Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: A theoretical study Behnaz Saed and Reza Omidyan Citation: The Journal of Chemical Physics 140, 024315 (2014); doi: 10.1063/1.4859255 View online: http://dx.doi.org/10.1063/1.4859255 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electronic and vibrational spectra of protonated benzaldehyde-water clusters, [BZ-(H2O)n5]H+: Evidence for ground-state proton transfer to solvent for n 3 J. Chem. Phys. 140, 124314 (2014); 10.1063/1.4869341 Photodissociation of van der Waals clusters of isoprene with oxygen, C5H8O2, in the wavelength range 213–277 nm J. Chem. Phys. 137, 054305 (2012); 10.1063/1.4737856 Electronically excited states of water clusters of 7-azaindole: Structures, relative energies, and electronic nature of the excited states J. Chem. Phys. 128, 214310 (2008); 10.1063/1.2928636 IR + vacuum ultraviolet (118 nm) nonresonant ionization spectroscopy of methanol monomers and clusters: Neutral cluster distribution and size-specific detection of the OH stretch vibrations J. Chem. Phys. 124, 024302 (2006); 10.1063/1.2141951 Solvent effects on the n * electronic transition in formaldehyde: A combined coupled cluster/molecular dynamics study J. Chem. Phys. 121, 8435 (2004); 10.1063/1.1804957

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THE JOURNAL OF CHEMICAL PHYSICS 140, 024315 (2014)

Protonation effect on the electronic properties of 2-pyridone monomer, dimer and its water clusters: A theoretical study Behnaz Saed1 and Reza Omidyan1,2,a) 1

Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran Centre Laser de l’Université Paris Sud (LUMAT, FR, 2764), Bât. 106, Univ. Paris-Sud 11, 91405 Orsay Cedex, France 2

(Received 24 August 2013; accepted 13 December 2013; published online 14 January 2014) The CC2 (second order approximate coupled cluster method) has been applied to investigate protonation effect on electronic transition energies of 2-pyridone (2PY), 2-pyridone dimer, and microsolvated 2-pyridone (0-2 water molecules). The PE profiles of protonated 2-pyridone (2PYH+ ) as well as monohydrated 2PYH+ at the different electronic states have been investigated. The 1 π σ ∗ state in protonated species (2PYH+ ) is a barrier free and dissociative state along the O-H stretching coordinate. In this reaction coordinate, the lowest lying 1 π σ ∗ predissociates the bound S1 (1 π π ∗ ) state, connecting the latter to a conical intersection with the S0 state. These conical intersections lead the 1 π π ∗ state to proceed as predissociative state and finally direct the excited system to the ground state. Furthermore, in presence of water molecule, the 1 π σ ∗ state still remains dissociative but the conical intersection between 1 π σ ∗ and ground state disappears. In addition, according to the CC2 calculation results, it has been predicted that protonation significantly blue shifts the S1 -S0 electronic transition of monomer, dimer, and microhydrated 2-pyridone. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4859255] I. INTRODUCTION

Excited states of protonated aromatic molecules, in particular small aromatic substituted molecules such as protonated adenine,1 benzaldehyde,2 salicylaldehyde,3 phenol4 and also polycyclic aromatic hydrocarbons (PAHs),5–10 have been extensively studied either theoretically or experimentally. As a common concluding remark in all of these studies, it was demonstrated that protonation leads to a red shift effect on the S1 -S0 electronic transition.2–8 Despite the importance of hetero-aromatic compounds, rarely reports are devoted to the protonation effect on the electronic properties of them. Recently, the electronic spectra of different isomers associated with protonated indole has been reported by Alata et al.11 in jet-cooled system. They have shown that, benzene ring is the most favored for protonation and N-heteroatom is in the second order of stabilization. However, for all of protonated isomers, it was demonstrated that S1 -S0 transition energies are far to the red compared to that of the S1 −S0 transition of neutral indole (4.38 eV).12, 13 This study represents new results on the topics of protonated hetero-aromatics molecules. The molecule of choice for this study is 2-pyridone, since of its important role in chemistry, biology, and biophysics.14–18 Numerous experimental19–21 and theoretical22–29 investigations have treated the keto-enol tautomerization reaction of 2-pyridone (2PY) to 2-hydroxypyridine (2HP). Also, 2PY has been considered as a good model for the excited state reactivity of the pyrimidine nucleo-bases uracil and thymine.30

a) Author to whom correspondence should be addressed. Electronic

addresses: [email protected] and [email protected]. Tel: (+98) 311 7932708. Fax: (+98) 311 6689732. 0021-9606/2014/140(2)/024315/10/$30.00

