DYNAMICAL STUDY OF THE DISSOCIATION AND ELIMINATION CHANNELS IN THE DECOMPOSITION OF METHYL NITRITE

The dynamics of the two unimolecular reactions that initiate the therm al decomposition of methyl nitrite were investigated by classical traj ectories and statistical variational efficient microcanonical sampling -transition state theory. These two channels are (I) O-N bond dissocia tion to produce CH3O and NO and (II) concerted elimination through a f our-center transition state to form CH2O and HNO. In order to perform both types of calculations, a potential energy function was developed, which reproduces reasonably well the energies, geometries, and freque ncies selected from the literature. Microcanonical rate coefficients a nd branching ratios were obtained by each method at total energies ran ging from 100 to 240 kcal/mol. The computed branching ratios indicated that reaction I is markedly faster than reaction II, which agrees wit h the experimental observations. It was found that for energies up to 160 kcal/mol. the dynamics of reaction I is intrinsically Rice-Ramsper ger-Kassel-Marcus (RRKM), but for the highest energies the behavior be comes intrinsically non-RRKM. The classical trajectories showed that t he elimination process takes place via a regular dynamics during the l ast moments before reaction, which is clear evidence for nonstatistica l behavior. Analysis of the trajectory rates computed for the deuterat ed species revealed that the dissociation process exhibits an inverse secondary isotope effect. (C) 1998 American Institute of Physics. [S00 21-9606(98)02744-5].