Origin of Ultrafast Excited State Dynamics of 1-Nitropyrene - The

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Origin of Ultrafast Excited State Dynamics of 1-Nitropyrene Sushant Murudkar,† Aruna K. Mora,† Prabhat K. Singh, and Sukhendu Nath* Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

bS Supporting Information ABSTRACT: Time-resolved emission measurements in subpicosecond time domain have been carried out for 1-nitropyrene in different solvents to understand the mechanism for the observed ultrafast decay of its first excited singlet state. Excited-state dynamics of 1-nitropyrene is found to be independent of the solvent viscosity. This result contradicts the proposition in the literature (J. Phys. Chem. A 2007, 111, 552) that the ultrafast decay in 1-nitropyrene is due to the large amplitude torsional motion of the nitro group around the pyrene moiety. Excited-state dynamics of 1-nitropyrene in solvents with different dielectric constants shows that excited-state lifetime suddenly increases after a certain value of the dielectric constant. Detailed quantum chemical calculations have been carried out to understand the process that is responsible for the observed effect of the dielectric constant on the excited-state dynamics of 1-nitropyrene. It is seen that the excited-state lifetime and the singlet
triplet energy gap follow similar variation with the dielectric constant of the medium. Such a correlation between the excited-state lifetime and the singlet
triplet energy gap supports the fact that the observed ultrafast decay for 1-nitropyrene is due to an efficient intersystem crossing rather than to the torsional motion of the nitro group as proposed in the literature.

’ INTRODUCTION Nitro polycyclic aromatic hydrocarbons (NPAHs), a class of environmental pollutant, mainly originated from the incomplete combustion of fuel and from the atmospheric nitration of the polycyclic aromatic hydrocarbons (PAH).1
3 Because of mutagenic and carcinogenic properties, NPAHs possess potential risk to human health.4 Once released because of the incomplete combustion of automobiles fuel, NPAHs are highly persistent in the environment and can be transported long distances from their original sources. Human exposure through inhalation of NPAHs in the atmospheric particulate phase could lead to serious respiratory and cardiovascular health problems.5,6 Although NPAHs are less abundant in ambient air than PAHs,7,8 the former are reported to be more poisonous compared to their parent hydrocarbons.9
13 The transformation of the NPAHs in the atmosphere is still debatable, and knowledge of their fates in the environment is still of larger interest. Understanding its photochemistry is very important because of the fact that the photochemical decomposition is considered to be the only way of natural removal of these NPAHs from the environment.14,15 Further, the mutagenic and carcinogenic properties of these class of molecules are mainly due to the formation of biologically active species generated by photochemical process.16 Thus, the knowledge of the excited-state dynamics of NPAH is necessary to understand their photochemical fate in the environment. 1-Nitropyrene (1NP) is among the most abundant NPAHs detected in urban air.17
20 It has been implicated to be the directacting mutagenic of diesel exhaust particles.21
25 From the literature, it is known that fluorescence quantum yield of 1NP is extremely low.26 Morales-Cueto et al. for the first time carried out the excited-state dynamics of several NPAHs, including 1NP, r 2011 American Chemical Society

using fluorescence upconversion technique.27 It has been shown that the excited-state decay of 1NP in methanol consists of an ultrafast component with an average lifetime of 1.6 ps at emission maxima (480 nm). Such ultrafast decay of the excited state of 1NP was proposed to be due to the relaxation of the Franck
Condon state to a structurally relaxed singlet state.27 It is also proposed that such a structurally relaxed state is formed because of the rotation of the nitro group relative to the pyrene moiety in the excited singlet state.27,28 Following the report from Peon and co-workers,27 other research groups have also studied the excited-state dynamics of 1NP using other time-resolved techniques like transient absorption method.29 From these timeresolved studies, authors have again hypothesized the formation of a structurally relaxed state from the Franck
Condon state followed by fast intersystem crossing to the nearby triplet state. Finally, the hypothesis for the formation of twisted state in the first excited singlet state of 1NP is supported by the quantum chemical calculation,29 which shows that the rotation of the nitro group relative to the pyrene aromatic rings results in the stabilization of the excited singlet state. Thus, from the literature reports, it is understood that merely on the basis of the quantum chemical calculations the ultrafast decay observed for 1NP by Morales-Cueto et al.27 has been assigned to the rotation of the nitro group relative to the pyrene moiety. However, so far there is no experimental evidence for the rotational motion of the nitro group in the excited state of the 1NP. In the present work, we have carried out detailed excited Received: June 24, 2011 Revised: August 23, 2011 Published: August 23, 2011 10762

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state dynamics of 1NP in solvents with different viscosities and dielectric constants to understand the fate of the Franck
Condon state of the 1NP molecule.