The keto/enol tautomeric equilibrium is one of the most important and widespread equilibrium in chemical and biological processes.31, 32 It is indeed shown by the nucleic acid bases33–42 and it has been related to the appearance of DNA mutations induced by proton transfer reactions.43 The 2HP/2PY system is a widely studied model system chosen for its extreme ease of experimental manipulation and the simplification as compared to real nucleo-base pairs. The relative energy of both isomers is determined to be 0.025 eV in the gas phase, 0.092 eV in solution, and 0.034 eV in solid state.44 Thus, for the 2HP/2PY system, the tautomeric equilibrium is slightly shifted to the hydroxy form.45 For this reason, both tautomers have an appreciable concentration at room temperature. Although, there is no intramolecular hydrogen bond in the 2HP/2PY, the transfer of hydrogen atom in this system does occur upon optical excitation in a low-temperature inert gas matrix, as it was confirmed via analysis and assignment of the IR spectra obtained before and after irradiation.46–53 Thus, it has been well established that photoinduced intramolecular proton transfer (PIPT) process in 2HP/2PY is fundamentally different from systems which involve intramolecular hydrogen bonding (nominally normal systems). The most common feature of the ESIPT reactions in the normal systems, involves transfer of a proton (or hydrogen) along the intramolecular hydrogen bond and the existence of a hydrogen bond is a necessary condition for the ESIPT process to occur.42, 54 The significantly different photophysics of typical PIPT systems, as mentioned above, lead to a new suggestion proposed by Sobolewski as photon-induced dissociation-association (PIDA)55 mechanism for UV photoinduced intramolecular proton (or hydrogen) transfer (PIPT) process in non-hydrogen bonded systems such as 2HP/2PY.

140, 024315-1

© 2014 AIP Publishing LLC

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FIG. 1. Optimized geometries of (a) 2-pyridone, (b) 2-hydroxypyridine, and (c) the most stable isomer of protonated 2-pyridone (2PYH+ ) and numbering pattern.

Later, this mechanism was confirmed to occur in formamide56 and also 4(3H)-pyrimidinone/4-hydroxypyrimidine57 as well. However, according to the results of Sobolewski and Adamowicz,22, 55 there is a large energy barrier for proton transfer reaction not only in the ground state (2 eV), but also in the S1 excited state (1.25 eV). This big barrier separates the 2PY-2HP in the intramolecular proton transfer coordinate at the S1 1 A (π π ∗ ) excited state. Also, the N-H dissociation reactions from the dark S2 state (1 A , nπ ∗ ) of 2HP-2PY have to pass a barrier of 1.5 eV to access a freely dissociative surface. Hence both tautomeric forms are well protected in 1 A state against dissociation of proton. Therefore, the protonation effect on such photo physical properties of monomer, dimer, and hydrated 2PY will be well addressed in this work. It is worthy to note that performing highly accurate calculations of geometric structures of titled compounds has not been the goal of the present work. We have rather been interested in the development of a clear qualitative picture of the basic mechanisms of the hydrogen-/proton-transfer processes in PYH+ and its water cluster as well. Our paper is organized as follows: after summarizing the computational methods, we present ground- and excited-state optimized structures and energies. We start with the neutral structures because they form a basis for understanding of protonated species. We discuss and explain the proton detachment and proton transfer in protonated 2-pyridone monomer and its cluster with water molecule, respectively. Finally, we attend to the protonation effect on electronic transition energies of 2-pyridone dimer. The CC2 is method of choice because it gives reasonable results for medium size organic molecules, either in bare form or in their complex forms (e.g., with water and ammonia) for a moderate computational time.2, 7, 57–60

II. COMPUTATIONAL DETAILS

The “ab initio” calculations have been performed with the TURBOMOLE program suit,61, 62 making use of the resolution-of-identity (RI) approximation for evaluation of the electron repulsion integrals. The equilibrium geometry of all systems (three classes) at the ground state has been determined at the MP2 (Moller-Plesset second order perturbation theory) level.63, 64 Excitation energies and equilibrium geometry of the lowest excited singlet states have been determined

at the RI-CC2 (the second-order approximate coupled-cluster method).65, 66 In order to evaluate our calculation level and find the best consistency between the adiabatic transition energy of neutral compounds with their corresponding UV-vis information of literature, several basis function have been employed in our calculations: the Dunning’s correlation consistent splitvalence double-ζ basis set (cc-pVDZ) and the augmented ccpVDZ by diffuse functions on all atoms (aug-cc-pVDZ)67 have been used accompanied to the triple-ζ valence polarized (TZVP) and def2-TZVPP basis sets. The abbreviations of 2PY and 2HP will be used here after, respectively, for 2-pyridone and 2-hydroxypiridine. All of systems belonging to each one of our three classes (monomer, dimer, and hydrated) are of Cs symmetry in the ground state. With exception of few calculations (we will address evidently), the Cs symmetry of the systems was taken to account for geometry optimization of ground and excited states. Within the Cs point group the wave function of the ground state and S1 (1 π π ∗ ) state transform according to the A representation and thus they are more distinguishable with the 1 π σ ∗ excited state (which is of A symmetry). The numbering pattern of the carbon atoms is shown in Figure 1. The charge distribution calculations were performed based on the Natural Population Analysis algorithm (NPA)68 implemented on TURBOMOLE program.

III. RESULTS AND DISCUSSION A. Neutral and protonated 2PY/2HP

The ground-state equilibrium structure of 2PY and 2HP have been determined by several authors at various levels of theory23, 69–73 and need not to be discussed here. It was well established that both of tautomers (2HP/2PY) are planar22 (Cs symmetry). Here, we only present the results of calculations relevant to protonated species. Similar to other organic molecules, there are several protonation sites on 2HP/2PY neutral molecules corresponding to different positions of carbon, oxygen, and nitrogen atoms. As the first step of this study, looking for the most stable isomer upon to protonation of 2PY and 2HP has been performed. Obviously, in 2HP/2PY, the azine and carbonyl groups are electron-reach species, so they are more attractive to proton rather than carbon cites.

