ref. 22 and 39, the bond lengths from the B3LYP functional with the mixed basis set are a little better than those from the Def2-QZVPD basis set. Besides, Table 2 shows that isomer 1A with a doublet state and isomer 1B with a quartet state are the first two lowest-energy structures of the anionic CoCN− cluster from both B3LYP/Def2-QZVPD and B3LYP/6-311+G(d)/LANL2DZ methods. Isomer 1B with a quartet
Table 1
state is 0.12 eV higher in energy than isomer 1A with a doublet state, either from the mixed or Def2-QZVPD basis set on the basis of the B3LYP functional. The ADEs and VDEs obtained from the mixed and Def2-QZVPD basis sets have minor differences for isomers 1A and 1B, but the Def2-QZVPD basis set is much more expensive than the mixed basis for late transition metal Con clusters. Therefore, the B3LYP functional in combination with the 6-311+G(d) basis set for C and N and the LANL2DZ basis set for Co was chosen for all the calculations of the ConCN−/0 (n = 1–5) system. The calculated energies of the optimized clusters are useful for probing their photoelectron spectroscopic properties. In our calculations, ADEs of clusters were calculated based on the energy differences between the neutral clusters and anionic clusters. Since photoelectrons detachment is a vertical process, after the electron is detached, the resulting one-electron less neutral structure will relax to the nearest local minimum; we optimized neutral clusters based on the anionic structures when we calculated the ADE. The total energy difference between the neutrals and anions both at the geometries of the anionic species gives the VDE for each low-lying isomer. The ADE of the anionic cluster is equal to the electron affinity (EA) of the neutral cluster if the ground state geometries of neutral and anionic clusters are similar. Otherwise EA is the energy difference between the neutral and anionic clusters both at their respective ground states.40 For the anionic clusters with multiplicity M, the neutral species with multiplicities M − 1 and M + 1 were considered in the VDE and ADE calculations. Based on the theoretically generalized Koopman theorem (GKT),41 the photoelectron spectral features can be viewed as photodetachment involving removal of an electron from the occupied molecular orbitals of the anionic cluster. Hence, we simulated the photoelectron spectra of the lowest-lying isomers of ConCN− (n = 1–5) clusters. In the simulation, we first set the transition related to the highest occupied molecular orbital (HOMO) of the anionic cluster to the position of
Table 2 The relative energies (RE), ADEs and VDEs (eV) of the anionic CoCN− cluster by the B3LYP functional
Mixed
Def2-QZVPD
Unit/eV
RE
ADE
VDE
1A 1B
0 0.12
1.55 1.42
1.55 1.42
1A 1B
RE
ADE
VDE
0 0.12
1.55 1.43
1.56 1.49
The Co–C and C–N bond lengths of neutral CoCN ground state geometry
B3LYP
WB97XD
M06-2X
Unit/Å
Mixed
Def2-QZVPD
Mixed
Def2-QZVPD
Mixed
Def2-QZVPD
Ref.
Co–C C–N
1.8879 1.1631
1.8819 1.1585
1.9008 1.1597
1.8881 1.1544
1.9599 1.1573
1.9565 1.1531
1.8827a 1.1677b
a The experimental value.22 b The highly accurate theoretical value.39 Mixed is the method of Co using LANL2DZ, C and N using the 6-311+G(d) basis set.
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the first VDE, and shifted the transitions of the deeper orbitals according to the HOMO transition. The relative energies of the orbitals (ΔEm) were calculated by the equation: ΔEm = E(HOMO) − E(HOMO−m), where E(HOMO) and E(HOMO−m) are the energies of the HOMO and (HOMO−m) orbitals from theoretical calculations. The higher VDEs were calculated by adding the ΔEm to the first VDE. The simulations were conducted by fitting the distribution of the transitions with unit-area Gaussian functions of 0.04 eV width. This method has been shown to be successful by comparison of the experimental photoelectron spectra with the simulated ones.42–45
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3. Results and discussion 3.1.
