Combined experimental and master equation investigation of the multiwell reaction H + SO2.
The temperature and pressure dependence of the rate coefficient for the reaction H + SO2 has been measured using a laser flash photolysis/laser-induced fluorescence technique, for 295 <or= T/K <or= 423 and for 3 <or= [He]/10(18) molecules cm(-3) <or= 23. Under these conditions, the reaction occurs exclusively to form HSO2. These data have been fitted to, and used to refine, a master equation (ME) model of the H + SO2 reaction system that includes the isomers HSO2 and HOSO and the high-temperature products OH + SO. The potential energy for the system is based on literature ab initio calculations. The three eigenvalues of lowest magnitude, obtained from solution of the ME, are the chemically significant eigenvalues that are related to the pressure and temperature dependent rate coefficients for the chemical system. The eigenvalues are generally well separated from each other and from the eigenvalues relating to collisional relaxation, except at high temperatures. The rate coefficients were extracted from the eigenpairs of the ME solution, using an analysis developed by Klippenstein and Miller (J. Phys. Chem. A 2002, 106, 9267) and an adaptation of an earlier analysis by Bartis and Widom (J. Chem. Phys. 1974, 60, 3474). The relationship between the temperature dependence of the chemically significant eigenvalues and combinations of rate coefficients was elucidated via an approximate analysis of the chemical rate equations and it was demonstrated that each eigenvalue relates primarily to passage over a single transition state. Some problems were encountered in equating ratios of rate coefficients for forward and reverse reactions to the equilibrium constant. This problem arises from the nonconservative nature of the reaction system; it was resolved by imposing orthogonality on the coupled rate equations of the chemical system, giving good agreement between rate coefficient ratios and equilibrium constants. The temperature and pressure dependences of the rate coefficients were parametrized according to Troe and modified Troe expressions. The key processes, from a combustion perspective, are the formation and dissociation of HOSO and the formation of OH + SO. Except at pressures >10(3) atm, the latter proceeds directly from H + SO2, via the energized states of HOSO. The derived rate coefficients rely heavily on measurements of the reverse reaction, OH + SO, which has only been determined at temperatures up to 700 K.
Blitz
Mark A
MA
School of Chemistry, University of Leeds, Leeds, LS2 9JT, U.K.
Hughes
Kevin J
KJ
Pilling
Michael J
MJ
Robertson
Struan H
SH