TUNNELING RRKM CALCULATIONS FOR THE H-2 LOSS REACTION FROM ETHANE IONS ON AN AB-INITIO POTENTIAL-ENERGY SURFACE

The potential energy surface for the loss of H-2 from ethane cations has been investigated by means of ab initio calculations. Several energy minima and transition states and the reaction path connecting them have been identified. The highest energy part of the reaction path involves the transfer of H atoms. The energy of the transition states governing the H-2 loss reaction is significantly higher than the experimental dissociation threshold. This discrepancy can be resolved by assuming that the reaction proceeds by tunneling of H atoms through a transfer barrier. Based on the ab initio potential energy surface and specific rate constant for the reaction has been calculated including tunneling of H atoms through this transfer barrier with the threshold energy for the reaction given by the thermochemical energy of the products. Calculation of k(E) predicts a large kinetic shift compatible with TPEPICO experiments. The extremely steep slope of the k(E) curve explains why the metastability of the reaction is difficult to observe in a TPEPICO experiment.