Excited State Charge Transfer Coupled Double Proton Transfer

Jun 30, 2010 - School of Physics and Optoelectronic Technology, College of Advanced ... Excited state charge transfer coupled excited state double pro...
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Excited State Charge Transfer Coupled Double Proton Transfer Reaction of 7-Azaindole Derivatives in Methanol: A Theoretical Study Xiaohong Zhao and Maodu Chen* School of Physics and Optoelectronic Technology, College of AdVanced Science and Technology, Dalian UniVersity of Technology, Dalian 116024, PR China ReceiVed: March 2, 2010; ReVised Manuscript ReceiVed: May 28, 2010

Excited state charge transfer coupled excited state double proton transfer (ESCT/ESDPT) reaction in methanol (MeOH) for 3-cyano-7-azaindole(3-CNAI), 5-cyano-7-azaindole(5-CNAI), and 3,5-dicyano-7-azaindole(3,5CNAI) were investigated using time-dependent density functional theory (TDDFT) method for the first time. The intermolecular hydrogen bonds of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes are demonstrated to be strengthened in the excited state and weakened in tautomer excited state, which indicates that reverse proton transfer reaction is not easy to take place. Due to the formation of intermolecular hydrogen bond, the absorption and excited state fluorescence spectra of the above three complexes are red-shifted in comparison with those of isolated molecules. The tautomer excited state fluorescence spectra that are induced by ESDPT reaction are also red-shifted relative to the excited state fluorescence for the above complexes. In addition, the sites where cyano group absorbed on 7-azaindole induces a large discrepancy of electron density distribution in excited state. Frontier molecular orbitals reflect that HOMO and LUMO orbitals of proton transfer PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes are different with HOMO and LUMO orbitals of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes, respectively. 1. Introduction The hydrogen bond (HB) as one of the important type of solute-solvent interaction has been used to investigate the properties of electronic excited states.1-6 It has been demonstrated for the first time by Zhao et al. that for the fluorenone (FN) and 4-dimethylaminobenzonitrile (DMABN) in methanol (MeOH), the deactivation of excited state via internal conversion (IC) can be facilitated by the excited-state hydrogen bonding strengthening in their benchmark works.4,7 In addition, excited state proton transfer (ESPT) reaction in which HB plays an important role has received more and more attentions because of its fundamental importance in molecular science. Among different ESPT reactions, excited state double proton transfer (ESDPT) as the most basic multiple-proton transfer reaction has been investigated by many researchers.8-15 For example, Chai et al. reconfirmed the stepwise mechanism of ESDPT in 2-aminopyridine/acid systems using time-dependent density functional theory (TDDFT) method.8 The ESDPT reaction could be used to investigate the time-resolved spectroscopy and the frequency-resolved spectroscopy, which could afford complementary excited state potential information.10,11,13 Recently, a large experimental and theoretical effort has been directed toward ESCT/ESDPT reaction with hydrogen bonding.16-20 Kiefer et al. found that intermolecular intrinsic barrier is largely determined by solvent reorganization because the charge redistribution is involved.20 Ultrafast ESCT/ESDPT reaction can be perturbed in many ways, and one of the most common causes for the perturbation arises from solvent interaction.16 Solvent plays a crucial role in many ESPT reactions.21-25 For example, in protic solvents, on one hand, the ESPT molecules with a relatively weak intramo* To whom correspondence should be addressed. Fax: 086-41184706100. E-mail: [email protected].

