Article pubs.acs.org/JPCA
Predictable Valence Excited States of Anions Ryan C. Fortenberry,* W. James Morgan, and Jordan D. Enyard Department of Chemistry, Georgia Southern University, Statesboro, Georgia 30460, United States ABSTRACT: The excited states of the 1 1A′ :CC−C̈ −R− family and related anions are investigated. Previous work has shown that 1 1A′ C3H− and CCSiH− each possess a rare valence excited state in addition to their dipole-bound excited states. A similar methodology to that employed previously shows that related anions (C3OH−, C3NC−, C4N−, C5H−, and HBCN−) also possess valence excited states. The valence states are the result of breaking the symmetry from C∞v to Cs. The half-filled π or π-type highest occupied molecular orbital (HOMO) is split into a′ and a″ pieces. The valence excitation takes place between these two pieces. If the anion HOMO is not a half-filled π-type orbital because of an increase in symmetry, cyclization, or both, the anion most likely does not exhibit the signs of a valence excited state even if the anion is an isomer of or isoelectronic to an anion that does possess a valence excited state. However, the :CC−C̈ −R− set is not the only classification of anions shown to possess valence excited states even though it is the most predictable grouping exhibiting this behavior found to date.
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INTRODUCTION Small, organic anions can support excited states. Such a fact is not new, but the number of anions and the number of states they can support continues to grow.1−6 The existence of dipolebound excited states of anions has proven to be quite a curiosity in the world of physical chemistry with excited states of CH2CN−7,8 and CH2CHO−9,10 experimentally observed for around 25 years with quantum chemical computations preceding the experiment by more than a decade for the enolate anion.11 A dipole-bound state in many ways resembles a Rydberg state. A Rydberg state exists when an electron is bound to the molecule by the monopole−monopole Coulombic attraction between the electron itself and the “hydrogen nucleus” of the remaining molecular core cation. Banishment of the extra electron to outer reaches of the molecular system in an anion does not retain such an attraction, often leading to the electron simply leaving the system. If a neutral molecule has a large enough dipole moment, however, an extra electron may be bound in a Rydberg-like fashion but through a weaker monopole−dipole interaction.1,2,12−14 As a result, the binding of most dipole-bound anions is tenuous, and the size of the dipole-bound orbital is larger than a typical Rydberg orbital. The minimum dipole moment required to bind the electron in a dipole-bound fashion was originally determined by Fermi and Teller nearly 70 years ago12 to be 1.625 D under ideal circumstances. Much more recent studies have put practical limits closer to 2.0 D, or even 2.5 D,8,14−18 with an understanding that this phenomenon may be environmentally or molecularly dependent to a certain degree.19−23 Several molecular and cluster anions have exhibited dipole-bound states,1,13,14,16,24−32 but it is the valence excited state that opens up the electronic properties of anions to more complex physical analysis similar to that of more stable molecular species.33 Because Fermi and Teller’s equations allow for only one dipole© 2014 American Chemical Society
bound state until a dipole moment of more than 9 D is reached,12 and again, the energy of a dipole-bound state is at the electron dissociation threshold, any anion with a dipolebound state as its ground state is metastable at best. If an anion possesses a valence state as its ground state, the dipole-bound state can exist as an excited state,1,2 as is the case for the aforementioned CH2CN− and CH2CHO− species. The easiest way for an anion to possess a valence ground state is simple removal of a hydrogen from the closed-shell neutral and replacing it with a lone electron to retain the spin-paired nature of the molecule even though now it is negatively charged. The dipole-bound excitation in the methylene nitrile anion has even been suggested as a carrier for one of the elusive diffuse interstellar bands,34−36 the ubiquitous interstellar absorption features labeled37 as the “longest standing problem in spectroscopy.” Additionally, it has been suggested that dipole-bound states may be necessary precursors to the formation of anions in the interstellar medium.38−40 As a result, excited states of anions are beginning to show nuanced physics that may be valuable for future applications. The fact that the dipole-bound excited state is quite nearly coincident with the energy required to remove the electron, or electron binding energy (eBE), a spectroscopic signature for this excitation produces a peak clearly indicating when the corresponding neutral has been created and what its relative energy is. Neumark and his collaborators have utilized this principle as well as the valence ground state itself to produce stability and structural data for numerous difficult radical species making use of slow electron velocity-map imaging spectroscopy.41−43 As such, the lone dipole-bound excited state of an anion can serve as a spectral signature, an intermediate in Received: September 19, 2014 Revised: October 20, 2014 Published: October 21, 2014 10763
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Figure 1. CCSD(T)/aug-cc-pVTZ anion geometries in angstroms and degrees for (A) C3OH−, (B) C3NC−, (C) C4N−, (D) C5H−, (E) c-C3C2H−, (F) HBCN−, (G) HBNC−, and (H) H2BCC−.
the dipole-bound state.3−6 CH2SiN−, SiCCN−, C3H−, and CCSiH− all possess a single valence excited state in addition to the dipole-bound state, giving two electronically excited states. CCSiN− actually has two valence excited states and, potentially, a dipole-bound state. Additionally, the C2n−1N− (n = 4−7) family of molecules has been shown experimentally to possess valence excited states,46 but the term “small” may not be applied to some of these molecules by many physical chemists. It is currently unknown why some anions possess valence excited states and why some do not. Valence excited states exist independently of the magnitude of the neutral’s dipole moment, making their existence in anions more tantalizing. Initial studies indicated that all the valence excited states of anions with four or fewer heavy atoms contain silicon.3,4 To determine if something special was at work with the silicon
a reaction pathway, or a necessary indicator of daughter species classification. The limited electronic features of anions, especially coupled with the metastable nature of dipole-bound states, still casts shadows of limited novelty on excited states of anions. However, multiple excited states of anions would open up the possible role that anions may play in various circumstances. The only way in which to possess multiple excited states is for an anion to exhibit an additional valence state between the dipole-bound state and the valence ground state. However, this requires exceptionally low excitation energies. Valence excited states of neutrals beneath the energy of Rydberg states have been shown for small hydrocarbons,44,45 making such states possible for anions. In fact, further research has shown that several small anions can retain additional valence states below 10764
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Table 1. Vertically Excited-State Energies (in eV) Across Basis Setsa Based on the Anion CCSD(T)/aug-cc-pVTZ Ground-State Geometry molecule −
C3OH
C3NC− C4N− C5H− c-C3C2H− HBCN− HBNC− H2BCC− a
state
pVDZ
apVDZ
dapVDZ
tapVDZ
eBE
1
5.60 2.52 6.35 1.47 6.03 0.90 6.56 0.83 6.13 7.10 1.18 6.24 3.05 7.41 3.70
3.26 2.48 4.67 1.45 4.38 0.89 3.34 0.81 3.11 2.60 1.12 2.58 2.30 4.55 3.73
2.91 2.48 3.75 1.45 3.55 0.89 2.91 0.81 2.60 1.68 1.12 1.53 1.65 3.74 3.72
2.88 2.48 3.57 1.45 3.41 0.89 2.86 0.81 2.50 1.55 1.12 1.40 1.44 3.64 3.71
2.93 − 3.53 − 3.36 − 2.89 − 2.48 1.53 − 1.37 − 3.62 −
2 1 2 1 2 1 2 1 1 2 1 2 1 1 1
A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 B1
fb 2 6 1 2 1 9 9 5 5 3 1 2 2 1 7
× × × × × × × × × × × × × × ×
10−2 10−3 10−3 10−3 10−3 10−4 10−3 10−4 10−3 10−3 10−2 10−4 10−5 10−4 10−3
pVTZ
apVTZ
dapVTZ
5.26 2.49 6.11 1.46 6.30 0.88 6.02 0.82 5.60 5.61 1.14 5.38 2.92 7.50 3.80
3.33 2.45 4.58 1.45 4.33 0.88 3.37 0.81 3.15 2.42 1.11 2.40 2.32 4.55 3.73
3.07 2.45 3.90 1.45 3.73 0.88 3.07 0.81 2.75 1.78 1.11 1.66 1.79 3.85 3.78
The correlation consistent basis sets are abbreviated with d-aug-cc-pVDZ as dapVDZ, for example. bEOM-CCSD/tapVDZ values.
