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)O+, 1a+, and its tautomer ethylene phosphite, [OCH2CH2O]POH+, 1b+, 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+ into its more stable (by 34kcal/mol) “enol” isomer 1b+ is prevented by a substantial 1,2-H shift barrier (14kcal/mol relative to 1a+). 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)2P(H)O+/(CH3O)2POH+, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization into CH2O(CH3O)P(H)OH+, the barrier for this reaction in 1a+ is prohibitively high and the cyclic distonic 1,2-H shift isomer [OCH2CH2O(H)]PO+, 1c+, is not directly accessible.The 1,2-H shift barrier separating 1a+ and 1b+ is calculated to lie close to the thermochemical threshold for the formation of C2H4++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+ can undergo ring-opening by CH2O or CH2CH2 cleavage, yielding ion–dipole complexes of the type [C2H4]+/HOP(O)2, 1e+, and H-bridged radical cations CH2O⋯[HOPOCH2]+, 1f+, respectively. Moreover, communication of 1b+ with the distonic ion 1c+ 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, HCO and C2H3O from metastable ions [OCH2CH2O]P(H)18O+ and [OCH2CH2O]P18OH+, the 18O-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+ 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+. Experiments with benzonitrile-d5 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) 489].