lecular hydrogen bond could be influenced by solvent polarity, and hence a solute/solvent intermolecular hydrogen bonded complex can be formed by this ESPT molecule and protic solvent.26-28 On the other hand, the large dipolar change between the normal and tautomer state in protic solvent is normally coupled to solvent polarization effects, which may make the relative energetics between the above two states becomes a function of solvent polarization coordinates. Thus, solventinduced barrier resulted from long-range polarization interactions may channel into the proton-transfer reaction.29,30 Due to the above two reasons, the ESCT/ESDPT reactions are most limited in aprotic solvent. So it is interested to extend the concept toward the reaction of ESCT/ESDPT in protic solvents using a theoretical approach. Herein, we investigate the reaction of ESCT/ESDPT in protic solvents using TDDFT method. To avoid the competition between the intramolecular and intermolecular hydrogen bonds, 7-azaindole (7-AI) and its derivatives are selected as ideal models. The reason is that 7-AI and its derivatives are nonintramolecular hydrogen bonded molecules in which the excited state intermolecular hydrogen bond forms only via solvent catalysis. 7-AI has been well studied as prototypes of molecules showing ESDPT process.13,17,31-34 The reaction of 7-AI in solvent was investigated by many researchers. For example, the ab initio molecular dynamics simulations methods that have been used by many researchers31,35 were used to investigated the asynchronous concerted mechanism of 7-AI-(H2O)2. Resonanceenhanced two photo ionization spectra and IR-UV ion-dip spectra of the deuterated 7-AI(MeOH)n n ) (1, 2), indicated that charge transfer delocalization interaction plays an important role on intermolecular hydrogen bonded.33 A series of 7-AI derivatives: 3-cyano-7-azaindole (3-CNAI), 5-cyano-7-azaindole (5-CNAI), and 3,5-dicyano-7-azaindole (3,5-CNAI) have been described by Hsieh et al,17 and they found that the study of

10.1021/jp101867u  2010 American Chemical Society Published on Web 06/30/2010

ESCT/ESDPT Reactions in Methanol

J. Phys. Chem. A, Vol. 114, No. 29, 2010 7787 TABLE 1: Calculated Bond Lengths L (in Å), Bond Angles A (in deg), and Dipole Moments of the Ground State and Excited State for 3-CNAI, 5-CNAI, and 3,5-CNAI Moleculesa 3-CNAI

Figure 1. Geometric conformations of 3-CNAI, 5-CNAI, and 3,5CNAI molecules.

ESCT/ESDPT reaction in protic solvent possibly come true, which may be used to investigate the proton coupled electron transfer in living systems. However, the detailed mechanism of ESCT/ESDPT in protic solvent has not been described in above studies. In addition, for 3-CNAI, 5-CNAI, and 3,5-CNAI in MeOH, the role of charge transfer on intermolecular hydrogen bond and the influence of cyano group absorbed on different sites of 7-AI are still worth being confirmed further. In the present work, we theoretically studied the excited state properties of 3-CNAI, 5-CNAI, and 3,5-CNAI as well as their intermolecular hydrogen bonded 3-CNAI-MeOH, 5-CNAIMeOH, and 3,5-CNAI-MeOH complexes for the first time. The absorption and fluorescence spectra of these complexes are theoretically presented. The unambiguous excited state geometries of all the structures are also shown. Moreover, the detailed mechanism of ESCT/ESDPT in MeOH is theoretically described.

µ LH1-N2 LC3-N4 AH1N2C3 AN2C3N4 a

5-CNAI

3,5-CNAI

PC

PC*

PC

PC*

PC

PC*

3.93 1.00 1.32 124.2 125.5

4.33 1.01 1.33 121.8 122.5

6.45 1.00 1.33 124.3 125.7

11.44 1.01 1.35 122.9 122.5

6.77 1.00 1.33 124.2 125.6

10.52 1.01 1.34 122.4 122.5

PC refers to ground state; PC* refers to excited state.

2. Theoretical Methods In this work, the ground state geometries of 3-CNAI, 5-CNAI, and 3,5-CNAI as well as the hydrogen-bonded solute-solvent complexes were optimized using density functional theory (DFT) with Becke’s three-parameter hybrid exchange function with Lee-Yang-Parr gradient-corrected correlation functional (B3LYP hybrid functional).36 The excited state and tautomer excited state for all structures were optimized using timedependent density functional theory (TDDFT) with B3LYP hybrid functional.37 Many theoretical investigations have confirmed that TDDFT method is a useful tool to investigate the property of electronically excited state.38-50 For example, Zhao et al. have used the TDDFT method to investigate the intermolecular dihydrogen bonding of the dihydrogen-bonded phenol-BTMA complex30 and concerted hydrogen bond strengthening and weakening of thiocarbonyl chromophores in alcohols38 for the first time. The triple-ξ valence quality with one set of polarization functions (TZVP) was chosen as basis set throughout.51 Fine quadrature grids 4 were also employed.52,53 Optimized all of ground state and excited state geometries, the selfconsistent field (SCF) convergency thresholds of the energy were used the default setting (10-6). All the electronic structure calculations were carried out using the TURBOMOLE program suite.36,37,51-53 3. Results and Discussion Figure 1 shows the geometries of 3-CNAI, 5-CNAI, and 3,5CNAI. As shown in Figure 1 and Table 1, for 3-CNAI, from ground state to excited state, the bond length LN2-H1 and LC3-N4 increase by 0.01 Å, whereas the excited state bond angles AH1N2C3 and AN2C3C4 decrease by 2.4 and 3°, respectively. For 5-CNAI and 3,5-CNAI, the change tendency from ground state to excited state is the same as that of 3-CNAI. The geometric conformations of intermolecular hydrogen bonded 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes as well as their proton transfer tautomer (PT-3-CNAIMeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes) are described in Figure 2. For the 3-CNAI-MeOH

