Tautomerization and dissociation of ethylene phosphonate ions [OCH 2CH 2O]P(H)O [rad]+: an experimental and CBS-QB3 computational study

Abstract

The unimolecular gas-phase chemistry of the cyclic title ion, [OCH 2CH 2O]P(H)O +, 1a +, and its tautomer ethylene phosphite, [OCH 2CH 2O]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 34 kcal/mol) “enol” isomer 1b + is prevented by a substantial 1,2-H shift barrier (14 kcal/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, (CH 3O) 2P(H)O +/(CH 3O) 2POH +, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization into CH 2O(CH 3O)P(H)OH +, the barrier for this reaction in 1a + is prohibitively high and the cyclic distonic 1,2-H shift isomer [OCH 2CH 2O(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 C 2H 4 ++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 CH 2O or CH 2CH 2 cleavage, yielding ion–dipole complexes of the type [C 2H 4] +/HOP(O) 2, 1e +, and H-bridged radical cations CH 2 O ⋯[ H O P OCH 2] + , 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 C 2H 3O from metastable ions [ OCH 2 CH 2 O] P( H) 18 O + and [ OCH 2 CH 2 O] P 18 OH + , the 18 O -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-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) 489].