Geometries and dissociation energies
The optimized geometries of a few low-lying isomers of anionic and neutral ConCN (n = 1–5) clusters obtained with DFT calculations are displayed in Fig. 1 and 2, respectively. The calculated relative energies as well as ADEs and the first VDEs of the low-lying isomers of ConCN− (n = 1–5) clusters are listed in Table 3. A. CoCN− and CoCN. For the anionic CoCN− cluster, the first two lowest-energy isomers 1A with a doublet state and 1B with a quartet state (Fig. 1) are close in energy. Isomer 1B with a quartet state is only 0.12 eV higher in energy than isomer 1A with a doublet state. They are both linear structures with a Co atom interacting with CN via the C terminal. The C–N bond length of the linear isomer 1A with a doublet state is 1.17 Å and the Co–C bond length is 1.92 Å, longer than the covalent bond length (1.83 Å) of Co–C.46 It is interesting to compare the Co–CN with Co–Cl bond since CN radical is a pseudohalogen and its EA (3.86 eV)47 is close to that of Cl (3.6 eV).48 It is found that the calculated Co–CN bond length of anionic CoCN− is shorter than the Co–Cl average bond length in anionic [CoCl3(NCMe)]− (2.24 Å).49 Besides, we calculated bond dissociation energies of Co–CN and Co–Cl in anionic CoCN− and CoCl− by fragments of Co, CN− and Co, Cl−. The results show that the Co–CN dissociation energy (1.68 eV) is higher than that (1.35 eV) of the Co–Cl bond. These facts reveal that the Co–C bond of anionic CoCN− (isomer 1A) is stronger than the Co–Cl bond of the CoCl− cluster. The calculated VDEs of isomer 1A with a doublet state and 1B with a quartet state are 1.55 and 1.42 eV, respectively. Isomers 1C with a quartet state and 1D with a doublet state are 0.36 and 0.57 eV higher in energy than isomer 1A with a doublet state (Fig. 1). They are both composed of a Co atom interacting with CN via the N terminal. From the relative energies of isomers 1A–1D, we suggest that the ground state structure of the anionic CoCN− cluster is a linear cyanide configuration. Our calculations show that the ground state geometry of neutral CoCN (isomer 1a with a triplet state in Fig. 2) is also a linear cyanide structure, in agreement with the experimental results of the direct absorption techniques.22 The Co–C bond length of isomer 1a with a triplet state is 1.89 Å, in good agreement with the experimental measurement (1.8827 Å)22 and
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Fig. 1 Optimized geometries of the low-lying isomers of anionic ConCN− (n = 1–5) clusters. Bond lengths are given in angstrom.
ab initio molecular orbital studies (1.8540 Å).23,39 The C–N bond length (1.1677 Å) of the ground state structure CoCN from ab initio calculations23 differs from that (1.1313 Å) of the pure rotational spectroscopy experiment.22 Hirano et al.23
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Fig. 2 Optimized geometries of the low-lying isomers of neutral ConCN (n = 1–5) clusters. Bond lengths are given in angstrom and S represents spin multiplicities.
Table 3 Relative energies (RE, eV), spin multiplicities (S) of the lowlying structures of ConCN− (n = 1–5) as well as their ADEs and the first VDEs (eV) obtained by DFT calculations
Cluster CoCN−
Co2CN−
Co3CN−
Co4CN−
Co5CN−
1A 1B 1C 1D 2A 2B 2C 2D 3A 3B 3C 3D 4A 4B 4C 4D 5A 5B 5C 5D 5E 5F 5G 5H
RE
Sym
S
VDE
ADE
0 0.12 0.36 0.57 0 0.23 0.24 0.47 0 0.23 0.25 0.40 0 0.08 0.21 0.23 0 0.15 0.18 0.19 0.22 0.26 0.27 0.27
C∞v C∞v C∞v C∞v C∞v C∞v C∞v C∞v C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1 C1
2 4 4 2 5 3 5 3 8 8 6 6 9 9 9 9 12 12 8 10 10 10 12 12
1.55 1.42 1.60 1.39 1.91 2.86 1.83 2.89 2.07 2.04 2.54 2.55 2.49 2.53 2.08 2.73 2.63 2.73 3.29 1.50 1.93 2.21 2.39 2.54
1.55 1.42 1.60 1.39 1.91 2.86 1.83 2.89 1.96 1.95 2.44 2.19 1.96 2.14 1.97 2.19 2.34 1.87 2.89 1.43 1.31 1.30 1.35 2.24
suggested that this discrepancy is caused by neglecting the large-amplitude bending motion of CoCN when the C–N bond length was calculated from the rotational constant of the pure rotational spectroscopy. Our calculated C–N bond length is 1.16 Å, in good accord with the ab initio molecular orbital study (1.1677 Å) of a high-quality method.39 The calculated dipole moment of isomer 1a with a triplet state is 7.78 D, in accord with the value (7.48 D) of ab initio calculations.30
