H2NO2+ IONS IN THE GAS-PHASE - A MASS-SPECTROMETRIC AND POST-SCF ABINITIO STUDY

Gaseous H2NO2+ and CH4NO2+ ions have been obtained by chemical ionization (CI), at pressures ranging from 0.2 to 0.4 Torr. Various reactions have been employed for the preparation of the H2NO2+ ions, e.g., addition of water to nitrosonium ion, NO+, and protonation of ethyl nitrite and nitroethane by AH+ Bronsted acids (A = H-2 or CH4). The CH4NO2+ ions have been obtained by addition of methanol to nitrosonium ion and by protonation of methyl nitrite by AH+ (A = CH4 or H2O). Structurally diagnostic mass spectrometric techniques, e.g., MIKE and CAD spectrometry, have been employed to probe the H2NO2+ and CH4NO2+ populations from the above reactions. Strong evidences have been obtained for the existence of two CH4NO2+ isomers, which have been assigned the (CH3-NOH)+ and (CH3OH-NO)+ structures, whereas the single isomer detectable in the H2NO2+ populations has been identified as the nitrosohydronium ion, (H2O-NO)+. The results of molecular orbital calculations at the MP4(SDTQ) parallel-to 6-311G**/MP2(FU) parallel-to 6-31G** + ZPE (MP2(FU)/6-31G**) post-SCF level of theory identify six different conformers of the H2NO2+ ion as stable species (true minima on the surface), the nitrosohydronium ion being the most stable one. At the post-SCF level of theory, the latter species is viewed as an ion-molecule complex between NO+ and H2O, rather than as a normal cation. Employing the Gaussian-1 theory, recently outlined by Pople and co-workers, a heat of formation of 160 +/- 2 kcal mol-1 has been computed for this ion, which compares fairly well with the experimental heat of formation of the H2NO2+ ion, 159 +/- 1.5 kcal mol-1.