CONCERNING THE PROTON AFFINITY OF HYDRAZOIC ACID AND METHYL NITRATE

The proton affinity (PA) of hydrazoic acid has been reevaluated with experimental and ab initio methods in order to resolve the disturbing discrepancy between experiment and theory introduced by a recent computational study performed at the CISD (fc)/6-311G(df,p)//MP2(full)/6-+3l G* AZPVE (MP2(fu11)/6-31G*) level of theory, giving a value, 186.4 kcal mol-' at 0 K, corresponding to ca 187.9 kcal mol-' at 298 K, much higher than our earlier experimental value of 176.6 kcal mol-' at 298 K. Additional experimental measurements have been performed by ICR mass spectrometry, using the equilibrium and the bracketing methods and reference bases including propene, methanol, and benzene, whose PA is firmly established. From the results of these experiments, the PA of HN3 is estimated to be 180 f 2 kcal mol-' at 298 K, a value independently supported by additional mass spectrometric evidence obtained with the kinetic method based on the metastable decomposition of the [MeOH-H-NaHI+ proton-bound dimer. The results of an ab initio study of the HNs molecule and the H2N-N2+ ion performed with the Gaussian-1 method give a PA of 177.9 kcal mol-' at 0 K, corresponding to ca. 179.4 kcal mol-' at 298 K, in excellent agreement with the experimental measurements. The overall conclusion is that all presently available evidence from high-level ab initio methods, as well as from different experimental techniques, suggests that the PA of HN3 calculated at the CISD (fc)/6-311G(df,p)/MP2 (full)/6-31G* + AZPVE (MP2 (full)/6-31G*) was overestimated.