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Hence, to precisely find the relative energy of protonated isomers, the MP2 geometry optimization for all protonated isomers of 2PY and 2HP has been carried out. However, there are eight protonated isomers for 2PY and five different isomers for protonated 2HP, corresponding to different carbon sites on two molecules. The relative energy of isomers associated with protonation of 2PY and 2HP is presented in Figure S1 (supplementary material74 ). Also, Figure 1 shows the optimized geometries and numbering scheme of neutral 2PY/2HP molecules. Indeed, protonation of carbonyl group of 2PY, leads to an isomer which is at least 1.23 eV (118.7 kJ mol−1 ) more stable than other isomers (see Figure 1(c)). In addition, protonation of azine group in 2HP makes the most stable protonated isomer of this tautomer (nominally 2HPH+ ), which is 1.56 eV more stable than other isomers (≈150 kJ mol−1 ). Furthermore, the proton affinity (PA) of 2-pyridone and 2-hydroxypyridine has been calculated. The MP2/cc-pVDZ calculation, obtained 9.95 eV (960.31 kJ mol−1 ) for proton affinity of 2-pyridone which is comparable to the experimental value of 9.58 eV (924.7 kJ mol−1 ) reported by Michelson.75 Also at the same level, the proton affinity of 2HP has been calculated to 9.84 eV (949.52 kJ mol−1 ) which is reasonably greater than that of 2PY.

1. Structures and properties of protonated 2HP/2PY

Protonation of carbonyl group in 2PY or azine group in 2HP leads to one protonated structure; that is the most stable isomer for both tautomers, specifically 2PYH+ (Fig. 1(c)). The MP2 geometry optimization of 2PYH+ showed a planar structure in the ground state with the Cs symmetry, which have been confirmed to be energy-minimum points by vibrational frequency calculations that give all real frequencies. The MP2 and CC2 optimized bond parameters and calculated energies for the 2PYH+ in the ground and excited states are supplied in Table S1 in the supplementary material.74 The

MP2/cc-pVDZ optimized N1-H7, C2-O8, and O8-H9 bond lengths are about 1.025, 1.329, and 0.973 Å. The O-H bond length in the neutral and protonated 2HP is the same, while the N1-H7 in 2PYH+ is longer than the corresponding MP2 value of neutral analogue. The other bond lengths in the ring of 2PYH+ at the ground state are closer to corresponding values in neutral 2HP than those of 2PY. The S1 geometry optimization of 2PYH+ under Cs symmetry leads to slightly bond shortening of C2-O8 and N1-H7, respectively, from 1.329 Å to 1.327 Å and 1.025 to 1.022 Å, while a significant lengthening on the C-C bonds of pyridine ring has been predicted upon to S1 optimization (see Tables S1 and S2 in the supplementary material).74 The largest alterations on the bond angles upon to S1 geometry optimization are decreasing the C5-C4-C3 and C2N1-C6, respectively, from 120.5◦ and 123.9◦ to 115.8◦ and 118.8◦ .

2. Vertical and adiabatic transition energies of neutral and protonated 2HP/2PY

Vertical and adiabatic transition energies of the first A (π π ∗ ) and the first 1 A (nπ ∗ or σ π ∗ ) excited states have been calculated for neutral and protonated 2PY/2HP at the CC2 level, using several different basis sets: cc-pVDZ, augcc-pVDZ, TZVP, and def2-TZVPP. The results are presented in Table I (For more information, see Table S3 in the supplementary material).74 The vertical transition energies calculated on the optimized geometry of ground state, while the adiabatic transition energies relevant to the 1(1 A ) and 1(1 A ) excited states, were calculated at the corresponding 1(1 A ) and 1(1 A ) excited state optimized geometries, respectively. The 1(1 A ) state corresponds to the second (S2 -S0 ) electronic transition in neutral 2PY/2HP and it is the fifth electronic transition (S5 -S0 ) in protonated 2PYH+ (see Table S3 in the supplementary material).74 In order to evaluate our method and basis set, the electronic transition energies of neutral 2PY/2HP, for 1

TABLE I. Excited transition energies (vertical and adiabatic), of neutral and protonated 2PY/2HP, computed at the MP2/CC2 levels with four different basis sets. The values in parenthesis correspond to the oscillator strength. The notation of Ea deals with the theoretical values for vertical and adiabatic transition energies of 2PY/2HP have been taken from Ref. 22 (calculated at the CASPT2 method using DZVP basis set). 2PY

2PYH+

2HP

Basis set

S1 − 1(1 A ) π π ∗

S2 − 1(1 A ) nπ ∗

S1 − 1(1 A ) π π ∗

S2 − 1(1 A )nπ ∗

S1 − 1(1 A )π π ∗

S5 −1(1 A )σ π ∗

Vertical transitions

cc-pVDZ aug-cc-pVDZ TZVP def2-TZVPP Ea

4.49 (0.1045) 4.33 (0.1040) 4.44 (0.1131) 4.37 (0.1057) 4.44

4.64 (0.0001) 4.56 (0.0002) 4.63 (0.0000) 4.57 (0.0000) 5.06

5.01 (0.0595) 4.87 (0.0649) 4.92 (0.0614) 4.89 (0.0629) 4.75

5.64 (0.0036) 5.51 (0.0040) 5.62 (0.0044) 5.50 (0.0036) 5.57

4.97 (0.1400) 4.87 (0.1350) 4.91 (0.1454) 4.87 (0.1376)

7.87 (0.0020) 7.27 (0.0010) 7.85 (0.0026) 7.69 (0.0025)

Adiabatic transitions

cc-pVDZ [ZPE/eV] aug-cc-pVDZ TZVP def2-TZVPP Ea E(expt.)b

3.84 [−0.18] 3.64 3.76 3.73 3.75 3.71

4.08 4.01 4.04 3.98 4.07

4.72 [−0.18] 4.58 4.72 4.68 4.28 4.47

5.05 4.90 5.06 4.94 5.02

4.67 [−0.23] 4.57 4.67 4.64

7.05 ...a 7.07 6.93

a b

One of the calculations, for geometry optimization of S2 (1 A ) state of 2PYH+ did not converge. The experimental values for the 0-0 band of S1 -S0 transition of neutral 2PY/2HP, E (expt.), have been taken from Ref. 44.