Geometry optimizations of these states produces more physically interpretable results even if the complexities of the wave functions and differences in geometries can muddle the results.1 Adiabatic eBEs are computed from the difference between the energy of the CCSD(T)/aug-cc-pVTZ groundstate geometry of the anion and the unrestricted Hartree−Fock (UHF)54,55 CCSD(T)/aug-cc-pVTZ ground-state geometry of the corresponding neutral radical. UHF-CCSD/aug-cc-pVTZ dipole moments for the neutral radicals are computed from this optimized geometry. CCSD/d-aug-cc-pVDZ ground-state and EOM-CCSD/d-aug-cc-pVDZ excited-state geometries are computed; the difference in energies produces the adiabatic excitation energies. The use of both vertical and adiabatic data serves to describe the nature of excited states and to provide experimentally meaningful data, respectively. All of the computations in this work make use of the free and opensource PSI4 quantum chemistry package56 with the exception of the EOMIP computations which require the use of the CFOUR program.57
atom, similar heavier atoms (Al−S) were inserted in small, hydrocarbon anions, and their spectral features were computationally investigated.5 No new valence excited states for aluminum-, phosphorus-, or sulfur-containing anions were shown. However, C3H− and its silicon analogue, CCSiH−, do possess valence excited states. In the present work we explore anions structurally related to C3H− and CCSiH− with half-filled π-type highest occupied molecular orbitals (HOMO) that create bent geometry, singlet-state anions. We will limit ourselves to only “first-row” (B−O) atoms because these are more common in the universe and are more likely to appear in nature (wherever that may be) as a result. It is hoped that further examples of states with valence excited states will allow us to form better models for understanding how and when multiple electronic states of anions arise.
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COMPUTATIONAL DETAILS The classification of an excited state of an anion as either valence or dipole-bound can be clearly distinguished by the behavior of the excitation energy as the diffuseness of the basis set is increased.3−5 As the basis set becomes spatially larger and the energy decreases, a dipole-bound state is present because more of the electron density of this highly diffuse s-type, Rydberg-like orbital is described. If little change is present, the excited state is valence in nature. Restricted Hartree−Fock reference wave functions47 and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]48 computations combined with the aug-cc-pVTZ basis set49−51 produce optimized geometries. From these geometries, equation-of-motion CCSD (EOM-CCSD) vertical excitation energies52 are computed with the n-aug-cc-pVDZ and n-aug-cc-pVTZ series of basis sets where n represents the even-tempered extrapolation of the diffuseness of the basis set from the cc-pVXZ level to the t-aug-cc-pVXZ level.45 Equationof-motion ionization potential (EOMIP) computations53 produce the vertical eBEs. Even though vertical computations are susceptible to error because of a lack of geometric rearrangement in response to the change in wave function and orbital occupation, the restriction of variables to only the change in wave function itself gives consistent results with clear conclusions as to the existence of any excited state, whether valence or dipole-bound.
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RESULTS AND DISCUSSION The anions examined here are based on the :CC−C̈ −R− general structure with 1A′ ground states. In the prototypical C3H− molecule, R = H, for example. It is known that the evennumbered carbon chains C2nH radicals are 2Σ+ ground states from n ≤ 4 and 2Π states for n ≥ 6.44,58−64 However, the odd n chains are all 2Π in their ground states. The major difference is that the corresponding anions for the even n radicals all fill their HOMOs, creating 1Σ+ ground states. The odd n radicals, however, have only one π electron in the HOMO leading to the possibility of triplet or singlet states in the corresponding anions. In the case of C3H−, Feshbach resonances are known to exist for the lower-energy triplet state,65,66 but the creation of the singlet state breaks the symmetry from C∞v in the radical to Cs in the singlet anion. As such, the π orbital splits into a′ and a″ pieces. The valence excited state is therefore the HOMO a′ → a″ excitation. To maintain this splitting of the π HOMO, any R-group substituting for the hydrogen atom in the :CC−C̈ − R− family must attach via only a single bond. For this study, common functional groups are added to test if these anions will retain this valence excitation. These groups include −OH, −CN/−NC, and −C2H. 10765
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1 1A′ C3OH− exists as two conformers on the potential energy surface (PES), one with the hydrogen cis and the other trans. Unsurprisingly, the trans is lower in energy and will be the only isomer examined here. This is depicted in Figure 1a. The CCSD(T)/aug-cc-pVTZ C2−C3−O bond angle is 111.0°, 1.9° greater5 than the C2−C3−H bond angle in C3H−. From a Lewis structure perspective, it is clear that the C3 atom in C3OH− possesses a lone pair like it does in C3H−. Additionally, the C1−C2 bond length implies a triple bond and carbide moiety. Hence, C3OH− should have electronic properties similar to those of C3H−. Table 1 gives clear indication that the hydroxy anion possesses a dipole-bound state, the 2 1A′, because the excitation energy decreases as the spatial extent of the basis set increases. Additionally, the eBE (2.93 eV) is close in energy to the 2.88 eV EOM-CCSD/t-aug-cc-pVDZ excitation energy. The valence excited state (1 1A″), lower at 2.48 eV, exhibits no decrease in energy after the first set of diffuse functions is included in the basis set. The dipole-bound excited state is the result of an electron being promoted out of the HOMO into a diffuse s-type orbital, whereas the valence excited state is an excitation out of the a′ HOMO into the corresponding π-pair a″ valence virtual orbital. This accepting orbital is not the lowest-energy a″ virtual orbital. Several exist below this energy and correspond to various p-type Rydberg orbitals. Both cases of excitation in C3OH− behave the same way that the dipole-bound and valence excitations do for C3H−. This is further corroborated by the adiabatic excitations reported in Table 2. C3OH− will support both excited states because the dipole moment for the corresponding radical is quite large at 5.41 D.
