J. Phys. Chem. A 2010, 114, 8017
Reply to “Comment on ‘The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radical from NO2 and H2O’” Xuebo Chen* and Weihai Fang* Department of Chemistry, Beijing Normal UniVersity Xin-wai-da-jie No. 19, Beijing 100875, People’s Republic of China ReceiVed: May 28, 2010; ReVised Manuscript ReceiVed: June 28, 2010 We recently reported a quantitative understanding on how to generate hydroxyl radicals from NO2 and H2O in the troposphere upon photoexcitation at 410 nm by using multiconfigurational perturbation theory.1 It was found that the conical intersections dominate the nonadiabatic relaxation processes after NO2 irradiated at ∼410 nm and further control the generation of OH radical by means of hydrogen abstraction. To respond to Blitz’s comment where Blitz tried to explore the implication of our calculated barrier of hydrogen abstraction in the ground state, we would like to make extended discussions on the basis of our computations.1 The barrier of hydrogen abstraction for NO2 from H2O in the ground state was carefully determined to be 51.6 kcal/mol at the CASSCF(13e/9o)/6-31G**/CASPT2 level of theory. As discussed in our publication,1 both the energetic difference between reactant of NO2-H2O (X˜2A1) and product of HONO (X˜) + OH (2Π3/2) and singly occupied electron redistribution caused by O-H bond fission are responsible for the high barrier of hydrogen abstraction in the ground state. Such high barrier definitely stops the wave packet propagation of vibrationally excited NO2 by photoexcitation at longer than 554 nm toward the direction of generation of OH radicals. As discussed in Blitz’s comment, our calculations are in agreement with experimental findings at 563.5, 567.5, and 532 nm,3,5 but do not support the experimental observation that OH was produced at the wavelengths g620 nm.2,4 * To whom correspondence should be addressed. E-mail: (X.C.)
[email protected]; (W.F.)
[email protected].
8017
Although Crowley and Carl proposed that the OH formation via excitation of NO2 at 432-449 nm was originated from efficient two photon dissociation of NO2 producing O(1D),5 the multiphoton mechanism for OH production was ruled out by Sinha et al. at long wavelengths >560 nm.4 It should be noted that vibrationally excited NO2 photoinitiated by 432-449 nm light could overcome the barrier of hydrogen abstraction from water to generate the OH radicals. Both the mechanism that is proposed by us and the two photon dissociation of NO2 to O(1D) can account for the observed OH at 432-449 nm in Crowley and Carl’s laboratory. The fission of NdO double bond is required when dissociation of NO2 to O(1D) occurs. Qualitatively, this reaction is much more difficult than that of hydrogen abstraction in which the O-H single bond cleavage followed by the formation of another O-H single bond. The OH radicals have already been generated by photoexcitation of NO2 in the presence of water. Although some excited NO2 molecules undergo one more excitation to get enough energy to trigger dissociation of NO2 to O(1D), the formation of OH radicals via hydrogen abstraction initiated by photoexcitation significantly decrease the probability of the reaction of dissociation of NO2 to O(1D). Anyway, the compelling evidence from both theory and experiment are required to evaluate the efficiency of above tworeactions.Theefficiencyoftwophotoexcitation(Franck-Condon factor) and the barrier of NdO bond fission upon two beam light irradiation will be calculated in our group. Moreover, the dynamic factor will be considered to determine how the efficiency of two reactions associated with wavelengths of excitation (one photo vs two photo processes) is different. It is dangerous to draw a final conclusion on the basis of present theoretical and experimental findings.1-5 References and Notes (1) Fang, Q.; Han, J.; Jiang, J.; Chen, X.; Fang, W. J. Phys. Chem. A 2010, 114, 4601. (2) Li, S.; Matthews, J.; Sinha, A. Science 2008, 319, 1657. (3) Carr, S.; Heard, D. E.; Blitz, M. A. Science 2009, 324, 336. (4) Li, S.; Matthews, J.; Sinha, A. Science 2009, 324, 336. (5) Crowley, J. N.; Carl, S. A. J. Phys. Chem. A 1997, 101, 4178.
JP1048937
10.1021/jp1048937 2010 American Chemical Society Published on Web 07/13/2010
