Thermochemical Information from Ion-Molecule Rate Constants

In the study of ion-molecule reactions, there has long been practice of inferring exothermicity from the fact that a reaction is observed to occur, or, on occasion, endothermicity from the non-occurrence of a particular reaction. This “bracketing” technique has, for example, been used to establish relative proton affinities by ascertaining whether or not the reaction: (1)$$M{H^ + } + B \rightleftarrows B{H^ + } + M$$ occurs preferentially from left to right or from the right to the left. Since the advent of the measurement of equilibrium constants for a bimolecular ion-molecule reactions in 1973 [1], most quantitative thermochemical information about ionmolecule reactions is derived from such measurements. Nevertheless, there are situations in which equilibrium constant determinations can not be made; this happens, for example, when one of the neutral bases is a free radical, or when a fast competing process precludes the establishement of an equilibrium.