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J. Chem. Phys. 140, 024315 (2014)

TABLE II. The highest occupied and the lowest unoccupied MOs of neutral and protonated 2PY/2HP. H and L represent to Homo and Lumo orbitals, respectively.

H-2

H-1

H

L

L+2

L+3

2PY

2HP

2PYH+

which, the jet cooled experimental data are available, have been recalculated. As shown in Table I, the best agreement between experimental44 band origin of S1 -S0 electronic transition and our calculated values for 2PY was obtained at the CC2/def2-TZVPP level of theory. Although, using the aug-ccpVDZ basis set, helps us to obtain a good result for adiabatic S1 -S0 (1 A , π π ∗ ) transition energy of 2HP, this basis function leads to obtain the worst values for S1 -S0 transition of 2PY in comparison to experiment,44 thus, the CC2/def2-TZVPP level has been selected as a more convenient basis function to perform further calculations on neutral and protonated of this system. In order to find the protonation effect on the electronic transition energies of 2HP/2PY tautomers, we performed calculations for determination of the vertical transition energies of eight singlet excitations, the results are presented in Table S3 (supplementary material).74 For neutral 2PY, there are four electronic transitions lying in the UV range (4.37–5.91 eV), among them the S1 -S0 and S4 -S0 have large oscillator strength, while in neutral 2HP, only S1 -S0 and S2 -S0 lie in UV range (4.89–5.50 eV) and other transitions are in higher energy range (E > 6 eV). Nevertheless, in protonated 2PYH+ , only the S1 -S0 electronic transition stays in the UV range (4.87 eV), and other electronic transitions are shifted to higher energy compared to neutral analogues. Furthermore, according to the RI-CC2 calculations, in both of neutral and protonated 2-pyridone, the first 1 A (1 π π ∗ ) state is dominated by the excitation from Homo (the highest occupied orbital) to Lumo (the lowest unoccupied orbital) (∼80%). Also, the first 1 A state has same nature in 2PY and 2HP (nπ ∗ ), which is a dark state, and they are mostly corresponding to the Lumo ← Homo − 1 (∼ 90%) transition (see Table II). Table II demonstrates the frontier MOs of neutral and protonated 2PY/2HP. Only the Homo − 2, Homo − 1, Homo, Lumo, and Lumo + 3 orbitals are depicted, since they are dominant in the 1(1 A ), 1(1 A ) and 2(1 A ) electronic states. The first electronic transition 1 A (S1 -S0 ) of neutral and protonated 2HP/2PY is of π π ∗ character, while the second transition (S2 -S0 ) in the neutral 2PY transforms as the A representation and that is of nπ ∗ character. However, in protonated species the S2 state represents as A . The first excited

state of A representation is S5 -S0 and it has 1 σ π ∗ nature for protonated species. Nevertheless, the 1 π σ ∗ state corresponds to S6 , 2(1 A ) state of 2PYH+ , and amounts to 7.98 eV (see Table S3 in the supplementary material).74 Although the first electronic transition of 2PYH+ , S1 -S0 , lies in the UV range (4.87 eV, 255 nm), the first electronic transition with the A representation (l σ π ∗ ) lies in the VUV (7.69 eV, 161.2 nm). The former transition corresponds almost (∼90%) to H-L (H and L indicate to Homo and Lumo orbitals, respectively). The 1(1 A , 1 σ π ∗ ) state mostly corresponds to (H − 2)-L transition (∼96%, see Table I). Also, in protonated species (2PYH+ ), the second 1 A state (1 π σ ∗ ), lies at 7.98 eV, which corresponds to the one electron transition from Homo to Lumo + 3 (a σ ∗ orbital localized on the O-H bond). In contrast to the first 1 A and 1 A states, the geometry optimizations at the CC2 level for the second 1 A state, which is of 1 π σ ∗ character, leads to proton detachment along the OH stretching bond of protonated 2-pyridone. Comparing the adiabatic S1 –S0 (π π ∗ ) electronic transition energy in 2PYH+ with the corresponding transition energy in neutral 2PY, it can be concluded that significant blue shift effect on the S1 -S0 transition is an essential consequence of protonation of 2PY. Nevertheless, the adiabatic S1 –S0 (π π ∗ ) electronic transition energy in 2PYH+ is very close to that of neutral 2HP as a parent system. The calculated value for adiabatic S1 (π π ∗ )– S0 transition of the 2PYH+ is 4.64 eV (CC2/def2-TZVPP,) while the origin band was reported to 3.71 eV for the neutral 2PY.44

