Theoretical investigation of nonstatistical dynamics, energy transfer, and intramolecular vibrational relaxation in isomerization reactions of matrix-isolated HONO/Xe

Abstract

Theoretical molecular dynamics studies of cis–trans isomerization, intramolecular vibration relaxation (IVR), and vibrational relaxation rates to lattice phonon modes of HONO isolated in a face-centered cubic (fcc) xenon matrix at 12 K are reported. The effect of the matrix environment upon the dynamics is obtained by comparison with the corresponding gas-phase results. Questions related to statistical vs nonstatistical behavior and the effect of lattice imperfections are also addressed. It is found that both cis→trans and trans→cis isomerization rates are significantly enhanced by the presence of the matrix in spite of the steric effects produced by the environment. It is shown that this result occurs because the matrix opens a (vibration→lattice phonon modes→rotation→torsional vibration) energy transfer path. The calculated isomerization rate coefficients indicate significant nonstatistical dynamics. The IVR rates in the matrix and in the gas phase are slow relative to the isomerization rates. Consequently, the isomerization cannot be statistical. The calculated cis→trans and trans→cis ratio is found to be significantly less than previously reported measurements indicate. Vibrational relaxation rates to the lattice phonon modes are found to be almost independent of the initial energy partitioning. It is suggested that this may be a result of the transfer rates approaching their limiting values determined by the Debye frequency of the lattice. The presence of lattice vacancies is found to exert a profound influence upon the dynamics. When the percentage of lattice vacancies approaches 20%, the calculated dynamics in the matrix are found to approach the gas-phase results.