’ EXPERIMENTAL SECTION 1-Nitropyrene obtained from Sigma was recrystallized twice from methanol. Further, 1NP was characterized through NMR and IR spectra. The NMR spectra of the sample recorded in CDCl3 is similar to that reported in the literature (cf. Supporting Information).2 The IR spectra of 1NP in KBr pellet and in dichloromethane solution show a singlet at 1511 cm
1 and a doublet at 1330 cm
1 because of antisymmetric and symmetric stretching of the NO2 group, respectively.30,31 All the solvents used in the present study were of spectroscopic grade and were obtained from Spectrochem India. Time-resolved fluorescence measurements were carried out using a femtosecond fluorescence upconversion instrument (FOG 100, CDP Inc. Russia) which has been described earlier.32 Briefly, a second-harmonic laser pulse (400 nm, 50 fs, 88 MHz) of a Ti-sapphire oscillator was used for the sample excitation. The residual fundamental laser beam, known as gate beam, was overlapped with the fluorescence light collected from the sample into the beta-barium borate (BBO) crystal after passing through an optical delay rail. The upconverted signal was dispersed into a double monochromator after passing through a band-pass filter to cut the fluorescence and the gate beam. The instrument response function (IRF) was independently measured through the cross correlation of the excitation and the fundamental laser pulse. The IRF was found to have a Gaussian intensity profile with a full width at half-maximum (fwhm) of 220 fs. Polarization of the excitation laser beam was set to the magic angle (54.7°) with respect to the horizontally polarized gate pulse to eliminate the effect of orientational motion of the molecule in the decay traces. Emission decay at each wavelength was collected at least twice to check the reproducibility of the measurements. The sample was taken into a rotating cell with optical path length of 0.4 mm to avoid the photodecomposition. Emission of 1NP was monitored at emission maximum for different solvents. At the monitoring wavelength of the fluorescence decays, the contribution of the scattered light (Raman scattering) from the medium was checked by placing a sample cell filled with the solvent only. The contribution of such scattering at the monitoring wavelength was negligible compared to the sample intensity. Transient emission decay for 1NP in all the solvents measured follows the nonexponential kinetics and was fitted with a multiexponential function using the standard convolute and compare nonlinear least-squares procedure.33 Viscosity of ethylene glycol-glycerol solvent mixtures were calculated using the following equation34,35 lnðηmix Þ ¼

∑i wi ln ηi

ð1Þ

where ηmix is the viscosity of the mixture, ηi is the viscosity of each component, and wi is the weighting factor of each component. Dielectric constants for mixed solvents (εMS) were calculated using the following relation36
38 εMS ¼

∑i fi εi

ð2Þ

where fi and εi represent the volume fraction and the dielectric constant of the cosolvents.

Figure 1. Emission transient decays for 1NP in ethylene glycol at different temperatures.

The ground-state geometry optimization of 1NP was performed using the density functional theory (DFT). Becke’s three-parameter hybrid exchange function with the Lee
Yang
Parr gradientcorrected correlated functional (B3LYP)39,40 was used in conjunction with 6-311++G(d,p) basis set as implemented in the GAUSSIAN 03 software package.41 The effect of bulk solvent dielectric on the ground-state geometry and on the excited-state vertical energies was modeled by performing self-consistent reaction field (SCRF) calculations using the polarizable continuum model (PCM). Time-dependent density functional theory (TDDFT) method using PBE1PBE/6-311++ G(d,p) basis set was used to calculate the energies in the excited states. The energy of the first excited singlet state (S1) was determined as the sum of the groundstate (S0) energy and the transition energy. The energy (expressed in eV) is relative to the minimum of the ground-state energy.

’ RESULTS AND DISCUSSION Time-Resolved Fluorescence Measurements. Effect of Viscosity. If the rotation of the nitro group in the

