HR: 0800h
AN: A51C-0080    [Abstracts]
TI: Master equation calculations to parameterize the pressure and temperature dependence of OH + C2H4 and OH + C2H2: consideration of pre-reaction complexes
AU: * Cleary, P A
EM: cleary.patricia@gmail.com
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Blitz, M A
EM: markb@chem.leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Hughes, K J
EM: k.j.hughes@leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Pilling, M J
EM: m.j.pilling@leeds.ac.uk
AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Wang, L
A51C-0080 AF: School of Chemistry, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: Many reactions in the atmosphere include 3-body stabilization of hot adduct. The complex pressure and temperature dependence of these reactions depend on the potential energy surface for the reaction and the efficiency of energy transfer during collisions. These reactions must be parameterized for the full extent of tropospheric conditions for use in models. Ethene and acetylene are both used as anthropogenic tracers in the upper troposphere. While many experiments can be conducted at various temperatures and pressures, a full fall-off of a pressure dependent reaction over the range of conditions found in the troposphere is not easily achieved. Master equation calculations have been conducted on the reactions of OH + C₂H₄ and OH + C₂H₂ over the temperature range of 200-400 K. The potential energy surfaces of both reactions show a shallow van der Waals complex formed before the addition of the OH to /pi-bond structure. The master equation calculations for OH + C₂H₄ show little stabilization of the van der Waals complex. Therefore, the pressure dependence of this reaction shows little influence from the van der Waals complex at low pressures, although at very high simulated pressures ( P > 760000 Torr) deviations from a flat high-pressure limit occur. However, the temperature dependence of the high pressure rate coefficient is influenced by the presence of the van der Waals well due to the interplay between two transition states: a flat transition state at the entrance channel and one of lower potential energy between the van der Waals complex and the addition complex. The temperature dependence was determined to be: k_inf= 5.01 x 10^-12 exp (148/ T). The influence from the van der Waals well on the pressure dependence is enhanced in the reaction of OH + C₂H₂, where there is a slight activation energy barrier to the addition reaction. At low temperatures (200 K) the presence of the van der Waals well influences the pressure dependence of the reaction, although slightly above 760 Torr, well above pressures found in the upper troposphere. By guiding the calculations with experiments conducted at 200-400 K, the activation energy barrier is determined to be 6.5 kJ mol^-1. The high pressure rate coefficient derived from the master equation calculations was: k_inf=2.48 x 10^-11( T/300)^-0.52exp(782/ T).
DE: 0317 Chemical kinetic and photochemical properties
DE: 0368 Troposphere: constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005