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Isomer 1b with a triplet state (Fig. 2) is 0.42 eV higher in energy than isomer 1a. It is an isocyanide CoNC structure. Isomer 1c with a quintet state (Fig. 2) is much higher (1.90 eV) in energy than isomer 1a with a triplet state. Therefore, the ground state geometry of neutral CoCN is a linear cyanide configuration. The ground state structure of the neutral CoCN is nearly identical to its anionic counterpart with slight changes in the bond lengths, and thus the ADE of the anionic CoCN− cluster equals the EA of the neutral CoCN cluster. B. Co2CN− and Co2CN. The first two stable structures (isomers 2A with a quintet state and 2B with a triplet state in Fig. 1) of the anionic Co2CN− cluster are both linear cyanide configurations. Isomer 2A with a quintet state is 0.23 eV lower in energy than isomer 2B with a triplet state. They can be described as a Co atom attaching to isomer 1A via a Co–Co bond. The Co–Co bond length of isomer 2A is about 2.42 Å, which is close to the Co–Co bond of the pure Co2 cluster (2.36–2.44 Å).50 The calculated VDE of isomer 2A is 1.91 eV. Isomers 2C with a quintet state and 2D with a triplet state (Fig. 1) are both isocyanide structures. They are 0.24 and 0.47 eV higher in energy than isomer 2A. Therefore, the most stable isomer (2A) of Co2CN− is a linear structure with Co2 attaching to the C terminal of CN. For the neutral Co2CN cluster, the most stable isomer 2a (Fig. 2) is composed of Co2 attaching to the C terminal of CN. The Co–Co, Co–C and C–N bond lengths of isomer 2a are 2.28, 1.96 and 1.16 Å, respectively, a little shorter than those (2.42, 1.97, 1.17 Å) of the anionic Co2CN− cluster. It implies that Co– Co, Co–C and C–N interactions of the neutral Co2CN cluster are stronger than those of the corresponding anionic cluster. The dipole moment of isomer 2a is 8.68 D, close to that of the typical ionic compound NaCl (9.0 D). In addition, we calculated bond dissociation energies of Co2CN and NaCl by fragments of Co2, CN and Na, Cl. The Co2–CN bond dissociation is 5.57 eV, much higher than that (3.99 eV) of NaCl. These facts show that the Co–CN bond of isomer 2a is strong and partly ionic. Isomer 2b with an isocyanide structure (Fig. 2) is 0.16 eV higher in energy than isomer 2a. Isomer 2c with a quartet state (Fig. 2) is much higher in energy (1.17 eV) than isomer 2a with a sextet state. Therefore, the most stable structure of the neutral Co2CN cluster is a linear cyanide configuration. The ground state geometry of neutral Co2CN is nearly identical to the corresponding anionic cluster except for minor differences in bond lengths, revealing that the ADE of the anionic Co2CN− cluster equals the EA of the neutral Co2CN cluster. C. Co3CN− and Co3CN. Our calculated Co3− is a quasi-linear structure, in agreement with the result from photoelectron spectroscopy and theoretical calculations.51 However, in the first two lowest-energy isomers (3A and 3B in Fig. 1) of the Co3CN− cluster, the three Co atoms form a triangle due to attaching a CN to Co3−. Isomer 3A is composed of the vertex Co atom of the triangular Co3 cluster attaching to the C terminal of CN. Isomer 3B is 0.23 eV in energy less stable than isomer 3A. It is composed of the vertex Co atom of the triangular Co3 cluster attaching to the N terminal of CN. The theoretical VDEs of isomers 3A and 3B are 2.07 and 2.04 eV,
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respectively. Isomer 3C with a sextet state (Fig. 1) is 0.25 eV higher in energy than isomer 3A with an octet state. It is formed by the triangular Co3 attaching the CN through the C terminal on its top site. Isomer 3D with a sextet state (Fig. 1) is 0.40 eV higher in energy than isomer 3A with an octet state. From relative energies of isomers 3A–3D, we suggest that isomer 3A is the most stable structure of the anionic Co3CN− cluster. The most stable neutral Co3 cluster is calculated to be an angle-like structure of about 144°. By attaching a CN to a Co3 cluster, the ground state structure (isomer 3a in Fig. 2) of the neutral Co3CN cluster is a quasi-planar five-member ring from the interaction of