Semiclassical tunneling corrections for classical dynamics of unimolecular reactions in polyatomic molecules

Scope and method of study. The tunneling effects in isomerization of HONO and HSiOH have been studied by using classical trajectories with semiclassical tunneling corrections. The potential energy surfaces were constructed by using the available ab initio, spectroscopic and thermodynamic data. A constrained classical trajectory method was employed to study the roles of the various vibrational modes and molecular rotation in intramolecular energy transfer in cis trans HONO.; Findings and conclusions. The results show that the rate constants for {dollar}cisto trans{dollar} are generally larger than {dollar}transto cis{dollar} for HONO isomerizations. Excitations of the OH and N=O stretches yield the smallest rate constants while the N-O stretch excitation gives the largest rate for both {dollar}cisto trans{dollar} and {dollar}transto cis.{dollar} The rate of energy transfer from bath modes to the torsional mode is found to be a dominant factor for determining the tunneling rate. Similar behaviors are found in the isomerization of HSiOH. The Si-O stretch excitation gives the largest rate constants and the OH stretch excitation yields the smallest rate constants for both {dollar}cisto trans{dollar} and {dollar}transto cis{dollar} isomerizations. The ratio of the decay rate with tunneling correction to that without tunneling increases with decreasing total energy of the system. Furthermore, the rate difference between the {dollar}cisto trans{dollar} and {dollar}transto cis{dollar} decreases with increasing the excitation energy. The values of the effective mass along the tunneling coordinate can facilitate or hinder the tunneling probability. We find that tunneling effects plays an important role in the isomerization of HSiOH. Our results also elucidate that the ONO bending and torsional modes are important for the relaxation of the excited OH stretch. Removing the torsional motion eliminates the influence of rotation on the intramolecular vibrational energy relaxation. The effects of constraining the ONO bending plays a much smaller role than does constraining the HON bending for decreasing the magnitudes of the rate. The interaction between the out-of-plane torsional and HON bending modes is a major factor in causing cis-HONO to isomerize at significantly greater rates than that of trans-HONO.