are small at 1.21 and 0.03 D, respectively. These neutrals actually exhibit a C1 minimum geometry but are close to linear. As a result, the C4N dipole moment could well be considered 0.0 D if the bond angles were actually all 180.0°. With the loss of the lone pair in the radical, the C4N radical has bond angles of ∠C4−C3−C2 = 168.8°, ∠C3−C2−C1 = 176.8°, and ∠C2− C1−N = 178.3°, making it nearly, but not actually, linear. The dihedral potential scans are very flat because of these nearlinear bond angles, but the most pronounced deviation from planarity in 1 2A C4N is τ(C3−C2−C1−N) at 63.0°. Even though the adiabatic eBEs for these anions are higher than the adiabatic dipole-bound state excitation energies (Table 2), the t-aug-cc-pVDZ vertical excitation energies and vertical eBEs (Table 1) indicate that these two states are nearly coincident, a necessary case for the state to be dipole-bound. Regardless, the small dipole moments present in each radical will not support a dipole-bound state in either of the corresponding C3NC− or C4N− molecules. Only the 1 1A′ valence state, again defined as the excitation out of the a′ HOMO into the other π-pair a″ orbital, will be observable or accessible for either of these anions. The dipole moment for the isoelectronic C5H radical, however, is quite substantial at 4.72 D. Additionally, Table 1 shows how the 2 1A′ state of C5H− (Figure 1d) is clearly a dipole-bound state with the 1 1A″ state clearly valence in nature. The 1 1A″ ← 11A′ excitation takes place at 0.81 eV vertically and 0.46 eV adiabatically. This excitation is energetically well into the infrared region of the electromagnetic spectrum and even into the range typically associated with hydride stretches. The C4N− 1 1A″ ← 11A′ adiabatic excitation is also in this same region at 0.47 eV. C5H− may be thought of as part of the :CC−C̈ −R− family with R = C2H or as C3H− with a carbide group attached to the end opposite the hydrogen atom. The latter description is fitting because ∠C4−C5−H is 113.3° and the lone pair is localized on the C5 atom. Previous computations put the ∠C5−C4−C3 much lower than our values creating a cyclic structure for the Cs structure,67 but our methods include more electron correlation and describe a deeper well than this other minimum. Regardless, the spinpaired, double occupation of the π-type HOMO and resulting symmetry breaking from this singlet state create a very low energy valence excitation. Another isomer of C5H, cyclic-C3C2H given in Figure 1e, is tested here because it is known to have a large dipole moment.68 However, it has also been quite a problem for electronic structure methods because of symmetry-breaking and multireference effects.69 Our interests lie in the electronic properties of the anion, thus, alleviating many of these issues. c-C3C2H− actually lies 2.43 eV lower (from CCSD(T)/aug-ccpVTZ) on the PES than C5H−. Because of the C2v structure of c-C3C2H− and the resulting orbital occupation of this anion, the half-filled π orbital is no longer present and is no longer the HOMO. This is replaced by the 4b2 orbital as the HOMO, out of which the electron is excited into a diffuse s-type orbital to create the lone dipole-bound state clearly shown in Table 1. The previously computed dipole moment is large enough to support this state, and no low-lying valence excited states are present. The adiabatic computations of the radical could not be properly carried out in this study, but the vertical computations showcase a dipole-bound excited state. Regardless, the loss of the π-splitting in the HOMO negates the presence of a valence excited state in this anion even though it is isomeric to C5H−.
Table 2. Adiabatic Excitation Energies, eBEs, and Dipole Moments molecule
state
C3OH−
2 1 2 1 2 1 2 1 1 2 1 2 2
C3NC− C4N− C5H− c-C3C2H− HBCN− HBNC− H2BCC−
1
A′ A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 A′ 1 A″ 1 A′ 1 B2 1
excitation energy (eV)
eBE (eV)
dipole momenta (D)
2.24 2.00 3.17 1.00 3.06 0.47 2.20 0.46 2.51 1.41 0.70 1.86 3.68
2.30 − 2.97 − 2.91 − 2.26 − 3.3c 1.53 − 1.87 0.79
a
For the corresponding neutral radical. bThe adiabatic eBE is not trustworthy because of multireference problems inherent in the radical’s electronic structure. See refs 68 and 69. cThis is the minimum value reported in ref 68.
The isomeric pair C3NC− and C4N− (Figure 1b,c) also show clear dipole-bound excited-state character for their respective 2 1 A′ states and valence character for their 1 1A′ states. C4N− is more stable than C3NC− by 0.96 eV from the ground-state CCSD(T)/aug-cc-pVTZ geometry optimizations. The C3− C2−R bond angles are larger in these two cases but are still less than 120°, indicating a lone pair is once more located on the central carbon (C2 in both cases here). However, the dipole moments for these two species’ corresponding neutral radicals 10766
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All indications are that creating singlet-state anions from 2Π or closely related radicals with a singly occupied π-type HOMO is a reliable means of producing valence excited states in anions. Deviation from this model will likely not produce valence excited states. However, CH2SiN−, SiCCN−, and CCSiN− also possess valence excited states. As such, the structure of 1A′ :CC−C̈ −R− and related isoelectronic, quasi-linear anions appear to be adequate but not necessary for an anion to possess valence excited states. There still exists much to learn about valence excited states of anions, and the development of a more complete model for anionic valence excitation is still warranted.
