GAS-PHASE NITRONIUM ION AFFINITIES

Evaluation of nitronium ion-transfer equilibria, L(1)NO(2)(+) + L(2) = L(2)NO(2)(+) + L(1) (where L(1) and L(2) are ligands 1 and 2, respectively) by Fourier-transform ion cyclotron resonance mass spectrometry and application of the kinetic method, based on the metastable fragmentation of L(1)(NO2+)L(2) nitronium ion-bound dimers led to a scale of relative gas-phase nitronium ion affinities. This scale, calibrated to a recent literature value for the NO2+ affinity of water, led for 18 ligands, including methanol, ammonia, representative ketones, nitriles, and nitroalkanes, to absolute NO2+ affinities, that fit a reasonably linear general correlation when plotted vs. the corresponding proton affinities (PAs). The slope of the plot depends to a certain extent on the specific nature of the ligands and, hence, the correlations between the NO2+ affinities, and the PAs of a given class of compounds display a better linearity than the general correlation and may afford a useful tool for predicting the NO2+ affinity of a molecule based on its PA. The NO2+ binding energies are considerably lower than the corresponding PAs and well below the binding energies of related polyatomic cations, such as NO+, a trend consistent with the available theoretical results on the structure and the stability of simple NO2+ complexes. The present study reports an example of extension of the kinetic method to dimers, such as L(1)(NO2+)L(2), bound by polyatomic ions, which may considerably widen its scope. Finally, measurement of the NO2+ affinity of ammonia allowed evaluation of the otherwise inaccessible PA of the amino group of nitramide and, hence, direct experimental verification of previous theoretical estimates.