B. Neutral and protonated 2-pyridone-(H2 O)n clusters with (n = 1,2)

The equilibrium structure of protonated 2-pyridoneH2 O ([2PY-W1 ,2 ]H+ ) complexes optimized at the electronic ground and excited states are shown in Figure 2. The [2PYW]H+ complex is bound by a fairly strong hydrogen bond, while there are two hydrogen bonds in [2PY-W2 ]H+ complex. We obtain an OH···O(w) bond length of 1.598 Å in monohydrated 2PYH+ ([2PY-W]H+ ), while for dihydrated complex, [2PY-W2 ]H+ , the NH···O(w) is 1.694 Å and OH···O(w) is

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FIG. 2. Optimized geometries of (a) mono- and (b) di-hydrated 2PYH+ at the ground (left panel), the first 1 A state (middle), and the first 1 A state (right panel). Only the hydrogen bond lengths (in Å) are shown.

charge distributions presented in Table S4 in the supplementary material).74 Furthermore, in the case of dihydrated ([2PY-W2 ]H+ ), the OHO(w) bond length significantly contracts, while the NH···O(w) slightly elongates after excitation to the S1 1  1 A ( π π ∗ ) state (Figure 2). Obviously, the Cs constrained geometry of dihydrated 2PYH+ does not represent the global minimum of the S0 state and without the symmetry consideration, the geometry optimization may lead to a significantly more stable structure. However, within the Cs symmetry the π π ∗ and π σ ∗ states transform clearly according to the different A ands A representations, respectively. The proton transfer from of the hydroxyl group of 2PYH+ to the water molecule and substantial elongation of NH···O(w) are two important alterations after excitation the system to the first 1 A excited state. Thus, we did not find a local minimum for this state and no value has been reported for corresponding adiabatic transition energies in Table III. In addition, the second 1 A state in mono- and di-hydrated

slightly shorter (1.634 Å) at the MP2 level. In addition, our MP2/cc-pVDZ calculations determined the OH···OH(w) hydrogen bond length of 1.872 Å in neutral monohydrated 2PY. This verifies the hydrogen bonding in protonated monohydrated 2PY is significantly stronger. So far, several papers have been devoted to hydrogen bond weakening or strengthening upon to electronic excitations.76–78 It has been verified that hydrogen bond lengths in excited state is a good sign on hydrogen bond strengthening or weakening upon to electronic excitations. The hydrogen bond lengths associated with each electronic state is presented in Figure 2. As shown, in [2PY-W]H+ as a monohydrated species, the OHO(w) bond length decreases from 1.599 Å (in the ground state) to 1.493 Å in the S1 (1 A -π π ∗ ) excited state. Also, the first 1 A state (which is of 60% nπ ∗ and 30% σ π ∗ characters), is dissociative along the OH coordinate, meanwhile, the CC2 optimization of the first 1 A state leads to proton transfer from 2PYH+ to the H2 O molecule, producing H3 O+ and neutral 2PY species (see

TABLE III. Excited transition energies (vertical and adiabatic), for neutral and protonated of mono and dihydrated 2PY/2HP, computed at the MP2/CC2 levels with three different basis sets. The values in parenthesis correspond to the oscillator strength. 2PY-W

[2PY-W1 ]H+

2PY-W2

[2PY-W2 ]H+

Basis set

S1 − 1(1 A ) ππ∗

S2 − 1(1 A ) σπ∗

S1 − 1(1 A ) ππ∗

S2 − 1(1 A ) σπ∗

S1 − 1(1 A ) ππ∗

S2 − 1(1 A ) nπ ∗ , σ π ∗

S1 − 1(1 A ) ππ∗

S2 − 1(1 A ) nπ ∗

Vertical transitions

cc-pVDZ aug-cc-pVDZ def2-TZVPP

4.49 (0.1017) 4.35 (0.1001) 4.38 (0.1021)

4.87 (0.0001) 4.79 (0.0001) 4.80 (0.0000)

4.51 (0.1047) 4.36 (0.1036) 4.39 (0.1054)

4.93 (0.0001) 4.86 (0.0001) 4.87 (0.0000)

4.90 (0.1564) 4.79 (0.1531) 4.79 (0.1548)

7.64 (0.0013) 7.07 (0.0014) 7.48 (0.0020)

4.91 (0.1417) 4.80 (0.1385) 4.81 (0.1402)

7.62 (0.0014) 7.02 (0.0012) 7.46 (0.0018)

Adiabatic transitions

cc-pVDZ [ZPE/eV] aug-cc-PVDZ def2-TZVPP E(expt.)b

3.90 [−0.22]

3.54

3.94 [−0.22]

3.64

4.56 [−0.23]

5.85

4.59 [−0.15]

5.76

3.74 3.83 3.78

3.45 3.53

3.77 3.87 3.81

3.56 3.63

4.44 4.51

a

4.46 4.53

a

a b

5.79

5.57

The asterisks represent the cases for which the CC2 cycles cease to converge. The experimental values, E(exp) , for the 0-0 band of S1-S0 transition of water complexes (neutral), were taken from Ref. 60.

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J. Chem. Phys. 140, 024315 (2014)

TABLE IV. The highest occupied and the lowest unoccupied MOs of neutral and protonated 2PY-(H2 O)n Clusters with (n = 1,2).

H-3

H-2

H

L

2PY-W

2PY-W2

[2PY-W]H+

[2PY-W2]H+

addition, one may compare the S1 -S0 electronic transition energy of individual 2PYH+ with those of micro hydrated. The adiabatic S1 -S0 transition energy of 2PYH+ has been determined at CC2/def2-TZVPP level, amounts to 4.64 eV. Comparing this value with corresponding values of mono and dihydrated 2PYH+ (4.51 eV and 4.43 eV, respectively), slightly red shift appears upon to micro-hydration of 2PYH+ .

