Ab initio study of the phenol-water cation radical

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Abstract
The phenol‐water cation radical has been investigated by ab initio theory using the spin‐restricted open‐shell Hartree–Fock and spin‐restricted open‐shell second‐order Mo/ller–Plesset theories with 3‐21G*(O) and 6‐31G* basis sets. The full geometrical optimization was performed for several hydrogen‐bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol‐localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero‐kinetic‐energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol‐water cation radical are compared with those of the corresponding neutral complex.