INTERPRETATION OF THE INFLUENCE OF HYDROGEN-BONDING ON THE STRETCHING VIBRATIONS OF THE PO2-MOIETY

Infrared spectroscopic studies of different phosphate-containing molecules, above all DNA, reveal that the O...P...O stretching vibrations are influenced by the degree or state of hydration. Increasing the water activity of the sample dramatically reduces nu-approximately a, the wavenumber of the band due to the antisymmetric PO-2 stretching vibration, whereas nu-approximately a due to the corresponding symmetric stretch decreases only slightly. A previous suggestion that these changes are caused by hydrogen bonding of water molecules can be rationalized by the results of semi-empirical CNDO/2 calculations and coupled normal coordinate analyses performed for phosphate model fragments. Accordingly, hydrogen bonding of a water molecule to phosphate oxygen results in both a decrease in the P...O bond strength and a closure of the O...P...O bond angle. For nu(a) and nu(s), these two parameters show consignate and contrary effects on the wavenumber, respectively, which may explain the drastic difference in the magnitude of the wavenumber shifts observed experimentally.