Isotope effects in liquid water by infrared spectroscopy. IV. No free OH groups in liquid water.

The presence of free OH (OH not H-bonded) in bulk water is a key element for the determination of its molecular structure. The OH covalent bond infrared (IR) absorption is highly sensitive to the molecular environment. For this reason, IR spectroscopy is used for the determination of free OH. A workable definition of this is obtained with methanol (MeOH) in hexane where minute quantities of free OH are present. These absorb at 3654 cm(-1) (a 27 cm(-1) redshift from the gas position) with a full width at half height of 35 cm(-1). The IR spectrum of water between room temperature and 95 °C does not display such a band near 3650 cm(-1). This indicates that we do not see, in the IR spectra, the "free" OH group. From this we conclude that it is not present in liquid water at least down to the 1000 ppm level which is the limit of detectivity of our spectrometer. Other spectroscopic considerations of methanol and water in acetonitrile solutions indicate that weak H-bonds are also not present in liquid water.