C. Potential energy profiles and internal conversions

1. Protonated 2-pyridone (2PYH+ )

The CC2 geometry optimization of 2PYH+ at the first A ( π σ ∗ ) state leads to a bond breaking on the O-H group. Thus we have been motivated to investigate the PE profiles of 2PYH+ along this coordinate. In Figure 3, the CC2 PE 1

 1

10

8

Energy (eV)

2PYH+ has 1 π σ ∗ nature and that is also dissociative along the O-H coordinate. Considering the values of oscillator strength, in protonated and neutral 2-pyridone-(H2 O) and 2-pyridone-(H2 O)2 clusters, the S2 −S0 transition is much weaker than the S1 −S0 transition. The vertical and adiabatic excitation energies of the 1 π π ∗ and of the 1 nπ ∗ , 1 σ π ∗ states of the [2PY-W]H+ complex are given in Table III. According to RI-CC2 calculations, in both of neutral and protonated forms of mono- and dihydrated 2PY, the first electronic transition of 1 A (S1 −S0 ) corresponds (∼ 85%) to Lumo ← Homo single electron transition which has the π π ∗ nature. While, the first 1 A electronic transition has 60% nπ ∗ and 30% σ π ∗ nature. It mainly corresponds to the Lumo ← Homo − 2 and Lumo ← Homo − 3 electronic transitions (see Table IV). The optimized geometry of [2PY-W]H+ belongs to the Cs symmetry point group and its S1 −S0 transition could be known as the 1 π π ∗ . The calculated S1 −S0 adiabatic transition energies of neutral 2PY-W1 and 2PY-W2 species, at the CC2/def2-TZVPP level, are 3.83 and 3.87 eV, respectively (see Table III). The experimental origin of the S1 (0−0) band of neutral 2PY-W and 2PY-W2 in the gas phase molecular beam were reported by Held and Pratt79 to 3.78 eV (30– 488 cm−1 ) and 3.81 eV (30–730 cm−1 ), respectively. Thus, the calculated value of the present study at the CC2/def2TZVPP level of theory is in good agreement with experiment. Also, the band origin of the S1 -S0 electronic transition of 2HP-W1 and 2HP-W2 , in the jet cold experiment, have been recorded, respectively, at 4.40 eV (30–408 cm−1 ) and 4.37 eV (30–730 cm−1 ) by Nilmos et al.44 Comparing the S1 -S0 electronic transition energy of neutral mono- and di-hydrated 2PY (3.78 and 3.81 eV, respectively, for 2PY-W1 and 2PY-W2 ) with those of protonated species (4.51 eV and 4.43 eV, respectively, for [2PY-W]H+ and [2PY-W2 ]H+ ), a significant blue shift (>0.6 eV) have been occurred upon to protonation of micro-hydrated 2PY. In

6

4

2

0 1.0

1.2

1.4

1.6

1.8

2.0

2.2

OH/ [A] FIG. 3. CC2 PE profiles of protonated 2-pyridone (2PYH+ ) in the electronic ground state (circles), the lowest 1 π π ∗ excited state (squares), and the lowest 1 A (1 π σ ∗ ) state (triangles), as a function of the OH stretching coordinate.

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profiles calculated along the minimum-energy path for detachment of proton from of the OH group of 2PYH+ are presented. For clarity, only the lowest 1 π π ∗ , 1 π σ ∗ , and 1 nπ ∗ states and the electronic ground state are shown. The geometries of the excited states have been optimized along the reaction path, while the ground-state energy is computed at the 1 π σ ∗ optimized geometries. From inspection of results presented in Figure 3, one sees that the PE profiles of the ground state, the lowest valence 1 π π ∗ and 1 nπ ∗ excited states rise with increasing OH distance in an approximately parallel manner, while the PE profile of the 1 π σ ∗ state is essentially repulsive. The repulsive 1 π σ ∗ PE profile crosses with the 1 nπ ∗ at the beginning of reaction coordinate and then the 1 π π ∗ state at the longer distance of OH bond. In a multidimensional picture, the 1 nπ ∗ -1 π σ ∗ and 1 π π ∗ -1 π σ ∗ curve crossings in Figure 3 develops into the conical intersections (CIs). Because, the potential energy curves of the different electronic states (1 π π ∗ , 1 π σ ∗ , and 1 nπ ∗ ) have been determined independently at different geometry optimizations, the energetic position of these CIs cannot be precisely determined from Fig. 3. Nevertheless, the CC2 calculations are reliable,80–82 and our results relevant to the CIs are good signs for the region of conical intersections qualitatively. The resulting lower adiabatic PE sheet of the coupled 1 π π ∗ -1 π σ ∗ states will exhibit a barrier in the vicinity of the conical intersection. We have evaluated this barrier by breaking the Cs symmetry of the system; (to allow vibronic coupling between 1 π π ∗ and 1 π σ ∗ states). The barrier has been determined amount to 1.5 eV, consequently, a wave packet prepared in the 1 π π ∗ state by optical excitation with sufficient excess energy (1.5 eV) will bypass this barrier and then evolve on the 1 π σ ∗ surface. The low-energy part of the 1 π π ∗ surface is separated from the region of strong non-adiabatic interactions with the ground state by this barrier on the PE surface of the lowest excited singlet state. It is seen in Figure 3 that the repulsive 1 π σ ∗ PE profile intersects the PE function of the ground state at an OH distance of about 2.2 Å, resulting in another conical intersection. This conical intersection is expected to lead the excited system to an ultrafast internal conversion to the ground state.

