Nonadiabatic unimolecular reactions. III. Dissociation mechanisms of methylnitrite and deuterated methylnitrite ions

At low energies, methylnitrite ions dissociate via two channels giving rise to CH3O + NO+ or to [CH3O+] + NO fragments. Peculiar characteristics have been detected in the dissociation of energy-selected parent ions, viz., remarkably low rate constants, one of which is found to remain insensitive to an increase of the internal energy, and large isotope effect. These peculiarities are accounted for by a statistical, nonadiabatic model. Ab initio calculations, confirmed by multipolar expansions reveal that the potential energy curves which correlate to these two dissociation asymptotes cross. The crossing takes place at a large value along the reaction coordinate R, indicating a long-range interaction. Production of the CH3O + NO+ fragments results from a simple bond cleavage taking place on a single diabatic surface. On the other hand, production of [CH3O+] + NO fragments is brought about by a transition from one diabatic surface to the other. It leads to deformed methoxy ions which immediately rearrange to the much more stable H2COH + structure. The nonadiabatic rate constant has been calculated by a statistical method. The contribution of each channel is weighted by a transmission coefficient which is equal to the nonadiabatic transition probability. Implementation of this statistical treatment requires partitioning the set of degrees of freedom as follows: {R, y, d, v}. It is necessary to withdraw the isomerization mode y from the statistical treatment, because the equilibrium positions along this coordinate are very different in each electronic state. Physically, this means that the reaction involves tunneling from one surface to the other along the displaced degree of freedom y. The large isotope effect has a double origin: part of it (a factor of ∼9) results from tunneling along γ; the remainder (an additional factor of 3) comes from the usual RRKM-like effect on the densities of states. The degrees d constitute a set of four low-energy bending modes which form a sink for the internal energy. The nonadiabatic transition probability is determined by the off-diagonal matrix element V12(Rc). Effective potential energy curves have been calculated by extending the Quack and Troe method to nonadiabatic reactions. It turns out that the excitation of the set d leads to a decrease of V12(Rc) and hence to a decrease of the nonadiabatic transmission coefficient. This accounts for the weak dependence of the rate constants kCH3O+ and kCD3O+ vs the energy (at least above a certain energy threshold). © 1988 American Institute of Physics.