The electronic excited states of the H-bonded complex of the phenol cation and water

Abstract

Complete active space multireference first-order configuration interaction (FOCI) calculations are used to determine the excitation energies of organic radicals. Application is made to the analysis of the effect of H-bonded interactions on the spectra of the phenoxyl radical and the phenol cation. Although earlier tests have been successful, the FOCI excitation energies of the benzyl radical, which is isoelectronic with phenoxyl, is compared to both experiment and earlier multiconfiguration self-consistent field (MCSCF) values providing additional corroboration of the accuracy of FOCI methods. The well-known 398 nm transition in the pheoxyl radical is represented well by the FOCI calculation. Two additional lower energy transitions are also predicted. The transition at 461 nm in vacuo is assigned to a broad experimental fluorescence band observed in the 425 nm region. No spectrum of the phenol cation has been reported. A broad transition around 373 nm has been attributed to the phenol cation complex with a single water molecule. Shifts in. the spectra of the phenol cation and phenoxyl complexes relevant to the analysis of this transition are calculated to be of the order of 500cm −1. This result supports the spectral assignment to the phenol cation complex and the conclusion that proton transfer does not occur in the ground state of this complex.