Fluorobenzene•••water and difluorobenzene•••water systems:: An ab initio investigation

Recently, Brutschy and co-workers have reported the spectra of (substituted benzene)...(H2O)(n) systems. To investigate the possibility of these systems exhibiting a pi-H kind of bonding interaction as observed in benzene ...(H2O)(n) systems, we have carried out extensive ab initio calculations on different conformations of the fluorobenzene ...(H2O) and p-difluorobenzene ...(H2O) systems using various basis sets. Our results indicate that unlike the pi interaction observed in benzene ...(H2O)(n) the O-H of the water molecule is involved in the formation of a six-membered ring system with the F-C-C-H of the aromatic ring. This six-membered ring which results from the formation of two H-bonds (water hydrogen and fluorine, water oxygen and benzene hydrogen), is extensively stabilized by electrostatic interactions. The strength of this sigma-bonding interaction of water to fluorobenzene in C6H5F ... H2O is nearly equal to the corresponding pi-bonding interaction of water to benzene in C6H6... H2O. However the sigma interaction of water to difluorobenzene in p-C6H4F2... H2O is somewhat higher than the pi interaction in C6H6... H2O and slightly higher than the corresponding interaction in C6H5F ... H2O. The frequency shifts of the predicted OH stretching modes are in reasonable agreement with the experimental vibrational frequency shifts for both C6H5F ... H2O and p-C6H4F2... H2O. (C) 1999 American Institute of Physics. [S0021-9606(99)31016-3].