Gas-phase NO+ affinities


The National Academy of Sciences of the USA


A scale of relative gas-phase NO+ binding energies (BEs) has been constructed by evaluation of NO+-transfer equilibria L1NO+ + L2 ⇌ L2NO+ + L1 by Fourier-transform ion cyclotron resonance mass spectrometry and by application of the kinetic method, based on the metastable fragmentation of L1(NO+)L2 nitryl-ion bound dimers. The relative scale, anchored to the NO+ affinity of water, for 52 ligands, including alkyl halides, alkyl nitrates, alcohols, nitroalkanes, nitriles, aldehydes, ketones, and aromatic and heterocyclic compounds, led to an absolute NO+ affinity scale. The results are compared with those of an earlier study, and the apparent discrepancies are traced to a different choice of the absolute BE value used as the reference standard. The NO+ BEs fit a satisfactorily linear correlation when plotted versus the corresponding proton affinities (PAs). The NO+ BEs, while much lower than the PAs, are nevertheless higher than the corresponding BEs of the strictly related NO2+ cation, a result consistent with the experimental and theoretical results currently available on the structure and the stability of NO+ and NO2+ complexes. The NO+ BE vs. PA correlation allows one to estimate within 1–2 kcal·mol−1 the NO+ BE of the molecules included in the comprehensive PA compilations currently available. For example, the correlation gives the following NO+ affinities of the DNA bases, in kcal·mol−1 (1 kcal = 4.18 kJ): adenine, 40.3; cytosine, 40.4; guanine, 40.1; and thymine, 34.9. The experimental NO+ BE of thymine, the only one accessible to direct measurement, amounts to 35.6 ± 2 kcal·mol−1, which underlines the predictive value of the correlation. This study reports the second successful extension of the kinetic method to the evaluation of the absolute BEs of polyatomic cations, following our recent application to the strictly related NO2+ ion.

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