科研之友
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摘要
Triplet ketene exhibits a steplike structure in the experimentally observed dissociation rate, but its mechanism is still unclear despite many theoretical efforts. A previous surface-hopping simulation at the CASSCF level suggests that nonadiabatic transition from the S(0) to T(1) states creates the T(1) species in a highly nonstatistical manner, which raises the question of whether the use of statistical rate theory is valid in itself for the T(1) state. Here, we study this problem by performing ab initio trajectory simulation at the multireference second-order Moller-Plesset perturbation (MRMP) level of theory. Since the MRMP theory is too expensive for such a trajectory calculation, we first construct dual-level potential energy surfaces (PESs) for the S(0) and T(1) states by calibrating the PESs at the B3LYP level with a limited set of MRMP energies. We then introduce the assumption'of vibrational equilibrium on the S(0) surface and characterize the S(0)(->)T(1) crossing points using the conditional microcanonical distribution on the S(0)/T(1) seam surface. The latter distribution is obtained by running a constrained trajectory on the seam surface by use of an efficient SHAKE-like method. Subsequently, we propagate a number of T(1) trajectories from the seam surface to obtain the dissociation rate. The result shows that (i) the S(0)(->)T(1) crossing points are localized mainly in the Ti reactant region; (ii) the lifetime on the T(1) surface is about 30 Ps at the MRMP level, which is 2 orders of magnitude greater than the previous estimate obtained from the surface-hopping simulation at the CASSCF level (similar to 100 fs); and (iii) the calculated Ti dissociation rate agrees reasonably well with classical transition state theory. These results suggest that the T(1) dissociation is rather statistical, given that the T(1) trajectories are initiated from the conditional microcanonical distribution on the seam surface.
- 出版日期2011-8
