The role of exact exchange on the structure of water dimer radical cation: Hydrogen bond vs hemibond.

Understanding the structure and chemical bonding in water dimers is central to the study of many (photo-)electrochemical oxidation reactions. Two structures of the water dimer radical cation, namely, proton-transfer and hemi-bonded structures, have been suggested using density functional theory (DFT) and coupled cluster singles, doubles, and perturbative triples [CCSD(T)]. Both structures are identified by us as local minima, and their relative stability strongly depends on the level of theory. The exact exchange correlates linearly to the energy difference between both local minima. DFT functionals with less than 20 percent exact exchange predict the hemi-bonded structure to be more stable, while more than 20 percent of the exact exchange stabilizes the proton-transfer structure. The latter structure is also confirmed by CCSD(T) benchmark computations. These computations, furthermore, indicate that the oxidized water dimer consists of a hydronium cation (H3O+) and an HO· radical. These results are reproduced by DFT functionals with more than 50% of exact exchange (BHandH, M06-2X, and M06-HF). The transition barrier for the interconversion from the proton-transfer to the hemi-bonded structure is 0.6 eV, while the reverse reaction has a barrier of 0.1 eV.