Water Dimer Radical Cation: Structures, Vibrational Frequencies, and Energetics

Fourteen stationary points for the water dimer radical cation on its doublet electronic state potential energy surface have been characterized using coupled cluster theory with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)]. This is done in conjunction with Dunning's correlation consistent polarized valence basis sets (cc-pVXZ and aug-cc-pVXZ, X = D, T, Q). Two stationary points are found to be local minima, isomer 1 (C-1 symmetry) with H3O+center dot center dot center dot OH character (hydrogen-bonded system), and isomer 7 (C-2 symmetry) with [H2O center dot center dot center dot H2O](+) character (hemibonded system). Among the other stationary points, seven are transition states, and the remaining five are higher order saddle points. The fourteen water dimer radical cation structures lie within 45 kcal mol(-1) of isorner 1. Structure 1, transition states 2 (C-s symmetry) and 3 (C-s symmetry) are related through torsion of the OH group; these three stationary points fall within one kcal mol(-1), demonstrating the low energy barrier of the OH torsional mode. Adiabatic ionization energies of (H2O)(2) to 1 and 7 are determined to be 10.81 and 11.19 eV, respectively; the former is in excellent agreement with the experimental value of 10.8-10.9 eV. The critical dissociation energy of 1 to H3O+ + OH center dot is predicted to be 26.4 kcal mol(-1), while the dissociation energy of isomer 7 to H2O+ + H2O is determined to be 34.7 kcal mol(-1). At the aug-cc-pVQZ CCSD(T) level of theory, the hydrogen-bonded I and hemibonded 7 minima are separated by 8.8 kcal mol(-1) with an interconversion barrier (1 -> 10 -> 7) of 15.1 kcal mol(-1). A careful comparison is made with the recent experiments of Gardenier, Johnson, and McCoy oil (H2O)(2)(+)center dot Ar and (H2O)(2)(+)center dot Ar-2.