Unimolecular reactions of N2O and CO2 at high pressure

Abstract

An apriori calculation in the framework of the theory of radiationless transitions is presented for the non-adiabatic spin-forbidden reactions of N2O and CO2. The calculation is two-dimensional, incorporating the two stretching degrees of freedom, and the microscopic reaction process is formulated using the relative coordinate system of Rosen as modified by Beswick and Jortner. All microscopic parameters required for the calculation are derived from spectroscopic data; no adjustable parameter is included. For each reaction, a purely theoretical reaction probability function k(E) is synthesised and used to derive a theoretical Arrhenius expression for the infinite-pressure reaction rate. The calculated high-pressure unimolecular dissociation rates for both reactions are in reasonable agreement with experiment. High-pressure recombination rate coefficients are also presented for CO2; they exhibit positive activation energies which increase from 0.5 kcal/mol at room temperature to 8 kcal/mol at 5000 K.