Ground and Excited States Of OH – (H 2 O) n Clusters
Résumé
We present an ab initio study of OH–(H2O)n (n = 1–7) clusters in their lowest three singlet and two triplet electronic states, calculated with the RASPT2 method. Minimum energy structures were obtained by geometry optimization for both (a) the 11Σ+ ground state and (b) the 13Π excited state. From these structures, vertical detachment energies (VDEs), transition energies, and atomic charges were calculated. (a) We found that ground-state geometries present the hydroxide at the surface, accepting three and four H bonds from water. The excess charge is strongly stabilized by water up to a VDE of 6.7 eV for n = 7. Bound singlet excited states for ground-state geometries exist for n ≥ 3, and their VDE increases up to 1 eV for n = 7. (b) The 13Π state equilibrium geometries completely differ from the ground-state geometries. They are characterized by the hydroxide acting as a single H bond donor to a water molecule, which then donates a H-bond to two others, forming a “tree” pattern. All minimum energy structures present this “tree” pattern and a constant total number of 2n – 2 H bonds, or equivalently 3 dangling hydrogens. The excess charge stabilizes from n = 2 and goes mainly at the surface, on the dangling hydrogens of water. An almost neutral OH radical is then formed. Resulting structural resemblances with the neutral system make the VDEs of the first excited states weakly geometry dependent but size sensitive because of additive polarization effects. In contrast, the 11Σ+ state at the 13Π geometries is strongly sensitive to structural patterns. We bring out existing correlations between these patterns and the corresponding 11Σ+ state energy increase, which leads to couplings with excited states and possibly to an inversion of the state energy order. From these assessments, we propose a scenario for recombination of aqueous hydroxide following excitation in a charge-transfer-to-solvent state.