Figure 2. Geometric conformations of 3-CNAI-MeOH, 5-CNAIMeOH, and 3,5-CNAI-MeOH complexes as well as their proton transfer complexes.

complex, the bond lengths LN2-H1 and LC3-N4 are slightly lengthened due to the formation of intermolecular hydrogen bond, compared with corresponding isolated 3-CNAI (see data in Table 1 and 2). From ground state to excited state, the bond lengths LN2-H1 and LC3-N4 increase by 0.02 Å, and LH5-O6 decreases by 0.01 Å for 3-CNAI-MeOH complex. The intermolecular hydrogen bonding N2-H1 · · · O6 and N4 · · · H5-O6 largely decrease from 1.97 to 1.81 Å and from 2.01 to 1.85 Å, respectively. The bond angle AN2H1O6 and AN4H5O6 increase by 2.9 and 3.0°. The change of bond angles is agreement with that of bond length. The change of bond angle and bond length indicated that the intermolecular hydrogen bonding in excited state is stronger than that in ground state. Since the proton transfer takes place via intermolecular hydrogen bond, progress of excited state proton transfer may be facilitated by the intermolecular hydrogen bond strengthening, and then the ESDPT should be initiated. In tautomer excited state, due to the proton transfer from pyrrolic nitrogen to pyridinyl nitrogen, the bond lengths of intermolecular hydrogen bond N2 · · · H1-O6 and N4-H5 · · · O6 are 2.04 and 1.98 Å, respectively. The bond angles of AN2H1O6 and AN4H5O6 are 140.5 and 140.9°. These suggest that the intermolecular hydrogen bond N2 · · · H1-O6 and N4-H5 · · · O6 at tautomer excited state for 3-CNAI-MeOH complex is weak, so the reverse reaction from tautomer excited state to excited state hardly occurs. For the 5-CNAI-MeOH and 3,5-CNAI-MeOH complexes, the varieties of bond length, bond angle, and intermolecular hydrogen bond are the same as those of 3-CNAI-MeOH. So the intermolecular hydrogen bond is strengthened in excited state and weakened in tautomer excited state. Many earlier reports have shown that the HOMO and LUMO orbitals of 7-AI are largely localized on pyrrole and pyridine

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Zhao and Chen

TABLE 2: Bond Lengths L (in Å), Bond Angles A (in deg), and Dipole Moments of the Ground State, Excited State, and Tautomer Excited State for 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH Complexesa 3-CNAI-MeOH PC µ LN2-H1 LN2 · · · H1 LC3-N4 LH1 · · · O6 LH1-O6 LN4 · · · H5 LN4-H5 LH5-O6 LH5 · · · O6 AN2H1O6 AN4H5O6

PC*

5.11 1.02

3.58 1.04

1.33 1.97

1.35 1.81

2.01

1.85

0.98

0.99

5-CNAI-MeOH PT* 3.59 2.04 1.38

PC

3,5-CNAI-MeOH

PC*

5.26 1.02

9.80 1.03

1.33 1.97

1.36 1.83

2.04

1.91

0.98

0.98

PT* 5.53 2.02 1.38

0.97

138.9 147.4

1.98 140.5 140.9

PC*

7.26 1.02

9.61 1.03

1.33 1.94

1.35 1.78

2.05

1.93

0.97

0.98

0.98

1.02 136 144.4

PC

137.6 144.1

1.98 142.6 142.3

6.78 2.10 1.38 0.97

1.02 135.9 141.9

PT*

1.02 136.5 140.6

139.2 142.2

1.90 136.1 143.4

a PC refers to ground state; PC* refers to excited state; PT* refers to tautomer excited state coming from excited state intermolecular proton transfer.