an acute triangle Co3 with C and N atoms of CN. Isomer 3b (Fig. 2) is 0.25 eV higher in energy than isomer 3a. It is formed by the vertex of the triangle Co3 cluster adsorbing CN via the C terminal. Isomer 3c (Fig. 2) is much higher in energy (0.47 eV) than isomer 3a. It is composed of the vertex of the triangle Co3 cluster adsorbing CN via the N terminal. Therefore, the lowest-energy structure of neutral Co3CN is a five-member ring configuration. The ground state structure of neutral Co3CN is vastly different from the anionic species. This suggests that the ADE (1.96 eV) of the anionic Co3CN− cluster is not equal to the EA (1.71 eV) of neutral Co3CN. D. Co4CN− and Co4CN. We optimized the structures of the anionic Co4CN− cluster by considering all possible geometries of the Co4 cluster. The results show that the first four stable isomers of Co4CN− differ only by 0.08, 0.21 and 0.23 eV in energy. They all have a nonet spin state. Isomer 4A (Fig. 1) is a quasi-planar structure of the CN adsorbed on the top site of the distorted rhombus Co4 cluster through the C terminal. Isomer 4B (Fig. 1) is a structure composed of a Co–Co bond of the distorted rhombus Co4 cluster bridged by the C terminal of CN. Isomer 4C (Fig. 1) is a six-member ring composed of a four-member ring Co4 and the CN. The structure of isomer 4D (Fig. 1) is similar to isomer 4A, but the CN attaches to the Co4 cluster through the N terminal. The calculated VDEs of isomers 4A–4D are 2.49, 2.53 2.08, 2.73 eV, respectively. Because the relative energies of isomers 4A and 4B are very close, we predict either isomer 4A or 4B to be the ground state structure of the anionic Co4CN− cluster. The most stable structure (isomer 4a in Fig. 2) of neutral Co4CN is composed of the distorted rhombus Co4 adsorbing the C terminal of CN on its bridge site. The two Co–C bond lengths are 1.98 and 2.17 Å, respectively. Isomers 4b with a dectet state and 4c with an octet state (Fig. 2) are 0.02 and 0.22 eV higher in energy than isomer 4a with a dectet state, respectively. They are both composed of the distorted rhombus Co4 adsorbing the CN on its bridge site via C and N atoms. Since isomer 4b is only 0.02 eV less stable than isomer 4a, we suggest that either isomer 4a or 4b is the ground state structure of neutral Co4CN. We optimized the most stable structures of bare Co4 neutral and anionic clusters to be planar four-member configurations; therefore, attaching a CN to neutral and anionic Co4 makes little changes to their structures. E. Co5CN− and Co5CN. The most stable structure of the anionic Co5CN− cluster is isomer 5A (Fig. 1), composed of CN
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adsorbed on the top site of the distorted bipyramid Co5 cluster via the C terminal. The calculated VDE of isomer 5A is 2.63 eV. Isomer 5B (Fig. 1) is 0.15 eV higher in energy than isomer 5A. It is composed of CN adsorbed on the top site of the W-like Co5 cluster via the C terminal. The calculated VDE of isomer 5B is 2.73 eV. Isomers 5C with an octet state and 5F with a dectet state (Fig. 1) have similar structures with isomer 5A with a 12-et state. They are 0.18 and 0.26 eV higher in energy than isomer 5A. Isomer 5D with a dectet state (Fig. 1) is 0.19 eV higher energetically than isomer 5A with a 12-et state. It is a structure composed of CN adsorbed on the bridge site of the W-like Co5 cluster via both C and N atoms. Isomer 5E with a dectet state, 5G with a 12-et state and 5H with a 12-et state are 0.22, 0.27, and 0.27 eV higher in energy than isomer 5A with a 12-et state, respectively. As a result, the lowest-energy structure of Co5CN− is composed of the distorted bipyramid Co5 adsorbing the C terminal of CN on its top site. We calculated the most stable Co5 anionic cluster to be a W-like structure, thus attaching a CN makes the structure of the Co5CN anionic cluster very different from that of bare Co5−. The most stable Co5 neutral structure calculated by us is a W-like configuration, in agreement with the result of Sebetci et al.52 For neutral Co5CN, the ground state structure (isomer 5a in Fig. 2) is composed of CN adsorbed on the bridge site of the W-like Co5 cluster via both C and N atoms, which indicates that the attached CN has a