Microhydration Effects on the Electronic Properties of Protonated Phenol: A Theoretical Study

The CC2 (second-order approximate coupled cluster method) has been a employed to investigate microhydration effect on electronic properties of protonated phenol (PhH+) According to the CC2 calculation results on electronic excited states of microhydrated PhH+, for the S-1 and S-2 electronic states, which are of (1)pi pi* nature and belong to the A' representation of molecular C-s point group, a significant blue shift effect on the SI and S2 electronic states, which are of 1 pi pi* nature and belong to the A' representation of molecular C, point group, in comparison to corresponding transitions on bare cation (PhH+), has been predicted. Nevertheless, for the S-3-S-0 (1A '', 1 sigma pi*) transition, a large red shift effect has been predicted. Furthermore, it has been found that the lowest (1)sigma pi* state plays a prominent role in the photochemistry of these systems. In the bare protonated phenol, the (1)sigma pi* state is a bound state with a broad potential curve along the OH stretching coordinate, while it is dissociative in microhydrated species. This indicates to a predissociation of the S-1((1)pi pi*) state by a low-lying (1)sigma pi* state, which leads the excited system to a concerted proton-transfer reaction from protonated chromophore to the solvent. The dissociative (1)sigma pi* state in monohydrated PhH+ has small barrier, while increasing the solvent molecules up to three removes the barrier and consequently expedites the proton-transfer reaction dynamics.