Semiempirical and spectroscopic study of the thermolysis of tetranitromethane

The thermolysis of tetranitromethane has been studied by the semiempirical methods AM1 and PM3 from the stable molecules to the final products NO, NO2, CO and/or CO2. In agreement with the experimental data, the rate-limiting step has been found to be the monomolecular activation of a C-N bond, which can proceed either towards dissociation or towards isomerization to nitrite. A detailed reaction mechanism is proposed and every transition state has been identified. In addition to using the usual criteria to characterize transition states, we have found it useful to calculate their normal coordinates, because the coordinate related to the imaginary vibrational frequency provides insights into which vibrational degree of freedom becomes a translational one. The activation energy and the pre-exponential factor in the rate constant have been computed and found to be in excellent agreement with experiment.