2. Monohydrated protonated 2-pyridone [2PY-W]H+

In Figure 4, the CC2 PE profiles calculated along the minimum-energy path for proton transfer between 2PYH+ and water are presented. Only the lowest 1 π π ∗ and 1 π σ ∗ states and the electronic ground state are considered. Similar to the bare 2PYH+ , geometries of the excited states have been optimized along the reaction path, while the groundstate energy is computed at the 1 π σ ∗ -optimized geometries. The reaction coordinate is defined as the difference of the O-H(2PYH + ) and OH(w) bond lengths and describes the position of the proton relative to the oxygen atom of 2-pyridone and water, respectively. It is seen that the PE profiles of the ground state and the lowest 1 π π ∗ excited state slightly rise with increasing reaction coordinate, while the 1 π σ ∗ profile steeply decreases. These trend cause to obtain a conical intersection of 1 π π ∗ -1 π σ ∗ at roughly near the end of reaction

J. Chem. Phys. 140, 024315 (2014)

FIG. 4. CC2 PE profiles of the [2PY-W]H+ complex in the electronic ground state (circles), the lowest 1 π π ∗ excited state (squares), and the lowest 1 π σ ∗ state (triangles) as a function of the proton-transfer reaction coordinate.

coordinate (where, proton entirely leaves 2-pyridone and attaches to water molecule). As shown in Figure 4, the most important effect of complexation of 2PYH+ with a single water molecule is the removal of the conical intersection of the 1 π σ ∗ state with the S0 state. In comparison with Figure 3, the 1 π σ ∗ energy is moved slightly downward, nevertheless, the S0 energy increases significantly less than in bare 2PYH+ for large OH2PYH + distances. As a result, the 1 π σ ∗ state shows a minimum at about RPT ∼ 0.6 Å and the intersection with the ground state is removed. At the minimum, the proton of 2PYH+ is transferred to the water molecule. The ultrafast internal-conversion channel which exists in bare 2PYH+ when the system has reached the 1 π σ ∗ state is thus eliminated in the [2PY-W]H+ complex. In addition, our results on the PE profiles of bare 2PYH+ and its monohydrated complex are in the well agreement with the conclusions drown on the photophysics of phenol in its individual form and its clusters with water and ammonia, reported by Sobolewski and Domcke.83 Where the dissociative trend of 1 π σ ∗ PE profile and its two conical intersections with 1 π π ∗ state and ground state (S0 ) in phenol and the removal of second conical intersection (1 π σ ∗ -S0 ) in the presence of water molecule were reported for the first time.83

D. Protonated 2-pyridone dimer

Comprehensive spectroscopic study on the (2PY)2 in the jet-cold molecular beam has been performed by Held and Pratt.84, 85 Also, Muller86 and co-workers analyzed intermolecular vibrations of jet-cold (2PY)2 using two-color resonant two-photon ionization spectroscopy. According to the results of Held, Muller and Sagvolden, the ground state structure of 2-pyridone dimer is planar, belonging to the C2h symmetry point group.85–89 In order to investigate the protonation effect on the electronic properties (mainly excitation energies) of the (2PY)2,

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J. Chem. Phys. 140, 024315 (2014)

FIG. 5. Optimized geometries of (a) neutral 2-pridone dimer and (b) protonated 2-pyridone dimer at the ground and the first excited electronic states. The values in each figure represent the hydrogen-bonding bond length in Å.

ror of +0.36 eV). Definitely, when we consider the difference between zero point vibrational energy of the S2 and S0 states (ZPE = −0.20 eV), the theoretical value of 3.98 eV will be more comparable with the experimental analogue. Figure 5(b) shows the optimized structure of protonated 2-pyridone dimer ([(2PY)2 ]H+ ), at the ground and S1 excited states. The MP2 optimized structure of ([(2PY)2 ]H+ ) at ground state shows a planar stature involving two strong hydrogen connecting the protonated (2PY) to the neutral molecule and belonging to the Cs symmetry point group. The O(2PY) . . . OH(2PYH + ) and O(2PY) . . . NH (2PYH+) bond lengths between neutral and protonated monomers at the ground state are 1.614 Å and 1.766 Å, respectively. More information about geometry properties of protonated 2-pyridone dimer are presented in Table S3 (supplementary material).74 The calculated vertical and adiabatic electronic transitions of protonated 2-pyridone dimer are presented in Table V. In spite of neutral 2-pyridone dimer, the CC2 method predicts large oscillator strength for the vertical S1 -S0 electronic transition of protonated species. Inspection of molecular orbital involving in the low lying electronic transitions of protonated 2-pyridone dimer reveals that the S1 -S0 , transforms as A representation, is mostly occurred between Homo and Lumo + 1 orbitals which are of the π and π ∗ natures,

