PROGRESS TOWARD AN ABSOLUTE GAS-PHASE PROTON AFFINITY SCALE

The temperature dependence of the proton transfer equilibrium constant s 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 ene rgy determinations of proton affinities as well as ab initio calculate d values is obtained. An important finding of this work is that the va lue of DELTAH(f)degrees for the tert-butyl cation must be significantl y higher than that derived from appearance energy measurements by Trae ger 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 isobu tene must be revised downward by approximately 4 kcal mol-1 with impor tant consequence for all proton affinities in the vicinity of isobuten e and above. In addition significant revisions are indicated for proto n affinities between those of propene and isobutene. In contrast, howe ver, the substantial upward revision of the proton affinity scale in t he basicity region above ammonia which had been proposed by Mautner an d Sieck (J. Am. Chem. Soc. 1991, 113, 4448) is not supported by the pr esent experiments.