Structure and dynamics of the phenol-water-argon cation radical

The phenol-water-argon system has been studied by ab initio methods and by ZEKE spectroscopy. Two structures of the cation radical were found; one has the argon sitting above the hydrogen bond, and the other has the argon linear with an OH bond in the water. It was found that the argon has little effect on the phenol-water moiety. The only notable change upon argon complexation is a shortening of the hydrogen bond. Two low-frequency vibrational progressions with spacings of 18.5 and 24.9 cm-1 were observed in the ZEKE spectrum. Ab initio calculations reveal that they are due to bending motions of the argon against the phenol-water moiety. The stretching mode of the argon is not seen in the ZEKE spectrum but is predicted to occur around 50 cm-1. The energies of the two minima are strongly dependent on the calculation used. At the UMP2/cc-pVDZ level, the structure with the argon sitting on the hydrogen bond has the lowest energy. On consideration of Franck-Condon factors, this structure is the most likely candidate for the true experimental structure. (Copyright (c) 1998 Elsevier Science B.V., Amsterdam. All rights reserved.)