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Tautomerization and Dissociation of Dimethyl Phosphonate Ions(CH3O)2P(H)=O•+:Theory and Experiment in Concert

The unimolecular gas phase chemistry of the title ion,(CH3O)2P(H)=O•+, (1•+) and its tautomer dimethyl phosphite, (CH3O)2P-OH•+, (2•+) was investigated using mass spectrometry based experiments in conjunction with computational quantum chemistry. A facile tautomerization of the "keto" ion 1•+ into its "enol" isomer 2•+ is prevented by a high 1,2-H shift barrier. Instead, 1•+ readily isomerizes via a 1,4-H shift to the very stable distonic ion CH2O-(CH3O)P(H)OH•+ (1a•+) and related ion-dipole complexes which serve as precursors for the low energy loss of CH2=O. Loss of CH2=O is also the major dissociation of the enol ion 2•+, which is more stable than 1•+ by 31 kcal/mol. The reaction involves a 1,3-H shift leading to 1a•+ and, at a marginally higher energy, a competing 1,4-H shift leading to the ion-dipole complex CH2O-P(OH)-O(H)CH3•+ (1b•+). The resulting product ions, viz (CH3O)P(H)OH•+ and P(OH)-O(H)CH3•+, are separated by a high 1,2-H shift barrier (44 kcal/mol). However, the CH2O moiety in 1a•+ and 1b•+ is calculated to reduce this barrier significantly by a mechanism coined as proton-transport catalysis. The identity of the ions was probed by tandem mass spectrometry methods. These include MI (metastable ion) or CID (collision induced dissociation) spectra, consecutive MI/CID and CID/CID spectra, NRMS (neutralization-reionization mass spectra), NR/CID and CIDI (collision induced dissociative ionization) spectra, time-resolved CID spectra and deuterium labelling. The energetics of the CH2=O loss from 1•+ and 2•+ was derived from ionization and appearance energies determined by VUV photoionization. The experimental results agree quite well with the computational findings. Heats of formation, isomerization barriers and dissociation energies of the various ionic and neutral species were obtained by the wavefunction-based CBS-QB3 method. Essentially identical energy profiles on the C2H7O3P•+ surface were obtained with the computationally less demanding novel MPW1K empirical DFT method in conjunction with the aug-cc-pVTZ basis set. Theory and experiment yield a consistent potential energy profile that describes the isomerization and low energy dissociation chemistry of ions 1•+ and 2•+ (Scheme 4). In the μs timeframe ions 1•+ have completely isomerized into distonic ions 1a•+ which do not significantly communicate with their more stable enol counterparts. However, ion-molecule reactions of 1•+ with benzonitrile lead to a complete enolization. This is by virtue of a dipole-assisted lowering of the 1,3-H shift barrier separating isomers 1a•+ and 2•+.