slight influence on the structure of the bare Co5 cluster. Isomer 5b (Fig. 2) is 0.33 eV higher in energy than isomer 5a. It is a structure of CN adsorbed on the bridge site of the W-like Co5 cluster via the N terminal. Isomer 5c (Fig. 2) is 0.48 eV higher in energy than isomer 5a. The ground state structure of neutral Co5CN is very different from that of the anionic Co5CN−. The large geometric difference between neutral and anionic Co5CN results in the ADE (2.34 eV) of anionic Co5CN− is not equal to the EA (1.54 eV) of neutral Co5CN. In summary, the most stable structures of neutral and anionic ConCN (n = 1–5) clusters are all planar or quasi-planar structures except the anionic Co5CN− cluster which is a three dimensional structure. In the most stable structures of anionic ConCN− (n = 1–5) clusters, the CN interacts with only one Co atom via its C terminal, and the C–N bond lengths are all 1.17 Å. In the most stable structures of neutral ConCN (n = 1–5) clusters, the CN interacts with one Co atom for n = 1–2, whereas the CN interacts with two Co atoms for n = 3–5. The C–N bond lengths (1.18 Å) of neutral ConCN (n = 3–5) are a little longer than those (1.16 Å) of neutral ConCN (n = 1–2), and they are longer than that (1.15 Å) of HCN. Thus Con (n = 3–5) clusters have greater effect on the C–N triple bond than Con (n = 1–2). F. Dissociation energies. To further investigate the stability of neutral and anionic ConCN (n = 1–5) clusters, we have calculated the dissociation energies of these clusters. Due to the complexity of dissociation channels for ConCN (n = 1–5) clusters, we only considered the channels of losing one Co or CN. For anionic clusters, due to the existence of extra electron, it has more dissociation pathways than the neutral. The stability
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of neutral and anionic ConCN (n = 1–5) clusters was determined via the energies of the following dissociation pathways. Anionic clusters Con CN ¼ Con þ CN
ΔEanion 1
ð1Þ
Con CN ¼ Con þ CN
ΔEanion 2
ð2Þ
Con CN ¼ Co þ ½Coðn1Þ CN
ΔEanion 3
Con CN ¼ Co þ Coðn1Þ CN
ð3Þ
ΔEanion 4
ð4Þ
Neutral clusters
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Con CN ¼ Con þ CN
ΔEneutral 5
Con CN ¼ Co þ Coðn1Þ CN
ð5Þ
ΔEneutral 6
ð6Þ
The dissociation energies of these six channels for neutral and anionic ConCN (n = 1–5) clusters are given in Table 4. The dissociation energies of neutral and anionic ConCN (n = 1–5) clusters in Table 4 reveal that these clusters are stable against fragmentation. From Table 4, we note that the dissociation channel of the anionic CoCN− cluster into Co and CN− is favorable to the dissociation channel into Co− and CN. This preferred channel is consistent with the fact that the EA (3.86 eV) of CN is much larger than that of Co (0.66 eV).53 For anionic ConCN (n = 2–5) clusters, channel (1) is favorable to channel (2), since the EA (3.86 eV) of CN is much larger than those of Con (n = 2–5) clusters (1.11 eV, 1.6 eV, 1.91 eV, 1.56 eV).53 Because of the EAs of ConCN (n = 1–5) clusters larger than that of Co, channel (3) is preferred to channel (4). The most favored dissociation pathway of anionic ConCN (n = 2–5) clusters is channel (3) since ΔEanion is the lowest dissociation energy. 3 This fact occurs due to the bond dissociation of Co–C54 being larger than the Co–Co bond.55 As seen in Table 4, the dissociation energies of Co atom loss channels exhibit a clear odd– even alternation in stability for neutral ConCN (n = 1–5) clusters, with odd n being more stable than the adjacent even n − 1 and n + 1 ones. A slight odd–even alternation in stability for anionic ConCN− (n = 1–5) clusters can also be observed from channel (2), with even n being more stable than the adjacent odd n ones. The odd–even alternation in stability for anionic ConCN− (n = 1–5) clusters is opposite for channel (4) with odd n being slightly more stable than the even n. These odd–even alternations may be due to electron parity effect of parent and fragment molecules or ions, which were also found for 3d transition metal carbides (from ScCn to CoCn clusters).56 The dissociation channel (6)
with the loss of a Co is preferred to channel (5) with the loss of a CN for neutral ConCN (n = 2–5) clusters.
