Energy and temperature dependent dissociation of the Na(+)(benzene)1,2 clusters: importance of anharmonicity.

Abstract

Chemical dynamics simulations were performed to study the unimolecular dissociation of randomly excited Na(+)(Bz) and Na(+)(Bz)2 clusters; Bz = benzene. The simulations were performed at constant energy, and temperatures in the range of 1200-2200 K relevant to combustion, using an analytic potential energy surface (PES) derived in part from MP2/6-311+G* calculations. The clusters decompose with exponential probabilities, consistent with RRKM unimolecular rate theory. Analyses show that intramolecular vibrational energy redistribution is sufficiently rapid within the clusters that their unimolecular dynamics is intrinsically RRKM. Arrhenius parameters, determined from the simulations of the clusters, are unusual in that Ea is ∼10 kcal/mol lower the Na(+)(Bz) → Na(+) + Bz dissociation energy and the A-factor is approximately two orders-of-magnitude too small. Analyses indicate that temperature dependent anharmonicity is important for the Na(+)(Bz) cluster's unimolecular rate constants k(T). This is consistent with the temperature dependent anharmonicity found for the Na(+)(Bz) cluster from a Monte Carlo calculation based on the analytic PES used for the simulations. Apparently temperature dependent anharmonicity is quite important for unimolecular dissociation of the Na(+)(Bz)1,2 clusters.