FURTHER-STUDIES OF THE METHYL NITRITE CIS-]TRANS ISOMERIZATION

Classical trajectory calculations have been performed to get more insi ghts into the dynamics of cis-->trans isomerization of methyl nitrite. Harmonic (diagonal and nondiagonal) and anharmonic potential energy s urfaces with parameter values based on the available experimental data and our own ab initio results have been used in these computations. T he ab initio calculations have been carried out at the MP2 and QCISD l evels of theory, employing the 6-311++G* basis set. Coupling of vibra tional modes is analyzed in detail. The influence of molecular rotatio n and isotopic substitution on the isomerization rate is investigated. Mode specific effects are clearly exhibited for the diagonal-harmonic potential, but they are significantly reduced by inclusion of anharmo nicity or interaction terms in the potential function. It is found tha t modes that exhibit specificity are coupled with the reaction coordin ate and with other vibrational modes that couple with the reaction coo rdinate as well. A comparison of rate coefficients computed for severa l deuterated species show an unexpected secondary isotope effect. (C) 1997 American Institute of Physics.