3.2.
The simulated photoelectron spectra
Fig. 3 displays the simulated photoelectron spectra of ConCN− (n = 1–5) clusters at 193 nm photons. From Fig. 3, we can see that the first VDEs of ConCN− (n = 1–5) become larger with the increase of Co atoms. For CoCN− (isomer 1A), we can observe that the VDEs are centered at about 1.56, 2.59, 2.68, 3.63, 3.76, 4.63, 5.89, 6.11 and 6.24 eV, respectively. The peaks centered at 1.97, 3.15, 3.33, 3.55, 3.7, 3.93, 4.58, 4.88, 5.26, 5.46, 5.87 eV can be observed in the simulated photoelectron spectrum of Co2CN− (isomer 2A). For larger size ConCN− (n = 3–5), the simulated photoelectron spectra are very congested. This might be due to a high density of states from 3d electrons of Con clusters and might be also the results of the presence of several isomers. EA is a significant factor for reactivity. Thus, we compared the EAs of bare Con and ConCN (n = 1–5). The calculated EAs of ConCN (n = 1–2) clusters are 1.55 eV and 1.91 eV, much higher than those (0.66 eV and 1.11 eV) of Con (n = 1–2)53 and much lower than that (3.86 eV) of CN, indicating that the interaction between Con (n = 1–2) and CN is strong and they both play a significant role in the photoelectron spectral features of ConCN− (n = 1–2) clusters. The calculated EAs of ConCN (n = 3–5) clusters are 1.71 eV, 1.96 eV and 1.54 eV, respectively, close to those (1.6 eV, 1.91 eV, and 1.56 eV) of Con (n = 3–5).53 It implies that Con (n = 3–5) clusters play an important role in the photoelectron spectral features of ConCN− (n = 3–5).
Table 4 Dissociation energies (eV) of neutral and anionic ConCN (n = 1–5) clusters
n
ΔEanion 1
ΔEanion 2
ΔEanion 3
ΔEanion 4
ΔEneutral 5
ΔEneutral 6
1 2 3 4 5
1.68 3.41 2.48 2.47 2.44
5.14 5.97 5.14 5.19 5.05
1.68 1.73 2.10 2.03 1.92
5.14 2.67 3.41 3.13 3.28
4.20 5.57 4.84 4.58 4.92
4.20 1.37 2.30 1.77 2.35
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Fig. 3 The simulated photoelectron spectra of the ground state structures (isomers 1A–5A) of anionic ConCN− (n = 1–5) clusters.