To determine the robustness of the hypothesis that anions with a half-filled π-type HOMO arranged in a singlet state can produce valence excited states, HBCN− is tested. It is isoelectronic with C3H−, and its optimized geometry (Figure 1f) resembles that of the hydrocarbon anion. The major exception is the B−N bond, which is 1.568 Å for HBCN− while the corresponding C−C bond in C3H− is 1.359 Å. The resulting molecules, therefore, differ to a certain extent, but the HOMO in both is still the a′ portion of the former half-filled π orbital. Consequentially, HBCN− exhibits a clear 2 1A′ dipolebound state and a 1 1A″ valence state, as shown in Table 1. However, the dipole moment of the corresponding radical is below the ∼2 D threshold, indicating that the dipole-bound excited state will not be present in this anion. As a result, the low-energy valence state is the only excited state in HBCN−. Adiabatically, the excitation energy is 0.70 eV, well below the 1.26 eV eBE, and vertically is one of the brightest of all of the excitations computed here with an oscillator strength on the order of 0.01. Simple isomerization to the 0.26 eV higher-energy (CCSD(T)/aug-cc-pVTZ) HBNC− anion does not produce a valence excited state or even a dipole-bound excited state. Although a second excited state (1 1A″) exists just above the first excited state (2 1A′) in Table 1, the lower-energy state is the dipolebound state. Additionally, the 1 1A″ state does not give a welldefined early excited-state energy convergence as one would expect for a valence excited state. The excitation energy varies by 1.61 eV from the cc-pVDZ basis set (3.05 eV) to t-aug-ccpVDZ (1.44 eV). The corresponding radical produces only a 1.87 D dipole moment, probably too low for HBNC− to retain a dipole-bound state. The lack of a valence state and the low dipole moment are both the result of cyclization in the structure of HBNC and HBNC− (Figure 1g for the anion). Similar behavior has previously been shown in the isoelectronic 1 1A′ HCCN molecule.70,71 H2BCC−, shown in Figure 1h, is isoelectronic to C3H−, HBCN−, and HBNC−, but its C2v structure decreases the dipole moment below the binding threshold and precludes the formation of the half-filled π HOMO. As a result, H2BCC− also does not possess valence or dipole-bound excited states.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Georgia Southern University is acknowledged for the provision of start-up funds necessary to perform this research. REFERENCES
(1) Simons, J. Molecular Anions. J. Phys. Chem. A 2008, 112, 6401− 6511. (2) Simons, J. Theoretical Study of Negative Molecular Anions. Annu. Rev. Phys. Chem. 2011, 62, 107−128. (3) Fortenberry, R. C.; Crawford, T. D. Theoretical Prediction of New Dipole-Bound States for Anions of Interstellar Interest. J. Chem. Phys. 2011, 134, 154304. (4) Fortenberry, R. C.; Crawford, T. D. Singlet Excited States of Silicon-Containing Anions Relevant to Interstellar Chemistry. J. Phys. Chem. A 2011, 115, 8119−8124. (5) Fortenberry, R. C. Singlet Excited States of Anions with Higher Main Group Elements. Mol. Phys. 2013, 111, 3265−3275. (6) Fortenberry, R. C. In IAU Symposium 297: The Diffuse Interstellar Bands; Cami, J., Cox, N. L. J., Eds.; Campbridge University Press: Cambridge, England, 2014. (7) Lykke, K. R.; Neumark, D. M.; Andersen, T.; Trapa, V. J.; Lineberger, W. C. Autodetachment Spectroscopy and Dynamics of CH2CN− and CH2CN. J. Chem. Phys. 1987, 87, 6842−6853. (8) Gutsev, G.; Adamowicz, A. The Valence and Dipole-Bound States of the Cyanomethide Ion, CH2CN−. Chem. Phys. Lett. 1995, 246, 245−250. (9) Mullin, A. S.; Murray, K. K.; Schulz, C. P.; Szaflarski, D. M.; Lineberger, W. C. Autodetachment Spectroscopy of Vibrationally Excited Acetaldehyde Enolate Anion, CH2CHO−. Chem. Phys. 1992, 166, 207−213. (10) Mullin, A. S.; Murray, K. K.; Schulz, C. P.; Lineberger, W. C. Autodetachment Dynamics of Acetaldehyde Enolate Anion, CH2CHO−. J. Phys. Chem. 1993, 97, 10281−10286. (11) Wetmore, R. W.; Schaefer, H. F., III; Hiberty, P. C.; Brauman, J. I. Dipole-Supported States. A Very Low Lying Excited State of Acetaldehyde Enolate Anion. J. Am. Chem. Soc. 1980, 102, 5470−5473. (12) Fermi, E.; Teller, E. The Capture of Negative Mesotrons in Matter. Phys. Rev. 1947, 72, 399−408. (13) Jordan, K. D.; Barrow, P. D. Temporary Anion States of Polyatomic Hydrocarbons. Chem. Rev. (Washington, DC, U.S.) 1987, 87, 557−588. (14) Jordan, K. D.; Wang, F. Theory of Dipole-Bound Anions. Annu. Rev. Phys. Chem. 2003, 54, 367−396. (15) Gutsev, G.; Adamowicz, A. Relationship between the Dipole Moments and the Electron Affinities for Some Polar Organic Molecules. Chem. Phys. Lett. 1995, 235, 377−381. (16) Gutowski, M.; Skurksi, P.; Boldyrev, A. I.; Simons, J.; Jordan, K. D. The Contribution of Electron Correlation to the Stability of
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CONCLUSIONS Anions of the :CC−C̈ −R− family reliably, and predictably, possess valence excited states. In every case of this family examined previously (C3H− and CCSiH−) and presently (C3OH−, C3NC−, C4N−, and C5H−), valence excited states are produced in the computations. Additionally, HBCN−, which is isoelectronic and structurally similar to C3H−, also possesses a valence excited state. The HOMO for the corresponding radicals to each of these anions is a singly occupied π-type orbital. By creating a singlet in the valence ground state of the anion, the two parts of this π-type orbital split breaking the symmetry of the system to Cs. Excitation can then take place between these two former pieces of a π-pair creating a a′ → a″ valence excitation. If the dipole moment is large enough in the radical, an additional dipole-bound state exists. The cases examined here that do not possess valence excited states (c-C3C2H−, HBNC−, and H2BCC−) either contain cyclic groups, higher symmetry, or both. As a result, the HOMO is no longer a recognizable part of a former π-pair even though these anions are isomers of and/or isoelectronic to anions with valence excited states. 10767
