The master equation (ME) provides a powerful technique for modeling reactions that involve at least one potential energy well. It can be widely applied to reactions with several connected energy wells and multiple product channels. The application of the technique is reviewed by reference to the H + SO(2) reaction, where phenomenological rate constants for use, for example, in a combustion model can be extracted through an analysis of the eigenvalues and eigenvectors of the collision matrix, M, that describes formation of the adducts HSO(2) and HOSO from the source H + SO(2), collisional energy transfer in the adduct wells and reaction via the product channel (sink) OH + SO. The approach is extended to systems with more than one sink and it is demonstrated that macroscopic (phenomenological) rate coefficients derived from a ME obey detailed balance if the original ME is appropriately constructed. The method has been applied to the 1-, 2-pentyl radical system, that includes isomerisation and dissociation via two channels to form C(3)H(6) + C(2)H(5) and C(2)H(4) + C(3)H(7). The calculations clearly demonstrate the importance of indirect dissociation channels, in which an isomer can dissociate to form the product set to which it is not directly connected, e.g. formation of C(3)H(6) + C(2)H(5) from 1-pentyl, via the energized states of 2-pentyl. As in previous studies of pentyl dissociation, there is a convergence of the chemically significant eigenvalues and the internal energy relaxation eigenvalues above approximately 1000 K; the consequences of this convergence are discussed.
Calculations of thermal rates for the reactions of the isomeric pentyl radicals involving (1,2), (1,3), (1,4), and
(1,5) intramolecular H-atom transfer, C−C bond scission, and H-atom elimination have been carried out.
Potential energy surfaces and associated properties for these reactions have been used for direct dynamics
studies within conventional and variational transition state theory formalism including nonclassical effects,
using the dual-level technique (PUMP-SAC2/6-311G**///AM1). We found that for C−C scission, the barrier
is broad, and a significant tightening of the loose transition state reduces the rate coefficients across a wide
temperature range. Converse behavior is predicted for the isomerization reactions where the optimal combination
of a low effective mass with a narrow barrier opens the best tunneling paths. High-pressure limiting rate
coefficients and kinetic parameters obtained in this study show good agreement with experimental measurements
and previous theoretical work.
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