CALCULATIONS ON THE RATE OF THE ION MOLECULE REACTION C2H2(+)+H2-]C2H3(+)+H

The minimum energy pathway for the ion-molecule reaction C2H2+ + H-2 --> C2H3+ + H has been calculated via ab initio quantum chemical techniques. The reactants first form a weakly bound entrance channel complex with H-2 in a bridged position perpendicular to the C2H2 linear structure. A transition state is then encountered with an asymmetric structure in which H-2 favors one of the two carbon atoms as it begins to come apart. After proceeding through a weakly bound exit channel complex, once again of bridged structure, the system produces the product ion C2H3+ in the nonclassical (bridged) position. The reactants do not access the deep potential well of C2H4+. At our highest level of calculation, the reaction is determined to be endothermic by approximately 2 kcal mol-1, in good agreement with thermochemical values. However, phase space calculations on the reaction dynamics, including tunneling, using the calculated potential energy surface are not in good agreement with measurements of the rate coefficient at both 300 and in the vicinity of 2 K. A revised potential energy surface, in which the products and intermediate stationary points are lowered in energy by 2 kcal mol-1, making the reaction thermoneutral and reducing the barrier over the transition state, leads to better agreement with measured rate coefficients. With the reduced energies, the calculated rate coefficients shows a temperature dependence similar to that obtained previously for the reaction between NH3+ and H-2.