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(38) Agúndez, M.; Cernicharo, J.; Guélin, M.; Gerin, M.; McCarthy, M. C.; Thaddeus, P. Search for Anions in Molecular Sources: C4H− Detection in L1527. Astron. Astrophys. 2008, 478, L19−L22. (39) Fortenberry, R. C.; Huang, X.; Crawford, T. D.; Lee, T. J. HighAccuracy Quartic Force Field Calculations for the Spectroscopic Constants and Vibrational Frequencies of 11A′ l-C3H−: A Possible Link to Lines Observed in the Horsehead Nebula Photodissociation Region. Astrophys. J. 2013, 772, 39. (40) F. Carelli, R. W.; Gianturco, F. A.; Satta, M. Formation of Cyanopolyyne Anions in the Interstellar Medium: The Possible Role of Permanent Dipoles. J. Chem. Phys. 2014, 141, 054302. (41) Neumark, D. M. Slow Electron Velocity-Map Imaging of Negative Ions: Applications to Spectroscopy and Dynamics. J. Phys. Chem. A 2008, 112, 13287−13301. (42) Yacovitch, T. I.; Garand, E.; Neumark, D. M. Slow photoelectron velocity-map imaging spectroscopy of the vinoxide anion. J. Chem. Phys. 2009, 130, 244309. (43) Yen, T. A.; Garand, E.; Shreve, A. T.; Neumark, D. N. Anion Photoelectron Spectroscopy of C3N− and C5N−. J. Phys. Chem. A 2010, 114, 3215−3220. (44) Fortenberry, R. C.; King, R. A.; Stanton, J. F.; Crawford, T. D. A Benchmark Study of the Vertical Electronic Spectra of the Linear Chain Radicals C2H and C4H. J. Chem. Phys. 2010, 132, 144303. (45) Mach, T. J.; King, R. A.; Crawford, T. D. A Coupled Cluster Benchmark Study of the Electronic Spectrum of the Allyl Radical. J. Phys. Chem. A 2010, 114, 8852−8857. (46) Grutter, M.; Wyss, M.; Maier, J. P. Electronic Absorption Spectra of C2nH−, C2n−1N− (n = 4−7), and C2nN (n = 3−7) Chains in Neon Matrices. J. Chem. Phys. 1999, 110, 1492−1496. (47) Scheiner, A. C.; Scuseria, G. E.; Rice, J. E.; Lee, T. J.; Schaefer, H. F., III Analytic evaluation of energy gradients for the single and double excitation coupled cluster (CCSD) wave function: Theory and application. J. Chem. Phys. 1987, 87, 5361−5373. (48) Raghavachari, K.; Trucks, G. W.; Pople, J. A.; Head-Gordon, M. A Fifth-Order Perturbation Comparison of Electron Correlation Theories. Chem. Phys. Lett. 1989, 157, 479−483. (49) Dunning, T. H. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (50) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. Electron Affinities of the First-Row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96, 6796−6806. (51) Dunning, T. H.; Peterson, K. A.; Wilson, A. K. Gaussian Basis Sets for Use in Correlated Molecular Calculations. X. The Atoms Aluminum through Argon Revisited. J. Chem. Phys. 2001, 114, 9244− 9253. (52) Stanton, J. F.; Bartlett, R. J. The Equation of Motion CoupledCluster Method. A Systematic Biorthogonal Approach to Molecular Excitation Energies, Transition-Probabilities, and Excited-State Properties. J. Chem. Phys. 1993, 98, 7029−7039. (53) Stanton, J. F.; Gauss, J. Analytic energy derivatives for ionized states described by the equation-of-motion coupled cluster method. J. Chem. Phys. 1994, 101, 8938−8944. (54) Gauss, J.; Stanton, J. F.; Bartlett, R. J. Coupled-Cluster OpenShell Analytic Gradients: Implementation of the Direct Product Decomposition Approach in Energy Gradient Calculations. J. Chem. Phys. 1991, 95, 2623−2638. (55) Watts, J. D.; Gauss, J.; Bartlett, R. J. Open-Shell Analytical Energy Gradients for Triple Excitation Many-Body, Coupled-Cluster Methods: MBPT(4), CCSD+T(CCSD), CCSD(T), and QCISD(T). Chem. Phys. Lett. 1992, 200, 1−7. (56) Turney, J. M.; Simmonett, A. C.; Parrish, R. M.; Hohenstein, E. G.; Evangelista, F. A.; Fermann, J. T.; Mintz, B. J.; Burns, L. A.; Wilke, J. J.; Abrams, M. L.; et al. PSI4: An Open-Source Ab Initio Electronic Structure Program. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 556−565. (57) CFOUR, a quantum chemical program package written by Stanton, J. F.; Gauss, J.; Harding, M. E.; Szalay, P. G.; with contributions from Auer, A. A.; Bartlett, R. J.; Benedikt, U.; Berger, 10768
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C.; Bernholdt, D. E.; Bomble, Y. J.; et al. For the current version, see http://www.cfour.de. (58) Koures, A. G.; Harding, L. B. Ab Initio Examination of the Electronic Excitation Spectrum of CCH. J. Phys. Chem. 1991, 95, 1035−1040. (59) Sobolewski, A. L.; Adamowicz, L. Ab Initio Characterization of Electronically Excited States in Highly Unsaturated Hydrocarbons. J. Chem. Phys. 1995, 102, 394−399. (60) Graham, W. R. M.; Dismuke, K. I.; Weltner, W. C2H Radical: 13 C Hyperfine Interaction and Optical Spectrum. J. Chem. Phys. 1974, 60, 3817−3823. (61) Dismuke, K. I.; Graham, W. R. M.; Weltner, W. Optical and ESR Spectra of the C4H Radical in Rare Gas Matrices at 4 K. J. Mol. Spectrosc. 1975, 57, 127−137. (62) Woon, D. E. A Correlated Ab Initio Study of Linear CarbonChain Radicals CnH (n = 2−7). Chem. Phys. Lett. 1995, 244, 45−52. (63) McCarthy, M. C.; Gottlieb, C. A.; Thaddeus, P.; Horn, M.; Botschwina, P. Structure of the CCCN and CCCCH Radicals: Isotopic Substitution and Ab Initio Theory. J. Chem. Phys. 1995, 103, 7820−7827. (64) Pino, T.; Tulej, M.; Güthe, F.; Pachkov, M.; Maier, J. P. Photodetachment Spectroscopy of the C2nH− (n = 2−4) Anions in the Vicinity of Their Electron Detachment Threshold. J. Chem. Phys. 2002, 116, 6126−6131. (65) Pachkov, M.; Pino, T.; Tulej, M.; Maier, J. P. Electronic Transition of C3H− in the Vicinity of the Lowest Photodetachment Threshold. Mol. Phys. 2001, 99, 1397−1405. (66) Pino, T.; Pachkov, M.; Tulej, M.; Xu, R.; Jungen, M.; Maier, J. P. Freshbach Resonances of the C3H− Anion: Laser Autodetachement Spectroscopy and the Ab Initio Calculations. Mol. Phys. 2004, 102, 1881−1889. (67) Sheehan, S. M.; Parsons, B. F.; Yen, T. A.; Furlanetto, M. R.; Neumark, D. M. Anion Photoelectron Spectroscopy of C5H−. J. Chem. Phys. 2008, 128, 174301. (68) Crawford, T. D.; Stanton, J. F.; Saeh, J. C.; Schaefer, H. F. Structure and Energetics of Isomers of the Interstellar Molecule C5H. J. Am. Chem. Soc. 1999, 121, 1902−1911. (69) Mintz, B. J.; Crawford, T. D. Symmetry Breaking in the cyclicC3C2H Radical. Phys. Chem. Chem. Phys. 2010, 12, 15459−15467. (70) Inostroza, N.; Huang, X.; Lee, T. J. Accurate Ab Initio Quartic Force Fields of Cyclic and Bent HC2N Isomers. J. Chem. Phys. 2011, 135, 244310. (71) Inostroza, N.; Fortenberry, R. C.; Huang, X.; Lee, T. J. Rovibrational Spectroscopic Constants and Fundamental Vibrational Frequencies for Isotopologues of Cyclic and Bent Singlet HC2N isomers. Astrophys. J. 2013, 778, 160.
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Predictable Valence Excited States of Anions Ryan C. Fortenberry,* W. James Morgan, and Jordan D. Enyard Department of Chemistry, Georgia Southern University, Statesboro, Georgia 30460, United States ABSTRACT: The excited states of the 1 1A′ :CC−C̈ −R− family and related anions are investigated. Previous work has shown that 1 1A′ C3H− and CCSiH− each possess a rare valence excited state in addition to their dipole-bound excited states. A similar methodology to that employed previously shows that related anions (C3OH−, C3NC−, C4N−, C5H−, and HBCN−) also possess valence excited states. The valence states are the result of breaking the symmetry from C∞v to Cs. The half-filled π or π-type highest occupied molecular orbital (HOMO) is split into a′ and a″ pieces. The valence excitation takes place between these two pieces. If the anion HOMO is not a half-filled π-type orbital because of an increase in symmetry, cyclization, or both, the anion most likely does not exhibit the signs of a valence excited state even if the anion is an isomer of or isoelectronic to an anion that does possess a valence excited state. However, the :CC−C̈ −R− set is not the only classification of anions shown to possess valence excited states even though it is the most predictable grouping exhibiting this behavior found to date.
