Transition state theory for ion-molecule reactions

Abstract

The standard formula for the rate coefficient given by transition state theory is not valid for exothermic ion-molecule reactions having no activation energy. A formula covering these is derived. For monatomic reactants Langevin’s formula is recovered. In the general case, account must be taken of the long-lived collision complex which arises because of the transfer of orbital energy into the rotational and vibrational modes of the reactants. The theory is applied to bimolecular reactions (most of which it gives to be rapid whether or not an electronic transition is involved), to termolecular association and to radiative association. The major conflict with experiment concerns N 2 + N 2 + + He---- >N 4 + + He which Bohme, Dunkin, Fehsenfeld & Ferguson (1968) have studied at a temperature of 80 K. According to their measurements, the apparent binary rate coefficient is a linear function of the helium pressure, if this is low, but above about a torr it reaches a saturation limit of only around one hundredth of the Langevin rate coefficient, the transition from the linear to the saturation region being quite sharp. Only the initial slope is in agreement with expectation.