excited state is mainly responsible for the observed ultrafast decay, it is quite expected that the viscosity of the medium should have profound influence on the excited-state dynamics of 1NP. To verify the hypothesis proposed in the literature about the rotation of the nitro group,27
29 we have recorded the excitedstate decay of 1NP in ethylene glycol at different temperatures. The fluorescence transient decays for 1NP in ethylene glycol at different temperatures are shown in Figure 1. Emission from the excited 1NP molecules shows an ultrafast decay with nonexponential kinetics. All emission decays were fitted with the multiexponential function, and the average lifetime was used to represent the gross dynamical feature of the excited 1NP molecules. Astonishingly, it is observed that the excited-state dynamics does not depend on the temperature and, hence, on the solvent viscosity. The viscosity of ethylene glycol at 23° and 70 °C are 18.5 and 3.99 cP, respectively.42 Thus, the average lifetime of 1NP in ethylene glycol is found to be ∼4.3 ps that remains invariant with temperature of the solution. If the excitedstate decay of 1NP is due to the rotation of the nitro group with respect to the pyrene moiety, it is expected to be faster with the decrease in the solvent viscosity. However, the variation in the viscosity of ethylene glycol because of the change in the temperature from 23° to 70 °C is relatively small (18.5 cP at 23° to 3.99 cP at 70 °C).42 To see the viscosity effect in the wider range, we have also recorded the 10763

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Figure 2. Emission transient decays for 1NP in ethylene glycol-glycerol solvent mixtures with different compositions: 10%, 20%, 30%, 40%, and 60% glycerol.

fluorescence transient decays in ethylene glycol-glycerol mixtures with different compositions at 23 °C. The experiment with neat glycerol could not be carried out because of sample degradation. This is in accordance with the result reported in the literature that 1NP undergoes photochemical decomposition in neat glycerol solution.43 Figure 2 shows the emission transients for 1NP in ethylene glycol-glycerol mixtures. It is quite evident from Figure 2 that even after varying the viscosity to a much wider range (18.7
160 cP) the excited-state decay of 1NP remains unaffected. Thus, the results in ethylene glycol and in ethylene glycol-glycerol solvent mixtures clearly indicate that the observed ultrafast decay for 1NP in different solvents is not due to any intramolecular mode with large amplitude motion as proposed by other research groups.27
29 Effect of Solvent Dielectric Constant. To understand the processes that are responsible for the observed ultrafast decay in the excited state of 1NP, we have also carried out the fluorescence transient measurements in solvents with different dielectric constants (ε). Figure 3A shows the transient emission decays of 1NP in solvents with different dielectric constants. The excited-state decay of 1NP is seen to be very fast in a solvent with low solvent polarity, like cyclohexane (ε = 2.03). Thus, the lifetime of 1NP in cyclohexane is 230 fs. The variation in the average fluorescence lifetime of 1NP with the solvent dielectric constants is shown in Figure 3B. It is evident from Figure 3B that the excited-state lifetime of 1NP is quite short in solvents with lower ε values and almost remains unchanged up to ε = 5. However, a sharp increase in the excited-state lifetime of 1NP is observed for solvents with ε > 5, and on further increase in the dielectric constant of the solvent, it reaches a constant value. Thus, the average lifetime of 1NP in solvents with ε = 6.9 (7:3 ethyl acetate
dichloromethane mixtures) is 4.38 ps, and no appreciable change in excited-state lifetime is observed with further increase in the ε values of the solvent. The present result clearly indicates that the excited-state lifetime of 1NP is largely controlled by the polarity of the solvent rather than by its viscosity. Quantum Chemical Calculation. To understand the reason behind the variation of the excited-state lifetime with the solvent dielectric constant, detailed quantum chemical calculations for 1NP in ground and excited states were carried out. To observe the effect of the dielectric constant on the energy levels of different excited states of 1NP, calculations were carried out in solvent media having different ε values. The optimized geometry

Figure 3. (A) Emission transient decays for 1NP in different solvents: (1) Cyclohexane (CH), (2) 80:20 CH
ethyl acetate (EA), (3) 60:40 CH
EA, (4) 40:60 CH
EA, (5) 10:90 CH
EA, (6) EA, (7) 70:30 EA
dichloromethane (DCM), (8) 50:50 EA
DCM, (9) 20:80 EA
DCM, and (10) DCM. (B) Variation in the average lifetime (b) and singlet
triplet energy gap (red triangle) of 1NP with the dielectric constant of the solvents.

Figure 4. Ground-state optimized geometry of 1NP in solvents with ε = 4.

for 1NP in the ground state is shown in Figure 4 for a nonpolar solvent with ε = 4. It is to be noted from Figure 4 that the nitro group in 1NP is not in the plane of pyrene moiety. The nitro group forms a dihedral angle of 31° with the plane of the pyrene moiety. This is in good agreement with the previous results.29 The energy for different excited states (singlet and triplets) was calculated using the geometry optimized in the ground state for different solvents. Energies of the first excited singlet and different triplet states are found to be dependent on the dielectric constant of the solvent. The variation in the energy gap between the first excited singlet state, S1(ππ*), and the nearest triplet state— which is T3(nπ*) for cyclohexane, T2(nπ*) for other nonpolar solvents with ε = 3
5, and T1 (ππ*) for solvents with ε > 5, with 10764