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INTRODUCTION Small, organic anions can support excited states. Such a fact is not new, but the number of anions and the number of states they can support continues to grow.1−6 The existence of dipolebound excited states of anions has proven to be quite a curiosity in the world of physical chemistry with excited states of CH2CN−7,8 and CH2CHO−9,10 experimentally observed for around 25 years with quantum chemical computations preceding the experiment by more than a decade for the enolate anion.11 A dipole-bound state in many ways resembles a Rydberg state. A Rydberg state exists when an electron is bound to the molecule by the monopole−monopole Coulombic attraction between the electron itself and the “hydrogen nucleus” of the remaining molecular core cation. Banishment of the extra electron to outer reaches of the molecular system in an anion does not retain such an attraction, often leading to the electron simply leaving the system. If a neutral molecule has a large enough dipole moment, however, an extra electron may be bound in a Rydberg-like fashion but through a weaker monopole−dipole interaction.1,2,12−14 As a result, the binding of most dipole-bound anions is tenuous, and the size of the dipole-bound orbital is larger than a typical Rydberg orbital. The minimum dipole moment required to bind the electron in a dipole-bound fashion was originally determined by Fermi and Teller nearly 70 years ago12 to be 1.625 D under ideal circumstances. Much more recent studies have put practical limits closer to 2.0 D, or even 2.5 D,8,14−18 with an understanding that this phenomenon may be environmentally or molecularly dependent to a certain degree.19−23 Several molecular and cluster anions have exhibited dipole-bound states,1,13,14,16,24−32 but it is the valence excited state that opens up the electronic properties of anions to more complex physical analysis similar to that of more stable molecular species.33 Because Fermi and Teller’s equations allow for only one dipole© 2014 American Chemical Society
bound state until a dipole moment of more than 9 D is reached,12 and again, the energy of a dipole-bound state is at the electron dissociation threshold, any anion with a dipolebound state as its ground state is metastable at best. If an anion possesses a valence state as its ground state, the dipole-bound state can exist as an excited state,1,2 as is the case for the aforementioned CH2CN− and CH2CHO− species. The easiest way for an anion to possess a valence ground state is simple removal of a hydrogen from the closed-shell neutral and replacing it with a lone electron to retain the spin-paired nature of the molecule even though now it is negatively charged. The dipole-bound excitation in the methylene nitrile anion has even been suggested as a carrier for one of the elusive diffuse interstellar bands,34−36 the ubiquitous interstellar absorption features labeled37 as the “longest standing problem in spectroscopy.” Additionally, it has been suggested that dipole-bound states may be necessary precursors to the formation of anions in the interstellar medium.38−40 As a result, excited states of anions are beginning to show nuanced physics that may be valuable for future applications. The fact that the dipole-bound excited state is quite nearly coincident with the energy required to remove the electron, or electron binding energy (eBE), a spectroscopic signature for this excitation produces a peak clearly indicating when the corresponding neutral has been created and what its relative energy is. Neumark and his collaborators have utilized this principle as well as the valence ground state itself to produce stability and structural data for numerous difficult radical species making use of slow electron velocity-map imaging spectroscopy.41−43 As such, the lone dipole-bound excited state of an anion can serve as a spectral signature, an intermediate in Received: September 19, 2014 Revised: October 20, 2014 Published: October 21, 2014 10763
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Figure 1. CCSD(T)/aug-cc-pVTZ anion geometries in angstroms and degrees for (A) C3OH−, (B) C3NC−, (C) C4N−, (D) C5H−, (E) c-C3C2H−, (F) HBCN−, (G) HBNC−, and (H) H2BCC−.
the dipole-bound state.3−6 CH2SiN−, SiCCN−, C3H−, and CCSiH− all possess a single valence excited state in addition to the dipole-bound state, giving two electronically excited states. CCSiN− actually has two valence excited states and, potentially, a dipole-bound state. Additionally, the C2n−1N− (n = 4−7) family of molecules has been shown experimentally to possess valence excited states,46 but the term “small” may not be applied to some of these molecules by many physical chemists. It is currently unknown why some anions possess valence excited states and why some do not. Valence excited states exist independently of the magnitude of the neutral’s dipole moment, making their existence in anions more tantalizing. Initial studies indicated that all the valence excited states of anions with four or fewer heavy atoms contain silicon.3,4 To determine if something special was at work with the silicon
a reaction pathway, or a necessary indicator of daughter species classification. The limited electronic features of anions, especially coupled with the metastable nature of dipole-bound states, still casts shadows of limited novelty on excited states of anions. However, multiple excited states of anions would open up the possible role that anions may play in various circumstances. The only way in which to possess multiple excited states is for an anion to exhibit an additional valence state between the dipole-bound state and the valence ground state. However, this requires exceptionally low excitation energies. Valence excited states of neutrals beneath the energy of Rydberg states have been shown for small hydrocarbons,44,45 making such states possible for anions. In fact, further research has shown that several small anions can retain additional valence states below 10764
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Table 1. Vertically Excited-State Energies (in eV) Across Basis Setsa Based on the Anion CCSD(T)/aug-cc-pVTZ Ground-State Geometry molecule −
C3OH
C3NC− C4N− C5H− c-C3C2H− HBCN− HBNC− H2BCC− a
state
pVDZ
apVDZ
dapVDZ
tapVDZ
eBE
1
5.60 2.52 6.35 1.47 6.03 0.90 6.56 0.83 6.13 7.10 1.18 6.24 3.05 7.41 3.70
3.26 2.48 4.67 1.45 4.38 0.89 3.34 0.81 3.11 2.60 1.12 2.58 2.30 4.55 3.73
2.91 2.48 3.75 1.45 3.55 0.89 2.91 0.81 2.60 1.68 1.12 1.53 1.65 3.74 3.72
2.88 2.48 3.57 1.45 3.41 0.89 2.86 0.81 2.50 1.55 1.12 1.40 1.44 3.64 3.71
2.93 − 3.53 − 3.36 − 2.89 − 2.48 1.53 − 1.37 − 3.62 −
2 1 2 1 2 1 2 1 1 2 1 2 1 1 1
A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 B1
fb 2 6 1 2 1 9 9 5 5 3 1 2 2 1 7
× × × × × × × × × × × × × × ×
10−2 10−3 10−3 10−3 10−3 10−4 10−3 10−4 10−3 10−3 10−2 10−4 10−5 10−4 10−3
pVTZ
apVTZ
dapVTZ
5.26 2.49 6.11 1.46 6.30 0.88 6.02 0.82 5.60 5.61 1.14 5.38 2.92 7.50 3.80
3.33 2.45 4.58 1.45 4.33 0.88 3.37 0.81 3.15 2.42 1.11 2.40 2.32 4.55 3.73
3.07 2.45 3.90 1.45 3.73 0.88 3.07 0.81 2.75 1.78 1.11 1.66 1.79 3.85 3.78
The correlation consistent basis sets are abbreviated with d-aug-cc-pVDZ as dapVDZ, for example. bEOM-CCSD/tapVDZ values.
