Ground and Excited States Of OH<sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> Clusters
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Abstract
We present an ab initio study of
OH<sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> (<i>n</i> = 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 1<sup>1</sup>Σ<sup>+</sup> ground state and
(b) the 1<sup>3</sup>Π 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 <i>n</i> = 7. Bound singlet excited
states for ground-state geometries exist for <i>n</i> ≥
3, and their VDE increases up to 1 eV for <i>n</i> = 7.
(b) The 1<sup>3</sup>Π 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 2<i>n</i> –
2 H bonds, or equivalently 3 dangling hydrogens. The excess charge
stabilizes from <i>n</i> = 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 1<sup>1</sup>Σ<sup>+</sup> state at the 1<sup>3</sup>Π
geometries is strongly sensitive to structural patterns. We bring
out existing correlations between these patterns and the corresponding
1<sup>1</sup>Σ<sup>+</sup> 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