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J. Phys. Chem. A 2010, 114, 8016
Comment on “The Conical Intersection Dominates the Generation of Tropospheric Hydroxyl Radicals from NO2 and H2O” Mark A. Blitz* NCAS - Composition, School of Chemistry, UniVersity of Leeds, Leeds, LS2 9JT, United Kingdom ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: June 8, 2010 In the recent publication by Fang et al.,1 the mechanism of how excited nitrogen dioxide, NO2*, can produce hydroxyl radical, OH, in the presence of water vapor, H2O, was explored. It was calculated that excitation at 410 nm leads to rapid relaxation within nitrogen dioxide via conical intersections to produce highly vibrationally excited ground-state, X 2A1. This vibrationally excited nitrogen dioxide was found to have enough energy to overcome the barrier of hydrogen abstraction from H2O to produce HONO (X) + OH (2Π3/2), see Figure 1. The existence of this barrier was not discussed in relationship to the literature however and hence to if this reaction is an important source of hydroxyl radical in the atmosphere. This comment explores the implications of this barrier. In recent work by Li et al.2 a new atmospheric source of OH was proposed in which nitrogen dioxide excited by visible light (NO2*) was found to react with water vapor to generate OH
NO2 + hν (λ > 420 nm) f NO2*
(1)
NO2* + H2O f OH + HONO
(2a)
NO2* + H2O f NO2 + H2O
(2b)
Li et al.2 observed that only one collision in a thousand between H2O and NO2* gives OH (reaction 2a) with the rest occurring via deactivation, reaction 2b. The solar flux in the visible region of the spectrum is sufficiently large however that this inefficient OH channel (OH yield ) 2a/2∼0.001) is still a major source of atmospheric OH: up to 50% of that produced from O(1D) and H2O at high solar zenith angles under polluted condition.2 This conclusion was made on the basis that reaction 2a occurs with little or no barrier and is therefore still operating up to the thermodynamic limit at ∼718 nm;3 OH was observed at wavelengths g620 nm together with vibrationally excited OH.2 In contrast to this, in experiments at Leeds we failed to observe any OH4 at wavelengths used by Li et al. (563.5 and 567.5 nm) but under conditions that better approximate solar flux. Our results are in agreement with a previous experiment by Crowley and Carl5 where no OH was observed from this reaction when NO2 was excited at 532 nm. This theoretical study by Fang et al. observed a barrier for reaction 2a. At the CASSCF(13e/9o)/6-31G**//CASPT2 level of theory (the highest carried out), the calculated barrier is equal to 554 nm, see Figure 1. This barrier is consistent with the study at Leeds4 where no OH was observed at 563.5 and 567.5 nm * To whom correspondence
[email protected].
should
be
addressed.
E-mail:
Figure 1. Schematic mechanism of hydrogen abstraction for NO2* from H2O photoinitiated by 410 nm light in line with CASSCF(13e/ 9o)//CASPT2 computations taken from Fang et al.1
and when the accuracy of the calculation is taken into account encompasses the Crowley and Carl work, that observed no OH at 532 nm. But the presence of a barrier for reaction 2a would appear to be in conflict with the results of Li et al.2 where OH (υ ) 0,1) was observed at wavelengths g620 nm. At the moment, the unresolved question is the energy of the barrier for reaction 2a. It might be the case that at a higher level of calculation this barrier might be located to a few kJ mol-1. However, this information on its own is not sufficient as the importance of reaction 2 in atmospheric chemistry requires information on collision energy transfer, which includes reaction 2b, and if the internal vibrational energy of NO2 is efficient at promoting reaction, dynamics. This suggests that more experiments over a range of wavelengths are required to locate the barrier of reaction 2a and over a range of pressures in order extrapolate the efficiency of the OH channel to atmospheric pressure. This on its own represents a major experimental challenge, however, at wavelengths below 488 nm5 efficient two photon dissociation of NO2 to O(1D) occurs; Crowley and Carl observed OH formation via laser excitation of NO2 between 432-449 nm in the presence of H2O but it was demonstrated that it was predominately occurring from the reaction of O(1D) with H2O and not reaction 2a. Therefore experiments to investigate reaction 2 must be done at low excitation fluences to decouple other potential OH sources from reaction 2a. To conclude, the calculated barrier from the study by Fang et al.1 implies that the reaction between photoexcited nitrogen dioxide and water vapor as a source of atmospheric hydroxyl radical is less important than suggested by Li et al.2 However, until the barrier for this reaction is located experimentally together with its efficiency in the presence of atmospheric colliders the importance of this reaction as a source of tropospheric OH is uncertain but is expected to be no more than minor compared to the reaction of O(1D) with H2O. (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) IUPAC Subcommittee for Gas Kinetic Data Evaluation, http:// www.iupac-kinetic.ch.cam.ac.uk/, accessed 2010. (4) Carr, S.; Heard, D. E.; Blitz, M. A. Science 2009, 324, 336. (5) Crowley, J. N.; Carl, S. A. J. Phys. Chem. A 1997, 101, 4178.
JP1033118
10.1021/jp1033118 2010 American Chemical Society Published on Web 07/13/2010