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3.3. The frontier molecular orbital analysis of anionic ConCN− (n = 1–5) clusters To investigate the interaction of Con clusters with CN, we examined the electron densities of their frontier molecular orbitals. The electron distributions of the first six highest occupied molecular orbitals (HOMO) of the lowest-energy structures of anionic ConCN− (n = 1–5) clusters are presented in Fig. 4. Fig. 4 demonstrates that HOMO of anionic CoCN− is composed of the 3dxz orbital of Co and the π orbital of CN. Similarly, HOMO−1 is contributed by the 3dyz orbital of Co and the π orbital of CN. HOMO−2 is only from the contribution of the 3dxy orbital of Co. HOMO−3 is a σ-orbital, primarily s–dz2 characteristic of C and Co. HOMO−4 is only contributed by the 3dx2−y2 orbital of Co. HOMO−5 is a π-orbital, primarily from the (p–d)π Co–CN bonding orbitals with the interaction of the 3dyz orbital of Co and the π orbital of CN. This indicates that the interaction between Co and CN is strong for CoCN−. For Co2CN−, HOMO and HOMO−1 are contributed by 3dx2−y2 and 3dxy orbitals of Co, respectively. HOMO−2 and HOMO−3 are due to the hybridization of 4s, 3dz2 between two Co atoms, revealing strong interaction between them. HOMO−4 is contributed by 3dxz orbitals of the two Co atoms and the π
Fig. 4
orbital of CN. HOMO−5 is contributed by 3dyz orbitals of the two Co atoms and the π orbital of CN. For ConCN− (n = 3–5), the HOMOs are mainly contributed by 3d orbitals of Con (n = 3–5) clusters. This indicates that the photoelectron spectra features of anionic ConCN− (n = 3–5) are mostly the contributions from 3d electrons of Con (n = 3–5). Perhaps it induces the photoelectron spectra of ConCN− (n = 3–5) to be very congested. 3.4. The calculated NPA charge, Wiberg bond order, and the simulated IR spectra To understand the chemical bonding in anionic and neutral ConCN (n = 1–5) clusters, we carried out natural population analysis (NPA) and Wiberg bond orders analysis. The NPA charge of Con and CN for the most stable structures of neutral and anionic ConCN (n = 1–5) clusters as well as the Co–C and C–N bond orders are displayed in Table 5. The NPA charge distribution of anionic and neutral ConCN (n = 1–5) clusters shows that the negative charge is mainly distributed on CN, indicating that the electrons always transfer from Con to CN, thus Co–C bonds are partly ionic. This is in agreement with the fact that the EA (3.86 eV) of CN is much larger than those (0.66, 1.11, 1.6, 1.91, 1.71 eV) of Con (n = 1–5) clusters.53 For
The frontier molecular orbitals of anionic ConCN− (n = 1–5) clusters (isomers 1A–5A).
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Table 5 Theoretical charge populations and bond order analysis for the most stable structures of anionic and neutral ConCN (n = 1–5) clusters (isomers 1A–5A and 1a–5a)
Bond order Anion cluster −
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CoCN Co2CN− Co3CN− Co4CN− Co5CN−
Charge
Bond order
Charge
Co–C
C–N
Con
CN
Neutral cluster
Co–C
C–N
Con
CN
0.57 0.55 0.58 0.61 0.62
2.83 2.84 2.86 2.86 2.87
−0.239 −0.256 −0.328 −0.361 −0.396
−0.761 −0.744 −0.672 −0.639 −0.604
CoCN Co2CN Co3CN Co4CN Co5CN
0.62 0.55 0.41 0.48 0.50
2.92 2.90 2.67 2.73 2.64
0.687 0.625 0.699 0.638 0.631
−0.687 −0.625 −0.699 −0.638 −0.631
anionic ConCN (n = 1–5) clusters, negative charges reside on both Con and CN, resulting in a repulsive electrostatic interaction between them. The bond order (Table 5) of Co–C in anionic ConCN (n = 1–5) clusters shows that the covalent interaction predominates the repulsive electrostatic interaction which makes these anionic clusters stable. Table 5 shows that the C–N bond orders of neutral ConCN (n = 3–5) clusters are smaller than those of the corresponding anions which reveals that neutral Con (n = 3–5) clusters have better activation effect on C–N triple bonds than those of anions. The C–N bond orders of ConCN (n = 3–5) are smaller than those of ConCN (n = 1–2) in neutral ConCN clusters. It implies that larger Con (n = 3–5) clusters have better C–N activation effect than Con (n = 1–2). The bond orders in combination with charge
analysis (Table 5) show the Co–C bonds of neutral and anionic ConCN (n = 1–5) clusters to be a mixture of ionic and covalent characters. The simulated IR spectra of CN radical and the ground state structures of neutral and anionic ConCN (n = 1–5) are presented in Fig. 5. The calculated Co–C stretching mode of neutral CoCN is at 441 cm−1, which is in reasonable agreement with that (478 cm−1) from the pure rotational spectrum of CoCN.22 The C–N stretching mode of CoCN is at 2222 cm−1, in good agreement with the experimental value of 2191 cm−1.22 For neutral Co2CN, the Co–Co, Co–C and C–N stretching frequencies are 237 cm−1, 425 cm−1 and 2228 cm−1, respectively. Fig. 5 shows that C–N stretching frequencies of neutral ConCN (n = 1–2) are larger than that of CN radical (2147 cm−1) at the
Fig. 5 The simulated IR spectra of CN radical and the ground state structures of anionic and neutral ConCN (n = 1–5) clusters (isomers 1A–5A and 1a–5a).
