MOLECULAR-DYNAMICS OF THERMAL-DISSOCIATION IN LIQUID N2O4

Molecular dynamics simulations were performed for the dissociation and association (D/A) reactions N2O4 reversible arrow 2 NO2 in the gas ph ase and in liquid N2O4. The trajectory was initialized from an equilib rium distribution of all variables in liquid Nz04, except the reactive mode, the NN distance of a reactant NO2 pair, was excited above the d issociation Limit of the Morse-like potential between NO2 fragments, a nd the dynamics were calculated for 500 fs both forward and backward i n time. Characteristics of the translational and vibrational energy re laxations of the reactant were studied in detail. Energy E(RT), which is defined to he the sum of the potential and kinetic energies of inte rfragment motion, is found to play a key role in the D/A dynamics; a r eactant pair is associated when E(RT)<0 and the pair is dissociated wh en E(RT)>0. The transition state to the D/A reactions is hence defined by the last associated phase curve E(RT)=0 in the phase space. Energy transfer between intrafragment vibrational modes and the interfragmen t translational mode, which occurs at the inner turning point of the i nterfragment 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 deactivati on 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 an d solvent modes. The reaction rates are determined essentially by the rates of energy transfers among relative translational mode, intrafrag ment vibrational modes, and solvent modes.