Dynamics simulations of the dissociation of vibrationally excited HOSO2 to OH and SO2: IVR, dissociation rates, and the OH + SO2 high pressure limit

PHYS 607

David R Glowacki, chmdrgl@leeds.ac.uk1, Michael J. Pilling, mikep@chem.leeds.ac.uk1, Dmitrii Shalashilin1, and Emilio Martinez Nunez2. (1) School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, United Kingdom, (2) Departamento de Quimica Fisica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain
Many association reactions of relevance to atmospheric chemistry occur in their high pressure limit, k∞; however, laboratory determinations of these reactions are often in the pressure dependent fall-off region. OH + SO2 → HOSO2 is one such reaction: it initiates tropospheric oxidation of SO2 and leads to formation of H2SO4, which plays a significant role in acid rain formation, visibility reduction, and climate modification. k∞ of OH + SO2 has been experimentally determined by examining OH(v=1) + SO2. So long as IVR is fast within the nascent HOSO2, then the rate of loss of OH(v=1) corresponds to the OH + SO2 k∞.

In this work, we constructed an analytical PES based on G3X and DFT electronic structure calculations to describe HOSO2 dissociation. We performed classical dynamics calculations in which the O-H mode was vibrationally excited in the nascent HOSO2 complex. For 1-10 quanta of initial O-H mode excitation (OH(ν*)), we used the trajectory simulations to investigate (1) the rate of IVR from the O-H stretch mode to the other HOSO2 bath modes, (2) the HOSO2 dissociation rates, and (3) the OH rovibrational energy distributions upon HOSO2 dissociation. The trajectory results were compared with the corresponding OH statistical product energy distributions and RRKM microcanonical rates. Significant non-statistical effects in the HOSO2 dissociation rate and the OH product energy distributions were observed with increasing energy. This nonstatistical behaviour derives from the fact that energy decay out of the initially excited O-H stretch into HOSO2 bath modes is slow, such that the ergodicity assumption in this system is not applicable; however, IVR always appears fast enough for the OH(ν*) + SO2 to give good estimates of k∞. Using the same PES, we have undertaken comparisons of the IVR rates calculated via classical trajectories with those obtained using coupled coherent states quantum dynamics simulations.

 

PHYS Poster Session - Computational Spectroscopy and Reaction Dynamics
7:30 PM-10:00 PM, Wednesday, April 9, 2008 Morial Convention Center -- Hall A, Poster

Division of Physical Chemistry

The 235th ACS National Meeting, New Orleans, LA, April 6-10, 2008