Theoretical studies of the effects of matrix composition, lattice temperature, and isotopic substitution on isomerization reactions of matrix-isolated HONO/Ar

Abstract

Theoretical molecular dynamics studies of matrix composition, lattice temperature, and isotopic substitution effects upon cis–trans isomerization rates and the vibrational relaxation rates to lattice phonon modes of matrix-isolated HONO, DONO, and H18ON18O systems are reported. The results show that isomerization is usually slower in an argon matrix than in xenon. The calculated ratios of the rates for different initial vibrational energy distributions correlate well with the ratio of the well-depth parameters for the lattice/HONO interactions. In all cases examined, the matrix-isolated isomerization rate is enhanced relative to the gas-phase rate. This behavior is attributed to a vibration → lattice phonon modes → rotation → torsional vibration) isomerization mechanism. Isomerization in both Xe and Ar matrices is nonstatistical with pronounced mode specificity present in both environments. In the gas phase, deuterium and 18O substitution produce small, positive enhancements of the isomerization rate by 13% and 26%, respectively, due to an increased kinetic coupling to the torsional modes. In the matrix, however, the isotope effects are negative and larger in magnitude. This reversal is attributed to a reduced rate of energy transfer from the lattice to rotation of DONO and H18ON18O due to the increased moment of inertia. In general, all of the present results support a matrix HONO isomerization mechanism via a (vibration→lattice phonon modes →rotation→torsional vibration) energy transfer pathway.