CONCERNING THE PROTON AFFINITY OF HYDRAZOIC ACID AND METHYL NITRATE

The proton affinity (PA) of hydrazoic acid has been reevaluated with e xperimental and ab initio methods in order to resolve the disturbing d iscrepancy between experiment and theory introduced by a recent comput ational study performed at the CISD (fc)/6-311G(df,p)//MP2(full)/6-31G + DELTAZPVE (MP2(full)/6-31G*) level of theory, giving a value, 186. 4 kcal mol-1 at 0 K, corresponding to ca 187.9 kcal mol-1 at 298 K, mu ch higher than our earlier experimental value of 176.6 kcal mol-I at 2 98 K. Additional experimental measurements have been performed by ICR mass spectrometry, using the equilibrium and the bracketing methods an d reference bases including propene, methanol, and benzene, whose PA i s firmly established. From the results of these experiments, the PA of HN3 is estimated to be 180 +/- 2 kcal mol-1 at 298 K, a value indepen dently supported by additional mass spectrometric evidence obtained wi th the kinetic method based on the metastable decomposition of the [Me OH-H-N3H]+ proton-bound dimer. The results of an ab initio study of th e HN3 molecule and the H2N-N2+ ion performed with the Gaussian-1 metho d give a PA of 177.9 kcal mol-1 at 0 K, corresponding to ca. 179.4 kca l mol-1 at 298 K, in excellent agreement with the experimental measure ments. The overall conclusion is that all presently available evidence from high-level ab initio methods, as well as from different experime ntal techniques, suggests that the PA of HN3 calculated at the CISD (f c)/6-311G(df,p)/MP2 (full)/6-31G + DELTAZPVE (MP2 (full)/6-31G*) was overestimated.