we recalculated the electronic transition energies of (2PY)2 at the CC2 method and three different basis sets. A planar structure, (C2h symmetry) involving two hydrogen bonds has been considered as the most stable configuration of (2PY)2 in ground state (see Figure 5(a)). Our calculated S1 -S0 adiabatic transition energy of (2PY)2 at the Cs symmetry is 3.86 eV (CC2/def2-TZVPP level). The S2 electronic state is of C2h symmetry, thus the adiabatic S2 -S0 electronic transition under C2h symmetry at the same level of theory obtained to 4.18 eV. In addition, the electronic spectrum of 2-pyridone dimer, in the jet cold molecular beam system, using a tunable pulsed dye laser was recorded by Held and Parat.84 The spectrum lies in the range of 30–500–31–150 cm−1 , including a sharp band at 30–776 cm−1 (3.815 eV). The sharp band of 30–776 cm−1 has been assigned to the 0-0 band of S2 -S0 electronic transition of 2-pyridone dimer,85, 86 since its S1 -S0 electronic transition symmetry is not allowed. Although our S1 -S0 adiabatic transition energy (3.86 eV) is in better agreement with the experimental origin band of 3.82 eV, the oscillator strength of this transition is too weak, meaning while that S1 state in 2-pyridone dimer is a dark state (in agreement with the suggestion of literature85 ). However, our theoretical value for the S2 -S0 adiabatic transition is still comparable with the experimental value of 3.82 (with an er-

TABLE V. Electronic excitation energies (vertical and adiabatic) of neutral and protonated 2-Pyridone dimer. Neutral

Vertical transitions

Adiabatic transitions

a

Protonated

Basis set

S1 − 1(1 A ) π π ∗

S2 (1 Bu )

S1 − 1(1 A )π π ∗

S3 − 1(1 A )σ π ∗

cc-pVDZ aug-cc-pVDZ def2-TZVPP

4.44 (0.0000) 4.27 (0.0000) 4.31 (0.0000)

4.55 (0.181) 4.40 (0.176) 4.42(0.180)

4.70 (0.180) 4.54 (0.190) 4.57 (0.190)

5.87 (0.0000) 5.73 (0.0003) 5.74 (0.0001)

cc-pVDZ [ZPE/eV] aug-cc-pVDZ def2-TZVPP E (expt.)a

3.95 3.77 3.86 ...

4.28 [−0.20] 4.07 4.18 3.82

4.22 [−0.18] 4.06 4.14 ...

4.73 4.57 4.64 ...

The experimental value for the 0-0 band of S2 -S0 transition of neutral 2-pyridone dimer was taken from Ref. 84.

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J. Chem. Phys. 140, 024315 (2014)

TABLE VI. Frontier MOs of neutral and protonated 2-pyridone dimer. (Only the MOs which are most contributed in the 1A and 1A excited states have been presented).

H-1

H

L

L+1

H-4

H

L

L+1

(2PY)2

(2PY)2H+

respectively. While, the first A excited state corresponds to the S3 -S0 electronic transition, mainly corresponds to the Lumo ← Homo − 4 electronic transition, indicating the nπ ∗ nature of this state (see Table IV). Also, the geometry optimization has been performed for the S1 excited state of protonated 2-py dimer. Upon to the S1 S0 electronic excitation, slightly lengthening in both hydrogen bond lengths has been predicted. The O(2PY) . . . .OH(2PYH + ) bond length increases from 1.614 Å to 1.692 Å and the O(PY) . . . NH(2PYH + ) bond length increases from 1.766 Å to 1.830 Å. In the B form, the S1 -S0 electronic transition is along with greater alterations in hydrogen bond length. The adiabatic S1 -S0 and S3 -S1 electronic transition energies of protonated 2-pyridone dimer have been calculated at the CC2 level, using three different basis sets. The results are also presented in Table V. The adiabatic S1 -S0 electronic transition energy of protonated 2-pyridone using different basis sets lies between 4.06 and 4.22 eV, which is significantly more than corresponding transition energy for neutral dimer (3.77–3.95 eV at the same level of theory). Thus one may conclude that protonation of 2-pyridone dimer as well as its monomer and microhydrated analogues has slightly blue shift effect on the S1 -S0 electronic transition (Table VI).

IV. CONCLUSION

Ab initio electronic-structure and reaction-path calculations have been performed to characterize the proton detachment process in protonated 2-pyridone (2PYH+ ) and intracluster proton-transfer processes in [2PY-W]H+ cluster. It has been found that the lowest 1 π σ ∗ state plays a prominent role in the photochemistry of these systems. In bare 2PYH+ , the 1 π σ ∗ state predissociates the bound S1 (π π ∗ ) state, connecting the latter to a conical intersection with the S0 state. An ultrafast internal-conversion channel thus opens above a certain excess energy in the S1 state. Clustering of 2PYH+ with water eliminates the 1 π σ ∗ -S0 conical intersection. Although, the red shift effect has been the common result of several previous studies relevant to protonation effect on the electronic transition energies of aromatic hydrocarbons (i.e., phenol,4 benzaldehyde,2 naphthalene,7 and other PAHs6, 90 ), in the 2-pyridone system, the calculations demonstrated that protonation is accompanied with a significant blue

shift effect on the S1 -S0 electronic transition energy. This effect has been verified for the S1 -S0 transition energy of dimer and micohydrated 2-pyridone as well. It was demonstrated that protonation leads to approximately 0.9, 0.7, and 0.3 eV blue shift effect on the S1 -S0 electronic transition energy, respectively, on monomer, dimer, and monohydrated 2pyridone.

ACKNOWLEDGMENTS

The research council of Isfahan University is acknowledged for financial support. We kindly appreciate the use of computing facility cluster GMPCS of the LUMAT federation (FR LUMAT2764). 1 N.

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