GAS-PHASE NO+ AFFINITIES

A scale of relative gas-phase NO+ binding energies (BEs) has been cons tructed by evaluation of NO+- transfer equilibria L1NO+ + L-2 reversib le arrow L2NO+ + L-1 by Fourier-transform ion cyclotron resonance mass spectrometry and by application of the kinetic method, based on the m etastable fragmentation of L-1(NO+)L-2 nitryl-ion bound dimers. The re lative scale, anchored to the NO+ affinity of water, for 52 ligands, i ncluding alkyl halides, alkyl nitrates, alcohols, nitroalkanes, nitril es, aldehydes, ketones, and aromatic and heterocyclic compounds, led t o an absolute NO+ affinity scale. The results are compared with those of an earlier study, and the apparent discrepancies are traced to a di fferent choice of the absolute BE value used as the reference standard . The NO+ BEs fit a satisfactorily linear correlation when plotted ver sus the corresponding proton affinities (PAs). The NO+ BEs, while much lower than the PAs, are nevertheless higher than the corresponding BE s of the strictly related NO2+ cation, a result consistent with the ex perimental and theoretical results currently available on the structur e and the stability of NO+ and NO2+ complexes. The NO+ BE vs. PA corre lation 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+ affini ties 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 experiment al NO+ BE of thymine, the only one accessible to direct measurement, a mounts to 35.6 +/- 2 kcal . mol(-1), which underlines the predictive v alue of the correlation. This study reports the second successful exte nsion of the kinetic method to the evaluation of the absolute BEs of p olyatomic cations, following our recent application to the strictly re lated NO2+ ion.