Tautomerization and dissociation of ethylene phosphonate ions [-OCH2CH2O-]P(H)=O•+:: an experimental and CBS-QB3 computational study

The unimolecular gas-phase chemistry of the cyclic title ion, [-OCH2CH2O-]P(H)=Ocircle+, 1a(circle+), and its tautomer ethylene phosphite, [-OCH2CH2O-]P-OHcircle+, 1b(circle+), was investigated using mass spectrometry-based experiments in conjunction with isotopic labelling and computational quantum chemistry, at the CBS-QB3 level of theory. A facile tautomerization of the "keto" ion 1a(circle+) into its more stable (by 34 kcal/mol) "enol" isomer 1b(circle+) is prevented by a substantial 1,2-H shift barrier (14 kcal/mol relative to 1a(circle+)). In line with this, the collision-induced dissociation (CID) and neutralization-reionization (NR) spectra of the two isomers are characteristically different. Unlike the corresponding acyclic dimethyl phosphonate/phosphite tautomers, (CH3O)(2)P(H)=Ocircle+/(CH3O)(2)P-OHcircle+, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization into CH2O-(CH3O)P(H)OHcircle+, the barrier for this reaction in 1a(circle+) is prohibitively high and the cyclic distonic 1,2-H shift isomer [-OCH2CH2O(H)-]P=Ocircle+, 1c(circle+), is not directly accessible. The 1,2-H shift barrier separating 1a(circle+) and 1b(circle+) is calculated to lie close to the thermochemical threshold for the formation of C2H4circle+ + HO-P(=O)(2). This reaction dominates the closely similar metastable ion (MI) spectra of these tautomers. At these elevated energies, the "enol" ion 1b(circle+) can undergo ring-opening by CH2-O or CH2-CH2 cleavage, yielding ion-dipole complexes of the type [C2H4](circle+)/HO-P(=O)(2), 1e(circle+), and H-bridged radical cations CH2=O... [H-O-P-OCH2](circle+), 1f(circle+), respectively. Moreover, communication of 1b(circle+) with the distonic ion 1c(circle+) now also becomes feasible. These computational findings account for the similarity of the MI spectra and provide a rationale for the observation that in the losses of CO, HCOcircle and C2H3Ocircle from metastable ions [-OCH2CH2O-]P(H)=O-18(circle+) and [-OCH2CH2O-]P-(OHcircle+)-O-18, the O-18-atom loses its positional identity. Theory and experiment yield a consistent potential energy profile for the cyclic phosphonate/phosphite system showing that non-dissociating ions 1a(circle+) retain their structure identity in the microsecond time-frame. However, the interaction of 1a with a benzonitrile (BN) molecule in a chemical ionization type experiment readily yields the more stable "enol" type ion 1b(circle+). Experiments with benzonitrile-d(5) support the proposal that this interaction does not involve the lowering of the 1,2-H shift barrier between the tautomers, via a proton-transport catalysis type mechanism. Rather, a "quid-pro-quo" mechanism is operative, analogous to that proposed for the benzonitrile-assisted enolization of acetamide [Int. J. Mass Spectrom. 210/211 (2001) 4891. (C) 2003 Elsevier Science B.V All rights reserved.