Gas-phase nitronium ion affinities.

Abstract

Evaluation of nitronium ion-transfer equilibria, L1NO' + L2 = L2NO' + L1 (where L1 and L2 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 L1(NO+)L2 nitronium ion-bound dimers led to a scale of relative gas-phase nitronium ion affinities. ligands, aimed at the construction of a gas-phase nitronium ion-affinity scale.To this end, we have applied the Fourier-transform ion cyclotron resonance mass spectroscopy equilibrium method largely used in proton affinity (PA) measurements (34) to NO' transfer reactions and extended the kinetic method, so far used exclusively in PA and gas-phase acidity determinations (35,36), to nitronium ion-bound dimers. MATERIALS AND METHODSAll chemicals were obtained as research-grade products from Aldrich and were used without further purification. The gases were purchased from Matheson with a stated purity >99.95 mol %, except NO2, for which purity was >99.5 mol %, and were used L,NO+ + L2 = L2NO2 + Ll, [1] allows direct determination of the corresponding free energy change, AG', provided that the equilibrium constant can be accurately measured, which requires attainment of true equilibrium, unperturbed by significant side reactions. In the specific application, CH3O(NO2)2, the nitrating reagent, is prepared in the external chemical ionization source of a Fourier transform ion cyclotron resonance mass spectrometer and driven into the resonance cell, where it nitrates both ligands, present in a known concentration ratio. One of the nitrated complexes is isolated by selective-ejection techniques and allowed to equilibrate in the mixture of the ligands. Application of the method is prevented in many cases by incursion of fast proton-transfer reactions that perturb the equilibrium-e.g., the nitronium ion transfer, [C2H5OH-NO2]+ + NH3 = [H3N-NO2]+ + C2H5OH, [la] undergoes competition by proton transfer to ammonia from both nitrated complexes, which react as the conjugate acids of ethyl nitrate and of nitramide. Furthermore, in the case of ligands of low ionization potential the study of equilibrium 1 Abbreviations: PA, proton affinity; BE, binding energy; RP, reference pair; MIKE, mass analyzed ion kinetic energy. tTo whom reprint requests should be addressed.The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. 8635

Gas-phase NO + affinities

“…The larger H 2 O-NO 2 ϩ separation (36) reflects the reluctance of NO 2 ϩ to form tightly bound complexes, where the close approach to the nucleophilic center entails the deformation of the cation. Another significant difference is that the BE vs. PA correlation displays a much better linearity in the case of NO ϩ than of NO 2 ϩ , and satisfactorily fits a single linear plot, whereas the NO 2 ϩ correlation has a composite character, significantly changing for different classes of compounds (23).…”

“…Following the application to the strictly related nitronium ion (23), the present work provides another example of successful extension of the kinetic method to the evaluation of the absolute BE of a polyatomic ion. In fact, all previously published studies of dimers bound by polyatomic cations, including CH 3 ϩ , CN ϩ , OCNCO ϩ , and NH 4 ϩ provide only the relative affinity order of the ligands investigated (24) plications and for modelling purposes.…”

“…However, the principal motivation of this work is to be found in the attempt to extend the NO ϩ BE scale to a significantly larger number of ligands, including molecules of great biochemical relevance, such as the DNA bases. To this end, we have applied the ICR equilibrium method, largely used in gas-phase proton affinity (PA) measurements (22) and successfully extended to NO 2 ϩ transfer reactions (23), complemented by the kinetic method (24). The latter, so far restricted to monoatomic ions, at least as concerns the measurement of absolute BEs, has recently been extended to the construction of an absolute BE scale of a triatomic species, the NO 2 ϩ cation (23).…”

Study of gas‐phase reactions of NO2+ with aromatic compounds using proton transfer reaction time‐of‐flight mass spectrometry

“…In the 1970s, gas‐phase reactions of NO 2 + with ArX were studied by ion cyclotron resonance mass spectrometry at a pressure of 10 −5 Torr, and the competing reaction mechanisms were found to be electron transfer to form ArX + and substitution to form ArXO + . Later, a few gas‐phase reactions between NO 2 + and organic compounds have been studied by ion cyclotron resonance mass spectrometry or triple quadrupole mass spectrometry and theoretical calculations with ab initio methods . However, these techniques are not sufficient for investigations of the gas‐phase ion chemistry for NO 2 + , especially gas‐phase reactions between NO 2 + and ArXs.…”

Gaseous [N2O5]H+, [N2O4]H+, and Related Species from the Addition of NO2+ and NO+ Ions to Nitric Acid and Its Derivatives

“…Gas-phase positive ion chemistry of nitric acid and related molecules is the focus of sustained interest, owing to its intrinsic fundamental interest and the relevance to atmospheric chemistry. − Particular attention has been devoted to protonated nitric acid, both in the isolated state and in water clusters, − and the investigation has been extended to its derivatives, i.e., protonated alkyl nitrates − and nitramide, leading to the construction of a scale of NO 2 + binding energies (BE) …”