PROGRESS TOWARD AN ABSOLUTE GAS-PHASE PROTON AFFINITY SCALE

The temperature dependence of the proton transfer equilibrium constants for approximately 80 pairs of bases ranging in proton affinity from N2 to tert-butylamine has been examined. These data provide the basis for formulation of a revised gas-phase proton affinity scale which now appears to have a firm basis. Excellent agreement with appearance energy determinations of proton affinities as well as ab initio calculated values is obtained. An important finding of this work is that the value of DELTAH(f)degrees for the tert-butyl cation must be significantly higher than that derived from appearance energy measurements by Traeger which had formed the basis for the proton affinity assignment for isobutene, an important reference point in the proton affinity scale. The data obtained here would suggest that the proton affinity of isobutene must be revised downward by approximately 4 kcal mol-1 with important consequence for all proton affinities in the vicinity of isobutene and above. In addition significant revisions are indicated for proton affinities between those of propene and isobutene. In contrast, however, the substantial upward revision of the proton affinity scale in the basicity region above ammonia which had been proposed by Mautner and Sieck (J. Am. Chem. Soc. 1991, 113, 4448) is not supported by the present experiments.