Geometry optimizations of these states produces more physically interpretable results even if the complexities of the wave functions and differences in geometries can muddle the results.1 Adiabatic eBEs are computed from the difference between the energy of the CCSD(T)/aug-cc-pVTZ groundstate geometry of the anion and the unrestricted Hartree−Fock (UHF)54,55 CCSD(T)/aug-cc-pVTZ ground-state geometry of the corresponding neutral radical. UHF-CCSD/aug-cc-pVTZ dipole moments for the neutral radicals are computed from this optimized geometry. CCSD/d-aug-cc-pVDZ ground-state and EOM-CCSD/d-aug-cc-pVDZ excited-state geometries are computed; the difference in energies produces the adiabatic excitation energies. The use of both vertical and adiabatic data serves to describe the nature of excited states and to provide experimentally meaningful data, respectively. All of the computations in this work make use of the free and opensource PSI4 quantum chemistry package56 with the exception of the EOMIP computations which require the use of the CFOUR program.57
atom, similar heavier atoms (Al−S) were inserted in small, hydrocarbon anions, and their spectral features were computationally investigated.5 No new valence excited states for aluminum-, phosphorus-, or sulfur-containing anions were shown. However, C3H− and its silicon analogue, CCSiH−, do possess valence excited states. In the present work we explore anions structurally related to C3H− and CCSiH− with half-filled π-type highest occupied molecular orbitals (HOMO) that create bent geometry, singlet-state anions. We will limit ourselves to only “first-row” (B−O) atoms because these are more common in the universe and are more likely to appear in nature (wherever that may be) as a result. It is hoped that further examples of states with valence excited states will allow us to form better models for understanding how and when multiple electronic states of anions arise.
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COMPUTATIONAL DETAILS The classification of an excited state of an anion as either valence or dipole-bound can be clearly distinguished by the behavior of the excitation energy as the diffuseness of the basis set is increased.3−5 As the basis set becomes spatially larger and the energy decreases, a dipole-bound state is present because more of the electron density of this highly diffuse s-type, Rydberg-like orbital is described. If little change is present, the excited state is valence in nature. Restricted Hartree−Fock reference wave functions47 and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]48 computations combined with the aug-cc-pVTZ basis set49−51 produce optimized geometries. From these geometries, equation-of-motion CCSD (EOM-CCSD) vertical excitation energies52 are computed with the n-aug-cc-pVDZ and n-aug-cc-pVTZ series of basis sets where n represents the even-tempered extrapolation of the diffuseness of the basis set from the cc-pVXZ level to the t-aug-cc-pVXZ level.45 Equationof-motion ionization potential (EOMIP) computations53 produce the vertical eBEs. Even though vertical computations are susceptible to error because of a lack of geometric rearrangement in response to the change in wave function and orbital occupation, the restriction of variables to only the change in wave function itself gives consistent results with clear conclusions as to the existence of any excited state, whether valence or dipole-bound.
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RESULTS AND DISCUSSION The anions examined here are based on the :CC−C̈ −R− general structure with 1A′ ground states. In the prototypical C3H− molecule, R = H, for example. It is known that the evennumbered carbon chains C2nH radicals are 2Σ+ ground states from n ≤ 4 and 2Π states for n ≥ 6.44,58−64 However, the odd n chains are all 2Π in their ground states. The major difference is that the corresponding anions for the even n radicals all fill their HOMOs, creating 1Σ+ ground states. The odd n radicals, however, have only one π electron in the HOMO leading to the possibility of triplet or singlet states in the corresponding anions. In the case of C3H−, Feshbach resonances are known to exist for the lower-energy triplet state,65,66 but the creation of the singlet state breaks the symmetry from C∞v in the radical to Cs in the singlet anion. As such, the π orbital splits into a′ and a″ pieces. The valence excited state is therefore the HOMO a′ → a″ excitation. To maintain this splitting of the π HOMO, any R-group substituting for the hydrogen atom in the :CC−C̈ − R− family must attach via only a single bond. For this study, common functional groups are added to test if these anions will retain this valence excitation. These groups include −OH, −CN/−NC, and −C2H. 10765
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1 1A′ C3OH− exists as two conformers on the potential energy surface (PES), one with the hydrogen cis and the other trans. Unsurprisingly, the trans is lower in energy and will be the only isomer examined here. This is depicted in Figure 1a. The CCSD(T)/aug-cc-pVTZ C2−C3−O bond angle is 111.0°, 1.9° greater5 than the C2−C3−H bond angle in C3H−. From a Lewis structure perspective, it is clear that the C3 atom in C3OH− possesses a lone pair like it does in C3H−. Additionally, the C1−C2 bond length implies a triple bond and carbide moiety. Hence, C3OH− should have electronic properties similar to those of C3H−. Table 1 gives clear indication that the hydroxy anion possesses a dipole-bound state, the 2 1A′, because the excitation energy decreases as the spatial extent of the basis set increases. Additionally, the eBE (2.93 eV) is close in energy to the 2.88 eV EOM-CCSD/t-aug-cc-pVDZ excitation energy. The valence excited state (1 1A″), lower at 2.48 eV, exhibits no decrease in energy after the first set of diffuse functions is included in the basis set. The dipole-bound excited state is the result of an electron being promoted out of the HOMO into a diffuse s-type orbital, whereas the valence excited state is an excitation out of the a′ HOMO into the corresponding π-pair a″ valence virtual orbital. This accepting orbital is not the lowest-energy a″ virtual orbital. Several exist below this energy and correspond to various p-type Rydberg orbitals. Both cases of excitation in C3OH− behave the same way that the dipole-bound and valence excitations do for C3H−. This is further corroborated by the adiabatic excitations reported in Table 2. C3OH− will support both excited states because the dipole moment for the corresponding radical is quite large at 5.41 D.