Figure 3. Frontier molecular orbitals of 7-AI, 3-CNAI, 5-CNAI, and 3,5-CNAI molecules.

moieties,54,55 which has been confirmed by our calculated results (see the frontier molecular orbitals of 7-AI in Figure 3). A cyano group is an electron withdrawing group, so the difference of electronic density at charge transfer excited state is largely caused by sites where the cyano group is absorbing on 7-AI. When the cyano group absorbs on the meta-pyrrole moiety, the electron, which is transferred to the pyridine moiety for 7-AI at the excited state, will partly transfer to the cyano group. This may make electron density distribution change little. A cyano group absorbed on meta-pyridine moiety will lead to the increasing of electron acceptability of the pyridine moiety, which causes the electron density distribution to vary largely. For 3,5CNAI, due to the two cyano groups absorbed on meta-pyrrole and meta-pyridine, respectively, the change of electron density distribution is in the range between the change of electron density distribution of 3-CNAI and 5-CNAI. Due to the above reasons, the dipole moment in the excited state is calculated in the order of 5-CNAI > 3,5-CNAI > 3-CNAI and 5-CNAIMeOH> 3,5-CNAI-MeOH> 3-CNAI-MeOH, which is consistent with the experimental results of Hsieh et al.17 In the tautomer excited state, the dipole moment is in the trend of 3,5-CNAIMeOH> 5-CNAI-MeOH> 3-CNAI-MeOH (see data in Table 2). The reason is that electron density distribution may be more influenced by proton transfer at the tautomer excited state. The change of dipole moment indicates that the ESCT takes place in the excited state. The frontier molecular orbitals (MOs) of 7-AI, 3-CNAI, 5-CNAI, and 3,5-CNAI are shown in Figure 3. All structures exhibit S0fS1 ππ* transition, and the S1 state corresponds mainly to the HOMO f LUMO transition. So Figure 3 only shows the HOMO and LUMO orbitals of all the molecules.

For the 3-CNAI, electron density spreads over molecule in frontier MOs. But for 5-CNAI and 3,5-CNAI, the electron density of the HOMO is mostly localized on the 7-AI, whereas the electron density of LUMO is delocalized over the 5-CNAI and 3,5-CNAI molecules. So the S1 state of 5-CNAI and 3,5CNAI is of intramolecular charge transfer (ICT) character. Many molecules with ICT character such as FN in alcohols1 and oxazine 750 in alcohols2 have been reported in previous studies.3 Figure 4 shows the frontier MOs of 3-CNAI-MeOH, 5-CNAIMeOH, and 3,5-CNAI-MeOH complexes as well as their proton transfer tautomer (PT-3CNAI-MeOH, PT-5CNAI-MeOH, and PT-3,5-CNAI-MeOH complexes). It is clear to see that the electron density of HOMO and LUMO orbitals for all the intermolecular hydrogen bonded complexes are localized on the 3-CNAI, 5-CNAI, and 3,5-CNAI moieties, respectively. So when photoexcitation to the S1 state occurs for all the complexes, only 3-CNAI, 5-CNAI, and 3,5-CNAI moieties are in the electronic excited state, and MeOH remains in the ground state. Therefore, S1 state of all the complexes should be assigned as a locally excited (LE) state. For the 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAIMeOH complexes, the distribution of frontier MOs on the 3-CNAI, 5-CNAI, and 3,5-CNAI moieties is similar with that of 3-CNAI, 5-CNAI, and 3,5-CNAI. As a result, both HOMO and LUMO orbitals are not significantly affected by MeOH, and the formation of intermolecular hydrogen bond does not largely influence the orbital transition. The change of electron density from HOMO to LUMO orbitals for the 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes is consistent with above analysis that electron density distribution is influenced by the location of cyano group in 7-AI at the excited

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Figure 5. Calculated absorption and fluorescence spectra of 3-CNAI, 5-CNAI, 3,5-CNAI and their intermolecular hydrogen bonded 3-CNAIMeOH, 5-CNAI-MeOH, 3,5-CNAI-MeOH complexes. The vertical lines show the absorption and fluorescence spectra in experiments.

