Combined experimental and ab initio molecular orbital studies on gaseous OH n2+ species ( n = 1–4)

Abstract

Ab initio molecular orbital calculations, executed at the MP4SDTQ/6–311 ++ G**//MP2/6–31G** level of theory, were performed for OH n 2+ cations ( n = 1–4). In line with experimental results, it is observed that the OH 2+· potential energy surface is purely repulsive. Consequently, it is obvious that no stable OH 2+· could ever be generated. Both OH 2 2+ as a triplet ( D ∞h and singlet ( C 2 v ) species were found to be stable minima, the latter being 68.3 kcal mol −1 less stable. However, proton loss from 3OH 2 2+ and 1OH 2 2+ is predicted to have an activation barrier as small as 1.5 and 2.5 kcal mol −1, respectively. H 3O 2+· ( D 3 h ) is found, in line with experiments, as a stable dication on the H 3O 2+· surface. A barrier of 20.7 kcal mol −1 prevents spontaneous exothermic dissociation (−86.9 kcal mol −1) of the dication to H 2O +· and H +. the unobserved OH 4 2+ ( T d) dication is also predicted to exist as a stable gas-phase species. Proton loss from OH 4 2+, while being exothermic (−59.0 kcal mol −1), has an activation energy of 39.6 kcal mol −1. The computed vertical ionization energies for the charge-stripping processes OH n + → OH n 2+ are in excellent-to-good agreement with experimentally determined Q min· values. In particular, they suggest that charge stripping from H 2O +· does not involve the ground state but the first excited electronic state ran 2O +·. Consideration of hypothetical proton transfer reactions to H 2O by using isodesmic reactions provide (i) approximate heats of formation for H 3O 2+· (Δ H f 0 = 684.6 kcal mol −1) and H 4O 2+ (Δ H f 0 = 563.7 kcal mol −1) and (ii) indicate that it is highly unlikely to ever generate H n O 2+ ( n = 3,4) as stable dications in solution. Both dications will either strip an electron from or transfer a proton to its solvation shell with avidity.