are small at 1.21 and 0.03 D, respectively. These neutrals actually exhibit a C1 minimum geometry but are close to linear. As a result, the C4N dipole moment could well be considered 0.0 D if the bond angles were actually all 180.0°. With the loss of the lone pair in the radical, the C4N radical has bond angles of ∠C4−C3−C2 = 168.8°, ∠C3−C2−C1 = 176.8°, and ∠C2− C1−N = 178.3°, making it nearly, but not actually, linear. The dihedral potential scans are very flat because of these nearlinear bond angles, but the most pronounced deviation from planarity in 1 2A C4N is τ(C3−C2−C1−N) at 63.0°. Even though the adiabatic eBEs for these anions are higher than the adiabatic dipole-bound state excitation energies (Table 2), the t-aug-cc-pVDZ vertical excitation energies and vertical eBEs (Table 1) indicate that these two states are nearly coincident, a necessary case for the state to be dipole-bound. Regardless, the small dipole moments present in each radical will not support a dipole-bound state in either of the corresponding C3NC− or C4N− molecules. Only the 1 1A′ valence state, again defined as the excitation out of the a′ HOMO into the other π-pair a″ orbital, will be observable or accessible for either of these anions. The dipole moment for the isoelectronic C5H radical, however, is quite substantial at 4.72 D. Additionally, Table 1 shows how the 2 1A′ state of C5H− (Figure 1d) is clearly a dipole-bound state with the 1 1A″ state clearly valence in nature. The 1 1A″ ← 11A′ excitation takes place at 0.81 eV vertically and 0.46 eV adiabatically. This excitation is energetically well into the infrared region of the electromagnetic spectrum and even into the range typically associated with hydride stretches. The C4N− 1 1A″ ← 11A′ adiabatic excitation is also in this same region at 0.47 eV. C5H− may be thought of as part of the :CC−C̈ −R− family with R = C2H or as C3H− with a carbide group attached to the end opposite the hydrogen atom. The latter description is fitting because ∠C4−C5−H is 113.3° and the lone pair is localized on the C5 atom. Previous computations put the ∠C5−C4−C3 much lower than our values creating a cyclic structure for the Cs structure,67 but our methods include more electron correlation and describe a deeper well than this other minimum. Regardless, the spinpaired, double occupation of the π-type HOMO and resulting symmetry breaking from this singlet state create a very low energy valence excitation. Another isomer of C5H, cyclic-C3C2H given in Figure 1e, is tested here because it is known to have a large dipole moment.68 However, it has also been quite a problem for electronic structure methods because of symmetry-breaking and multireference effects.69 Our interests lie in the electronic properties of the anion, thus, alleviating many of these issues. c-C3C2H− actually lies 2.43 eV lower (from CCSD(T)/aug-ccpVTZ) on the PES than C5H−. Because of the C2v structure of c-C3C2H− and the resulting orbital occupation of this anion, the half-filled π orbital is no longer present and is no longer the HOMO. This is replaced by the 4b2 orbital as the HOMO, out of which the electron is excited into a diffuse s-type orbital to create the lone dipole-bound state clearly shown in Table 1. The previously computed dipole moment is large enough to support this state, and no low-lying valence excited states are present. The adiabatic computations of the radical could not be properly carried out in this study, but the vertical computations showcase a dipole-bound excited state. Regardless, the loss of the π-splitting in the HOMO negates the presence of a valence excited state in this anion even though it is isomeric to C5H−.
Table 2. Adiabatic Excitation Energies, eBEs, and Dipole Moments molecule
state
C3OH−
2 1 2 1 2 1 2 1 1 2 1 2 2
C3NC− C4N− C5H− c-C3C2H− HBCN− HBNC− H2BCC−
1
A′ A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 A′ 1 A″ 1 B2 1 A′ 1 A″ 1 A′ 1 B2 1
excitation energy (eV)
eBE (eV)
dipole momenta (D)
2.24 2.00 3.17 1.00 3.06 0.47 2.20 0.46 2.51 1.41 0.70 1.86 3.68
2.30 − 2.97 − 2.91 − 2.26 − 3.3c 1.53 − 1.87 0.79
a
For the corresponding neutral radical. bThe adiabatic eBE is not trustworthy because of multireference problems inherent in the radical’s electronic structure. See refs 68 and 69. cThis is the minimum value reported in ref 68.
The isomeric pair C3NC− and C4N− (Figure 1b,c) also show clear dipole-bound excited-state character for their respective 2 1 A′ states and valence character for their 1 1A′ states. C4N− is more stable than C3NC− by 0.96 eV from the ground-state CCSD(T)/aug-cc-pVTZ geometry optimizations. The C3− C2−R bond angles are larger in these two cases but are still less than 120°, indicating a lone pair is once more located on the central carbon (C2 in both cases here). However, the dipole moments for these two species’ corresponding neutral radicals 10766
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All indications are that creating singlet-state anions from 2Π or closely related radicals with a singly occupied π-type HOMO is a reliable means of producing valence excited states in anions. Deviation from this model will likely not produce valence excited states. However, CH2SiN−, SiCCN−, and CCSiN− also possess valence excited states. As such, the structure of 1A′ :CC−C̈ −R− and related isoelectronic, quasi-linear anions appear to be adequate but not necessary for an anion to possess valence excited states. There still exists much to learn about valence excited states of anions, and the development of a more complete model for anionic valence excitation is still warranted.
To determine the robustness of the hypothesis that anions with a half-filled π-type HOMO arranged in a singlet state can produce valence excited states, HBCN− is tested. It is isoelectronic with C3H−, and its optimized geometry (Figure 1f) resembles that of the hydrocarbon anion. The major exception is the B−N bond, which is 1.568 Å for HBCN− while the corresponding C−C bond in C3H− is 1.359 Å. The resulting molecules, therefore, differ to a certain extent, but the HOMO in both is still the a′ portion of the former half-filled π orbital. Consequentially, HBCN− exhibits a clear 2 1A′ dipolebound state and a 1 1A″ valence state, as shown in Table 1. However, the dipole moment of the corresponding radical is below the ∼2 D threshold, indicating that the dipole-bound excited state will not be present in this anion. As a result, the low-energy valence state is the only excited state in HBCN−. Adiabatically, the excitation energy is 0.70 eV, well below the 1.26 eV eBE, and vertically is one of the brightest of all of the excitations computed here with an oscillator strength on the order of 0.01. Simple isomerization to the 0.26 eV higher-energy (CCSD(T)/aug-cc-pVTZ) HBNC− anion does not produce a valence excited state or even a dipole-bound excited state. Although a second excited state (1 1A″) exists just above the first excited state (2 1A′) in Table 1, the lower-energy state is the dipolebound state. Additionally, the 1 1A″ state does not give a welldefined early excited-state energy convergence as one would expect for a valence excited state. The excitation energy varies by 1.61 eV from the cc-pVDZ basis set (3.05 eV) to t-aug-ccpVDZ (1.44 eV). The corresponding radical produces only a 1.87 D dipole moment, probably too low for HBNC− to retain a dipole-bound state. The lack of a valence state and the low dipole moment are both the result of cyclization in the structure of HBNC and HBNC− (Figure 1g for the anion). Similar behavior has previously been shown in the isoelectronic 1 1A′ HCCN molecule.70,71 H2BCC−, shown in Figure 1h, is isoelectronic to C3H−, HBCN−, and HBNC−, but its C2v structure decreases the dipole moment below the binding threshold and precludes the formation of the half-filled π HOMO. As a result, H2BCC− also does not possess valence or dipole-bound excited states.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Georgia Southern University is acknowledged for the provision of start-up funds necessary to perform this research. REFERENCES
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CONCLUSIONS Anions of the :CC−C̈ −R− family reliably, and predictably, possess valence excited states. In every case of this family examined previously (C3H− and CCSiH−) and presently (C3OH−, C3NC−, C4N−, and C5H−), valence excited states are produced in the computations. Additionally, HBCN−, which is isoelectronic and structurally similar to C3H−, also possesses a valence excited state. The HOMO for the corresponding radicals to each of these anions is a singly occupied π-type orbital. By creating a singlet in the valence ground state of the anion, the two parts of this π-type orbital split breaking the symmetry of the system to Cs. Excitation can then take place between these two former pieces of a π-pair creating a a′ → a″ valence excitation. If the dipole moment is large enough in the radical, an additional dipole-bound state exists. The cases examined here that do not possess valence excited states (c-C3C2H−, HBNC−, and H2BCC−) either contain cyclic groups, higher symmetry, or both. As a result, the HOMO is no longer a recognizable part of a former π-pair even though these anions are isomers of and/or isoelectronic to anions with valence excited states. 10767
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