Figure 4. Frontier molecular orbitals of intermolecular hydrogen bonded 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes as well as their proton transfer tautomer.

state. At the same time, the charge density of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes in ground state and excited state is also agreement with the discussion about electron density distribution influenced by the location of cyano group in 7-AI (see Supporting Information). For the PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAIMeOH complexes, the electron density of the HOMO is mostly localized on the pyrrole moiety, whereas the electron density of the LUMO is mostly delocalized on pyridine moiety. Therefore, electron density distribution in tautomer excited state will be strongly influenced by the double proton transfer for PT-3-CNAI-MeOH, PT-5-CNAI-MeOH, and PT-3,5-CNAIMeOH complexes. Herein, the trend of dipole moment of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes at tautomer excited state can be explained. To distinctly see the absorption and fluorescence spectra of all the molecules, Figure 5 shows the calculated absorption and fluorescence spectra for all structures. In this work, when we considered the interaction between solute and solvent, only the methanol in the first solvation shell are involved without consideration of the bulk effect of the outer solvation shells. The reason is that only the methanol in the first solvation shell plays a dominant role on the intermolecular hydrogen bonding interaction and steady absorption and fluorescence spectra, and the reason has been confirmed through comparing the theoretical results with experimental results. This conclusion is in agreement with the results in the landmark studies of Zhao et al.4,5 When Zhao et al. investigated the excited state hydrogen-bonding dynamics of FN in MeOH4 and site-specific solvation of the photoexcited protochlorophyllide a in MeOH5 for the first time, the same conclusion was also confirmed. Figure 5 shows the absorption and fluorescence spectra of 3-CNAI, 5-CNAI, 3,5-CNAI, and their intermolecular hydrogen bonded complexes. The maximum absorption of 3-CNAI, 5-CNAI, and 3,5-CNAI are located at 205, 218, and 225 nm, and the relatively weak absorption peaks are distributed at 254, 278, and 270 nm, respectively. For 3-CNAI-MeOH, 5-CNAI-

MeOH, and 3,5-CNAI-MeOH complexes, the formation of the intermolecular hydrogen bonds induces the two absorption peaks red-shifted in comparison with the absorption peaks of their isolated configurations. The shoulder absorption peaks (255, 280, and 273 nm) of the three complexes are in good agreement with corresponding experimental absorption peaks (285, 297, and 294 nm). In experiment, only spectral values after 250 nm were given.17 This confirms that methanol in the first solvation shell plays an important role on the intermolecular hydrogen bonding interaction and steady spectra. Reflected by Figure 5, the excited state fluorescence maxima of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes are at 347, 374, and 372 nm, respectively, which is red-shifted compared with those of 3-CNAI, 5-CNAI, and 3,5CNAI (321, 350, and 345 nm) due to the formation of intermolecular hydrogen bonded. At the same time, a large Stokes shifted fluorescence with peaks at 546, 568, and 571 nm results from the excited state proton transfer. The dual fluorescence behavior of 3-CNAI, 5-CNAI, and 3,5-CNAI in MeOH has been found in experiment.17 In experiment, one of the fluorescence maxima at 343, 395, and 377 nm for the 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes, respectively, is attributed to normal emission. The other one of fluorescence maxima at 480 nm and 515 nm for 3-CNAIMeOH complex and 3,5-CNAI-MeOH complex, respectively, originates from the proton transfer tautomer emission. Herein, the assignments of dual fluorescence bands in experiments can be confirmed. In addition, the perfect match of steady-state spectra between calculated and experimented results indicates that using TDDFT method to calculate the excited state of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes can well describe the excited state of 3-CNAI, 5-CNAI, and 3,5-CNAI in MeOH. 4. Conclusion In this work, we extended the ESCT/ESDPT reaction from aprotic solvent to protic solvent using the TDDFT method. The mechanism of ESCT/ESDPT for 3-CNAI, 5-CNAI, 3,5-CNAI and their intermolecular hydrogen bonded complexes in MeOH were investigated in detail for the first time. For 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5-CNAI-MeOH complexes, the excited

