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

The gas-phase ion chemistry of H2N-NO2 (1) has been studied with mass spectrometric and ab initio theoretical methods. The H2N2O2.+ molecula r ion is formed upon electron-impact ionization of 1, whose photoelect ron (PE) spectra have been recorded. From the onset of the PE bands th e ionization energy of 1 has been estimated to be 11.02 +/- 0.06 eV. R eaction of 1 with gaseous Bronsted acid such as H-3+, CH5+, H3O+, and i-C3H7+, etc., gives protonated adducts[H2N-NO2]H+. Evaluation of prot otropic equilibria involving C6H6 and CH3OH as the reference bases giv es 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 diss ociation spectra of the [H2N-NO2]H+ populations from the protonation o f 1 with those of H3N-NO2+ model ions suggests that O-protonated proto mers are more stable and represent the predominant [H2N-NO2]H+ species at the time of structural analysis, i.e., ca. 10 mus after the proton ation of event. These conclusions are consistent with ab initio result s at the GI level of theory that identify the O atoms of the nitro gro up 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 satisfact ory agreement with the experimental value. In addition, on the basis o f the G 1 value of the H3N-NO2+ binding energy, one can estimate the e xperimentally unknown heat of formation of 1, 0 +/- 4 kcal mol-1. The higher stability of the O-protonated [H2N-NO2]H+ isomer that emerges f rom this study represents the first departure from the general trend p reviously observed in the protonation of other simple X-NO2 molecules, giving the HX-NO2+ adduct as the most stable isomer. The [HN-NO2]- an ion 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 3 38.3 +/- 2 and 345.3 +/- 2 kcal mol-1, respectively, at 300 K. The lat ter value is consistent with the results of G1 theoretical calculation s giving a PA of the [HN-NO2]- anion of 339.8 kcal mol-1 at 298 K.