Preferential deuterium bonding at the ice surface: A probe of surface water molecule mobility

Infrared spectra have been used to determine the temperature dependence of the preferential D-bonding of HDO at the single-donor (dangling O-H) sites on the ice surface. Data for ice nanocrystals containing H2O, D2O, and HDO were in the form of the relative peak intensities of the O-D stretch mode b ands of three-coordinated single-donor surface molecules of HDO and D2O. Th e magnitude of the enhanced stability of the D-bonded HDO molecules at thes e surface sites is estimated as 52 +/- 8 cm(-1). This value matches, within experimental error, the literature value for the D-bonded dimer of HDO [En gdahl and Nelander, J. Chem. Phys. 86, 1819 (1987)]. The thermal equilibriu m between the H-bonded and D-bonded configurations of HDO at these sites is lost upon cooling below 60 K. This identifies the temperature of onset of molecular rotation of the single-donor surface molecules as 60 K, on a time scale of 10(5) s. The possibility exists for determination of the onset te mperatures of more complex molecular motions on the surface of ice from rel ated data. In that respect, the constant intensity of the O-D stretch mode band of single-donor D2O molecules suggests that more complex surface motio ns, necessary for interchange with neighboring H-bonded HDO/H2O molecules, are frozen at temperatures below 130 K. (C) 2000 American Institute of Phys ics. [S0021-9606(00)72113-1].