Chemical Dynamics Simulations of Benzene Dimer Dissociation

Classical chemical dynamics simulations were performed to study the intramolecular and unimolecular dissociation dynamics of the benzene dimer, Bz₂ → 2 Bz. The dissociation of microcanonical ensembles of Bz₂ vibrational states, at energies E corresponding to temperatures T of 700-1500 K, were simulated. For the large Bz₂ energies and large number of Bz₂ vibrational degrees of freedom, s, the classical microcanonical (RRKM) and canonical (TST) rate constant expressions become identical. The dissociation rate constant for each T is determined from the initial rate dN(t)/dt of Bz₂ dissociation, and the k(T) are well-represented by the Arrhenius eq k(T) = A exp(-E_a/RT). The E_a of 2.02 kcal/mol agrees well with the Bz₂ dissociation energy of 2.32 kcal/mol, and the A-factor of 2.43 × 10¹² s^-1 is of the expected order-of-magnitude. The form of N(t) is nonexponential, resulting from weak coupling between the Bz₂ intramolecular and intermolecular modes. With this weak coupling, large Bz₂ vibrational excitation, and low Bz₂ dissociation energy, most of the trajectories dissociate directly. Simulations, with only the Bz₂ intramolecular modes excited at 1000 K, were also performed to study intramolecular vibrational energy redistribution (IVR) between the intramolecular and intermolecular modes. Because of restricted IVR, the initial dissociation is quite slow, but N(t) ultimately becomes exponential, suggesting an IVR time of 20.7 ps.