Thermal rate constants of the NO2 fission reaction of gas phase alpha-HMX:A direct ab initio dynamics study

The NO2 fission reaction of gas phase alpha-HMX has been studied using a di rect ab initio method within the framework of microcanonical variational tr ansition state theory (mu VT). The potential energy calculations were calcu lated using the hybrid nonlocal B3LYP density functional theory with the cc -pVDZ basis set. The calculated results show that the potential energy of b reaking the axial NO2 groups is lower than that of breaking the equatorial NO2 groups. No traditional transition state was found along the reaction pa th. Microcanonical rate constants calculation shows the variational transit ion state varies from 2.0 to 3.5 Angstrom of the breaking N-N bond length a s a function of the excess energy. The mu VT method was used for thermal ra te constants calculation over a temperature range from 250 to 2000 K. The f itted Arrhenius expression from the calculated data is k(T) = 1.66 x 10(15) exp(-18748K/T) s(-1), which is in good agreement with the experimental dat a at low temperatures.