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state intermolecular hydrogen bonds N2-H1 · · · O6 and N4 · · · H5-O6 are strengthened, and the tautomer excited state intermolecular hydrogen bonds N2 · · · H1-O6 and N4-H5 · · · O6 are weakened. This indicates that the proton transfer from tautomer excited state to excited state does not easily occur. The site where the cyano group absorbed on 7-AI results in obviously different charge transfer at the excited state for all of the structures. So, the dipole moments at the excited state are in the order of 5-CNAI > 3,5-CNAI > 3-CNAI and 5-CNAIMeOH > 3,5-CNAI-MeOH > 3-CNAI-MeOH. This can account for ESCT reaction taking place in the excited state. In the tautomer excited state, the dipole moment is in the trend of 3,5CNAI-MeOH > 5-CNAI-MeOH > 3-CNAI-MeOH because of the double proton transfer. Through analyzing the frontier molecular orbitals, we find that 5-CNAI and 3,5-CNAI are of ICT character, whereas the S1 state of 3-CNAI-MeOH, 5-CNAIMeOH and 3,5-CNAI-MeOH complexes as well as their proton transfer tautomer belongs to the LE state. At the same time, intermolecular hydrogen bonding does not obviously influence the orbital transition in excited state. Compared with 3-CNAI, 5-CNAI, and 3,5-CNAI, the absorption and excited state fluorescence of 3-CNAI-MeOH, 5-CNAI-MeOH, and 3,5CNAI-MeOH complexes are all red-shifted due to the formation of intermolecular hydrogen bonds, whereas the tautomer excited state fluorescence of complexes largely shift to red in comparison with the excited state fluorescence due to the double proton transfer. Systematic studies of the ESCT/ESDPT reactions in protic solvent using theoretical method may be useful to investigate the nature of proton-coupled electron transfer in living systems. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 10604012 and 10974023) and SRF for ROCS, SEM (2006). Supporting Information Available: Charge density of all molecules in ground state and excited state. This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Zhao, G. J.; Liu, J. Y.; Zhou, L. C.; Han, K. L. J. Phys. Chem. B 2007, 111, 8940. (2) Zhou, L. C.; Liu, J. Y.; Zhao, G. J.; Shi, Y.; Peng, X. J.; Han, K. L. Chem. Phys. 2007, 333, 179. (3) Chen, R. K.; Zhao, G. J.; Yang, X. C.; Jiang, X.; Liu, J. F.; Tian, H. N.; Gao, Y.; Liu, X.; Han, K.; Sun, M. T.; Sun, L. C. J. Mol. Struct. 2008, 876, 102. (4) Zhao, G. J.; Han, K. L. J. Phys. Chem. A 2007, 111, 9218. (5) Zhao, G. J.; Han, K. L. Biophys. J. 2008, 94, 38. (6) Zhang, H.; Wang, S. F.; Sun, Q.; Smith, S. C. Phys. Chem. Chem. Phys. 2009, 11, 8422. (7) Zhao, G. J.; Han, K. L. J. Comput. Chem. 2008, 29, 2010. (8) Ingham, K. C.; Abu-Elgheit, M.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1971, 93, 5023. (9) Zhao, G. J.; Han, K. L. J. Phys. Chem. A 2007, 111, 2469. (10) Zhao, G. J.; Han, K. L. J. Phys. Chem. A 2009, 113, 14329. (11) Douberly, G. E.; Walters, R. S.; Cui, J.; Jordan, K. D.; Duncan, M. A. J. Phys. Chem. A 2010, 114, 4570. (12) Relph, R.; Elliot, B.; Steele, R. P.; Kamrath, M.; Guasco, T.; Johnson, M. A.; McCoy, A. B.; Ferguson, E. E.; Viggiano, A. A.; Schofield, D. P.; Jordan, K. D. Science 2010, 327, 308. (13) Liu, Y. H.; Zhao, G. J.; Li, G. Y.; Han, K. L. J. Photochem. Photobiol. A: Chem. 2010, 209, 181.

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