Theoretical and experimental studies on the ground state potential energy surface of C₂H₄O dications

The C₂H₄O²⁺ ground state potential energy surface was examined by ab initio molecular orbital theory corrected for the effects of zero-point energies and for electron correlations by means of Møller-Plesset perturbation theory terminated to second order (MP2/6-31G) using 4-31G optimized geometries. The global minimum corresponds to H₃C-C-OH²⁺ (1, C_3v), which is 14.7 kcal mol^-1 more stable than the dication of ring-opened ethylene oxide, i.e. H₂C-O-CH₂²⁺ (2, D^2_d). Other stable C₂H₄O²⁺ isomers are the perpendicular forms of the anti- and syn-vinyl alcohol (3 and 4, 15.8 and 22.8 kcal mol^-1 less stable than 1, an oxygen analogue of allene, 5, i.e. H₂C-C-OH₂²⁺ (28.7 kcal mol^-1 relative to 1), and an oxygen analogue of propyne, 7, i.e. HC-C-OH₃²⁺, which is substantially less stable than 1 (99.4 kcal mol^-1). The end-on complex between water and acetylene, i.e HCCH-OH₂²⁺ (6), is also found to be a stable species on the 4-31G potential energy surface (38.0 kcal mol^-1 relative to 1), in contrast to its side-on isomer. However, investigations at higher levels of theory indicate that 6 should not be regarded as a true minimum structure. The dications of acetaldehyde (8), planar vinyl alcohol (9) and ethylene oxide (10) are not found to exist in potential minima. The thermochemical and kinetic stabilities of 1, 2 and 3 are evaluated in detail. Although these dications are thermochemically highly unstable, dissociation is prevented by substantial barriers, thus making observation in the gas phase feasible. In fact, charge-stripping mass spectrometry provides evidence that 1, 2 and 3 can indeed be generated, and a comparison of the experimentally derived Q_min values with the calculated vertical ionization energies shows good agreement for the ions 2 and 3. It seems, however, quite impossible that C₂H₄O²⁺ could ever be generated as viable chemical intermediates in solution because the dications will strip an electron or an atom from an adjacent neutral (or negatively charged) species with avidity.