Transition state and dynamics of unimolecular no-barrier fragmentation: Thermal dissociation of N2O4

Thermal dissociation reaction into polyatomic molecules on the ground state no-barrier potential energy surface is studied by classical molecular dynamics simulations of N2O4⇌2NO2. A phase space surface ET=Veff(Rl)≳0 is identified as the transition state (TS), where ET is the sum of the potential and kinetic energies of interfragment motion and Veff(Rl) is the orbital angular momentum-dependent effective barrier. By dividing the motion of the system into fragments’ vibrational (V), rotational (R), and interfragment (T) modes, where the T mode is composed of translational (TT) and orbital (TL) modes, a scheme of reactive energy transfer for fragmentation is presented. The present energy condition for the TS is in accord with the one of phase space theory (PST). The observed photofragmentation rates of NCNO and CH2CO which increase with energy slower than predicted by PST suggest that intrareactant energy redistribution may influence the rate. Dissociation is found to occur by energy redistribution among T−R−V modes followed by the one among TT−TL−R modes, which determine the product vibrational and rotational distributions, respectively. This scheme supports separate statistical ensemble method in reproducing the nascent distributions from unimolecular photofragmentation at excess energies above the vibrational threshold.