Protonated nitric acid. Structure and relative stability of isomeric H2N03+ ions in the gas phase

Gaseous H2N03+ ions have been obtained from direct protonation of nitric acid by H3+, CH5+, and H30+ as well as from the protonation of C2HSON02fo llowed by CzH4 loss. The proton affinity of nitric acid has been estimated to be 168 f 3 kcal mol-' from the results of bracketing experiments carried out by FT-ICR and CI mass spectrometry. Structural analysis by MIKE and CID spectrometry of H2N03+ ions and of their 180-labeled isotopomers obtained from the exchange with HZ1"0 provides strong evidence for the existence of two isomers, characterized by the (HO)zNO+ and the Hz0.NO2+ structure, and for the higher stability of the latter. The results are consistent with those of independent MO SCF calculations, pointing to the higher stability of the H20.N02+ structure