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same level. However, C–N stretching frequencies of neutral ConCN (n = 3–5) are smaller than that of CN radical. For anionic ConCN (n = 1–5), the calculated C–N stretching frequencies are all larger than that of CN radical. Therefore, compared to the stretching frequency of CN radical, the C–N stretching frequencies of neutral ConCN (n = 1–2) and anionic ConCN (n = 1–5) are blue-shifted while they are red-shifted for neutral ConCN (n = 3–5). This fact shows that the neutral Con (n = 3–5) clusters have better C–N bond activation effect than neutral Con (n = 1–2) and anionic Con− (n = 3–5) clusters. The cyanide radical (CN) is isoelectronic to the ethynyl radical (C2H), and they are both very reactive species in organic reactions. Hence, it is interesting to compare ConC2H and ConCN (n = 1–5) clusters. We investigated neutral and anionic ConC2H (n = 1–5) clusters by DFT calculations and photoelectron spectroscopy experiments previously.37 It was found that the ground state VDEs of ConCN− (n = 1–5) are all larger than those of ConC2H− (n = 1–5). Perhaps it is because the EA of CN (3.86 eV) is larger than that of C2H (2.97 eV). The structures of neutral ConCN and ConC2H (n = 1–5) clusters are similar and planar or quasi-planar structures. Anionic ConCN (n = 1–2) and ConC2H (n = 1–2) clusters are also similar and linear structures. However, the structures of anionic ConCN (n = 3–5) and ConC2H (n = 3–5) clusters are different with CN adsorbed on the top sites of Con and C2H adsorbed on the bridge sites of Con via the C terminal. The calculated magnetic moments of the most stable isomers of neutral ConCN (n = 1–5) clusters are 2, 5, 6, 9, and 10μB, respectively. These values are the same as those of ConC2H clusters which shows that adsorption of CN and C2H both have little effect on the magnetic moments of Con clusters.
4.
Conclusions
The anionic and neutral ConCN (n = 1–5) clusters were studied using density functional calculations. The calculations have shown that the CN prefers to be adsorbed on the top sites of Con clusters via the C terminal in the anionic ConCN− (n = 1–5) clusters. In neutral ConCN (n = 1–5) clusters, the CN is mainly adsorbed on the top sites of the Con (n = 1–2) clusters via the C terminal whereas it is adsorbed on the bridge sites of Con (n = 3–5) via both C and N atoms. The calculated C–N stretching frequencies of neutral ConCN (n = 3–5) are red-shifted while they are blue-shifted for neutral ConCN (n = 1–2), which is in agreement with the C–N bond lengthening more in neutral ConCN (n = 3–5) clusters than in ConCN (n = 1–2). The first vertical detachment energies (VDEs) of anionic ConCN− (n = 1–5) clusters were predicted to be 1.55, 1.91, 2.07, 2.49 and 2.63 eV, respectively.
Acknowledgements This work was supported by the Natural Science Foundation of the Education Department of Henan Province (no.
5524 | Dalton Trans., 2014, 43, 5516–5525
13B150987), the Natural Science Foundation of Zhengzhou City (no. 20120324), and the Natural Science Foundation of China (no. 21206049).
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