DIRECT DYNAMICS STUDY OF THE DISSOCIATION AND ELIMINATION CHANNELS INTHE THERMAL-DECOMPOSITION OF METHYL NITRITE

The dynamics of the two unimolecular reactions that initiate the therm al decomposition of methyl nitrite were investigated by direct dynamic s calculations. The two decomposition pathways are (I) O-N bond scissi on to form CH3O and NO and (Il) concerted elimination through a four-c enter transition state to produce CH2O and HNO. Structural data along the reaction paths were obtained from high-level ab initio methods. Sp ecifically, the elimination reaction path was achieved from MP2 result s scaled so that the height of the barrier coincided with the value gi ven by QCISD(T)//QCISD calculations. The dissociation path was first c alculated at the CASSCF(8,8) level of theory and then scaled to reprod uce the dissociation energy predicted by QCISD(T)//CASSCF(8,8) computa tions. All the ab initio calculations were performed with the standard 6-311++G(d,p) basis set. Thermal rate constants were evaluated by can onical variational transition-state theory(CVT). For the elimination p rocess, tunneling was taken into account by using the approximations z ero curvature tunneling (ZCT) and small curvature tunneling (SCT). The overall agreement between the calculated rate constants and the exper imental ones reported in the literature is reasonably good. The calcul ations indicate that the dissociation is remarkably faster than the el imination not only because the barrier height for the O-N bond scissio n is lower than that for the elimination reaction but also because the former process is entropically favored.