π-hydrogen bonding between water and aromatic hydrocarbons at high temperatures and pressures

Infrared OH stretching absorption of HDO isolated in aromatic hydrocarbons have been measured at temperatures of 473 and 523 K and at pressures in the 100-350 bar range. The peak frequencies are dependent on the solvents and their order, benzene>toluene>ethylbenzene>cumene>o-xylenesimilar tom-xylene>mesitylene, is exactly the same as the order for the ionization potentials of the hydrocarbons. Shifts of the frequencies from that of HDO in hexane, which was measured as a reference at the same temperature and pressure, were analyzed using a charge transfer theory for hydrogen bonding. Distances between the water molecule and a solvent phenyl ring were estimated to be 2.8+/-0.1 and 2.9+/-0.1 Angstrom at 473 and 523 K, respectively. These values are consistent with a structure of a water-benzene complex determined by a jet-cooled microwave spectroscopy. These facts suggest that the pi-hydrogen bond between water and aromatic hydrocarbons exists even at the high temperatures under pressure. (C) 2002 American Institute of Physics.