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Scheme 1. Excited-State Relaxation Pathways for 1-Nitropyrene in Nonpolar Solvents

Figure 5. Variation in the potential energy of 1NP in nonpolar solvents (ε = 4) with the dihedral angle between the nitro group and the pyrene moiety in the ground (S0) and the excited states (S1, T2, T3). The energy is relative to the minimum of the ground-state energy. The dotted horizontal line corresponds to the thermal energy at experimental temperature (23 °C).

the dielectric constant of the medium is shown in Figure 3B. It is evident from Figure 3B that the energy gap between the S1(ππ*) state and the nearest triplet state for solvents with ε < 5 is very small while the energy gap suddenly rises to reasonably higher values (>1 eV) for solvents with ε > 5. Thus, according to the energy gap law,44 the intersystem crossing (ISC) from the singlet excited state to the nearest triplet state can occur with high efficiency for solvents with low singlet
triplet energy gap. Thus, an efficient ISC might be taking place for 1NP in solvents with ε < 5. Further, as the singlet
triplet energy gap increases to relatively higher values, it is quite expected from the energy gap law that the ISC will be relatively less efficient for solvents with ε > 5. It is also evident from Figure 3B that the singlet
triplet energy gap of 1NP remains unchanged for solvents with ε > 7. Thus, it is expected that the efficiency of the ISC process in 1NP should remain constant for solvents with ε > 7. Qualitatively, the variation in the excited-state lifetime of the 1NP molecules with the solvent dielectric constant follows the similar trend as that of the singlet
triplet energy gap. Such correlation between the excited-state lifetime and the singlet
triplet energy gap clearly indicates that the efficient ISC process is mainly responsible for the observed ultrafast decay of the excited 1NP molecules. Further, we have also calculated the potential energy surfaces for different excited states (singlet and triplet states) of 1NP as a function of dihedral angle between the nitro group and the pyrene moiety, and the results are shown in Figure 5 for solvents with ε = 4. As mentioned earlier, in the ground state, the nitro group exists in a partially twisted configuration with a dihedral angle of 31°. The Franck
Condon state of 1NP for the solvent with ε = 4 is seen to be almost isoenergetic with the T2(nπ*) state. It is to be noted from Figure 5 that the energy of the fully twisted singlet state (with 90° dihedral angle) is relatively lower in energy compared to the Franck
Condon state. However, it is also evident from Figure 5 that the twisting in the singlet state is an energetically uphill process. Further, even a small extent of twisting in the S1(ππ*) state results in crossing of the potential energy surfaces of S1(ππ*) and T3(nπ*) states. Such crossing in the potential energy surfaces can lead to an efficient ISC from

S1(ππ*) to T3(nπ*) state. Also, the Franck
Condon state is nearly isoenergetic with T2(nπ*) state and prefers to undergo an efficient ISC crossing immediately after the photoexcitation. All these results clearly indicate that the twisting in the S1(ππ*) state is not favorable for 1NP molecule. The photophysical pathways for the relaxation of the excited state of 1NP in nonpolar solvents can be described by Scheme 1. The origin of the nonexponential nature in the fluorescence decay of 1NP can also be explained from Figure 5. The dotted horizontal line in Figure 5 corresponds to the thermal energy available at experimental temperature (23 °C). It is evident from Figure 5 that at the experimental temperature, 1NP molecules can exist with different dihedral angles between the nitro group and the pyrene moiety. Thus, at 23 °C, the NO2 group might have dihedral angles in the range of 12
43°. Molecules with different dihedral angles can undergo the ISC process with different efficiencies and could be responsible for the observed nonexponential behavior in the fluorescence decay curve.

’ CONCLUSIONS Time-resolved fluorescence measurements of 1NP in solvents with different viscosities indicate that no large amplitude torsional motion in the excited singlet state is responsible for the observed ultrafast decay. This result is in contradiction with the hypothesis previously proposed by several research groups in the literature. The excited-state lifetime of 1NP remains the same for solvents with ε < 5 and suddenly increases for solvent with ε > 5. The variation in the singlet
triplet energy gap with the ε follows a similar trend as that of the excited-state lifetime. Similar variation in the excited-state lifetime and the singlet
triplet energy gap with the solvent dielectric constant clearly indicates that the intersystem crossing is mainly responsible for the observed ultrafast decay for the 1NP molecules. ’ ASSOCIATED CONTENT

bS

Supporting Information. NMR and IR spectra of 1NP. This information is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel.: 91-22-25593771. Fax: 91-2225505151. Author Contributions †

Both authors contributed equally to this work.

