Molecular dynamics of thermal dissociation in liquid N2O4

Abstract

Molecular dynamics simulations were performed for the dissociation and association (D/A) reactions N2O4■2 NO2 in the gas phase and in liquid N2O4. The trajectory was initialized from an equilibrium distribution of all variables in liquid N2O4, except the reactive mode, the NN distance of a reactant NO2 pair, was excited above the dissociation limit of the Morse-like potential between NO2 fragments, and the dynamics were calculated for 500 fs both forward and backward in time. Characteristics of the translational and vibrational energy relaxations of the reactant were studied in detail. Energy ERT, which is defined to be the sum of the potential and kinetic energies of interfragment motion, is found to play a key role in the D/A dynamics; a reactant pair is associated when ERT<0 and the pair is dissociated when ERT≳0. The transition state to the D/A reactions is hence defined by the last associated phase curve ERT=0 in the phase space. Energy transfer between intrafragment vibrational modes and the interfragment translational mode, which occurs at the inner turning point of the interfragment potential, is found to be the dominant prompter of the D/A reactions. The vibration–translation (V–T) energy transfer is found to excite the relative translational motion between fragments or gives rise to dissociation, and T–V energy transfer often causes deactivation of the relative translational motion or association in both the gas and liquid phases. In minor cases, the D/A reaction is found to occur by an energy transfer between reactant relative translational mode and solvent modes. The reaction rates are determined essentially by the rates of energy transfers among relative translational mode, intrafragment vibrational modes, and solvent modes.