GAS-PHASE ION CHEMISTRY OF NITRAMIDE - A MASS-SPECTROMETRIC AND AB-INITIO STUDY OF H2N-NO2 AND THE H2N-NO2RADICAL+, [H2N-NO2]H+, AND [HN-NO2]- IONS

The gas-phase ion chemistry of H2N-NO2 (1) has been studied with mass spectrometric and ab initio theoretical methods. The H2N2O2.+ molecular ion is formed upon electron-impact ionization of 1, whose photoelectron (PE) spectra have been recorded. From the onset of the PE bands the ionization energy of 1 has been estimated to be 11.02 +/- 0.06 eV. Reaction of 1 with gaseous Bronsted acid such as H-3+, CH5+, H3O+, and i-C3H7+, etc., gives protonated adducts[H2N-NO2]H+. Evaluation of prototropic equilibria involving C6H6 and CH3OH as the reference bases gives a gas-phase basicity and a proton affinity (PA) of nitramide of 174.4 +/- 2 and 182.1 +/- 2 kcal mol-I at 300 K, respectively. Comparison of the metastable ion kinetic energy and collisionally activated dissociation spectra of the [H2N-NO2]H+ populations from the protonation of 1 with those of H3N-NO2+ model ions suggests that O-protonated protomers are more stable and represent the predominant [H2N-NO2]H+ species at the time of structural analysis, i.e., ca. 10 mus after the protonation of event. These conclusions are consistent with ab initio results at the GI level of theory that identify the O atoms of the nitro group as the thermodynamically preferred protonation site of 1 and give a value of the PA of nitramide, 180.2 kcal mol-1 at 298 K, in satisfactory agreement with the experimental value. In addition, on the basis of the G 1 value of the H3N-NO2+ binding energy, one can estimate the experimentally unknown heat of formation of 1, 0 +/- 4 kcal mol-1. The higher stability of the O-protonated [H2N-NO2]H+ isomer that emerges from this study represents the first departure from the general trend previously observed in the protonation of other simple X-NO2 molecules, giving the HX-NO2+ adduct as the most stable isomer. The [HN-NO2]- anion has been obtained by negative-ion chemical ionization of 1 either by dissociative electron attachment or via proton transfer to gaseous anions. The evaluation of the prototropic equilibrium of 1 with HCOO- leads to a gas-phase acidity of 1 and a PA of its anion amounting to 338.3 +/- 2 and 345.3 +/- 2 kcal mol-1, respectively, at 300 K. The latter value is consistent with the results of G1 theoretical calculations giving a PA of the [HN-NO2]- anion of 339.8 kcal mol-1 at 298 K.