RASPT2 Analysis of the F^-(H₂O) _n=1-7 and OH^-(H₂O) _n=1-7 CTTS States

We analyze the electronic structure of the lowest excited states of the F^-(H₂O) _n=1-7 and OH^-(H₂O) _n=1-7 anionic clusters in the framework of RASPT2 theory. At the ground-state geometry, these clusters can bind the excess electron in the first excited singlet and triplet states for n ≥ 3 for F^- and n ≥ 2 for OH^-. The geometry relaxation of the F^-(H₂O) _n=1-7 clusters in their lowest-energy triplet state produces two series of minima. A first series is made of a F radical weakly bound to a negatively charged water cluster to form F-(H₂O) _n^-. A second series associated with hydrogen transfer from a water molecule to the fluorine atom is built on a HF molecule and a OH radical bound to a negatively charged water cluster to form OH-HF-(H₂O) _n-1^-. This second series provides the lowest-energy isomers of F^-(H₂O) _n for the excited state. These two series of minima are inherited from the neutral fluorine water cluster structure only weakly perturbed by the excess electron. They are similar to the OH^-(H₂O) _n isomers obtained for the lowest-energy triplet state, which are also made of a neutral OH radical inserted in the water molecule network of a (H₂O) _n^- cluster. For all of these clusters in the lowest-energy excited state, the excess electron is localized outside of the cluster near unbound hydrogen atoms. Its binding energy is well correlated to the electric dipole of the cluster, and a lower limit of 4.1 D is necessary to bind it to the cluster. The two series of F^-(H₂O) _n isomers offer two very different routes for geminate recombination observed in water solutions. Our calculation suggests that the recombination takes place with the OH radical left after hydrogen transfer rather than with the F radical.