’ ACKNOWLEDGMENT We acknowledge Dr. T. Mukherjee, Dr. S. K. Sarkar, and Dr. H. Pal for their constant encouragement and support during this work. The authors are also thankful to Prof. A. Datta, Indian Institute of Technology, Bombay, for his help in the quantum chemical calculations. ’ REFERENCES (1) Maccrehan, W. A.; May, W. E.; Yang, S. D.; Benner, B. A. J. Anal. Chem. 1988, 60, 194. (2) Paputa-Peck, M. C.; Marano, R. S.; Schuetzle, D.; Riley, T. L.; Hampton, C. V.; Prater, T. J.; Skewes, L. M.; Jensen, T. E.; Ruehle, P. H.; Bosch, L. C.; Duncan, W. P. Anal. Chem. 1983, 55, 1946. (3) Kamens, R. M.; Zhi-Hua, F.; Yao, Y.; Chen, D.; Chen, S.; Vartiainen, M. Chemosphere 1994, 28, 1623. (4) Yu, H. J. Environ. Sci. Health, Part C 2002, 20, 149. (5) Chan, P. C. NTP technical report, U.S. Department of Health and Human Services: North Carolina, NIH publication 96-3383, 1996; p 3383. (6) Ardent Pope, C., III; Burnett, R. T., III; Thun, M. J.; Calle, E. E.; Krewski, D.; Ito, K.; Thurston, G. D. J. Am. Med. Assoc. 2002, 287, 1132. (7) Bamford, H. A.; Bezabeh, D. Z.; Shantz, M. M.; Wise, S. A.; Baker, J. E. Chemosphere 2003, 50, 575. (8) Hayakawa, K.; Tang, N.; Akutsu, K.; Murahashi, T.; Kakimoto, H.; Kizu, R.; Toriba, A. Atmos. Environ. 2002, 36, 5535. (9) Rosenkranz, H. S.; Mermelstein, R. Mutat. Res., Genet. Toxicol. 1983, 114, 217. (10) Chae, Y.-H.; Upadhyaya, P.; Ji, B.-Y.; Fu, P. P.; El-Bayoumy, K. Mutat. Res., Fundam. Mol. Mech. Mutagen. 1997, 376, 21. (11) Topinka, J.; Schwarz, L. R.; Kiefer, F.; Wiebel, F. J.; Gajdos, O.; Vidova, P.; Dobias, L.; Fried, M.; Sram, R. J.; Wolff, T. Mutat. Res., Genet. Toxicol. 1998, 419, 91. (12) Taga, R.; Tang, N.; Hattori, T.; Tamura, K.; Sakai, S.; Toriba, A.; Kizu, R.; Hayakawa, K. Mutat. Res., Genet. Toxicol. Environ. Mutagen. 2005, 581, 91. (13) Enya, T.; Susuki, H.; Watanabe, T.; Hirayama, T.; Hisamatsu, Y. Environ. Sci. Technol. 1997, 31, 2772. (14) In IPCS INCHEM Selected Nitro-and Nitro-Oxy-Polycyclic Aromatic Hydrocarbons. Environmental Health Criteria (EHC) Monographs; WHO: Geneva, 2003; No. 229. (15) Yu, H. J. Environ. Sci. Health C 2002, 20, 149. (16) Fukuhara, K.; Kurihara, M.; Miyata, N. J. Am. Chem. Soc. 2001, 123, 8662. (17) Gibson, T. L. Atmos. Environ. 1982, 16, 2037. (18) Gibson, T. L. J. Air Pollut. Control Assoc. 1986, 36, 1022. (19) Arey, J.; Zielinska, B.; Atkinson, R.; Winer, A. M. Mutat. Res. Lett. 1988, 207, 45. (20) Hayakawa, K.; Murahashi, T.; Butoh, M.; Miyazaki, M. Environ. Sci. Technol. 1995, 29, 928. (21) Hayakawa, K.; Nakamura, A.; Terai, N.; Kizu, R.; Ando, K. Chem. Pharm. Bull. 1997, 45. (22) Salmeen, I.; Durisin, A. M.; Prater, T. J.; Riley, T. Mutat. Res. Lett. 1982, 104, 17. (23) Kawanaka, Y.; Sakamoto, K.; Wang, N.; Yun, S.-J. J. Chromatogr., A 2007, 1163, 312. (24) Rosenkranz, H. S. Mutat. Res. 1982, 101, 1. (25) Tokiwa, H.; Ohnishi, Y. CRC Crit. Rev. Toxicol. 1986, 17, 23.

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