Raman Spectra from Partially Deuterated Water and Ice VI to 10.1 kbar at 28°C

Abstract

High-pressure argon-ion laser-Raman spectra (4880 A excitation) have been obtained from partially deuterated water and ice VI (20 volume % D2O) in the OD and OH stretching regions to pressures of 10.1 kbar at 28°C. The Raman spectra from ice VI are the first to be reported at room temperature, and they are similar to the liquid spectra obtained at 9.7 kbar. Raman shifts corresponding to contour intensity maxima were observed to change with pressure rise in the OD and OH stretching regions from \( \Delta \bar v = 2513 - 2490\,{\text{cm}}^{ - 1} \) and \( \Delta \bar v = 3402 - 3380\,{\text{cm}}^{ - 1} \) respectively, for pressures from 1 bar to 10.1 kbar (ice VI). In addition, a shoulder observed at 1 bar on the OD contour near \( \Delta \bar v = 2650\,{\text{cm}}^{ - 1} \) became less distinct and was visually absent for pressures from 6.4 to 10.1 kbar, although a shoulder on the OH contour at about \( \Delta \bar v = 3250\,{\text{cm}}^{ - 1} \) intensified gradually for pressures to 9.7 kbar, and abruptly upon freezing at 10.1 kbar. The small effects of pressure on the OD component percentages obtained from computer analysis indicate that hydrogen-bond breakage is not a significant effect of pressure rise, and a downward change in the position of the OD stretching component having the largest Raman shift indicates that the nonhydrogen-bonded OD units or broken O-D…O bonds that exist at 1 bar are probably transformed by close packing due to compression into weak O-D…O bonds that are angularly deformed. In addition, intensification of the OH component at \( \Delta \bar v = 3220\,{\text{cm}}^{ - 1} \) upon freezing or upon pressurizing the liquid to 9.7 kbar is indicated by the computer analyses, and an increase in intermolecular coupling is thus favored, as opposed to enhancement of Fermi resonance, because the positions of components at \( \Delta \bar v = 3220\,{\text{cm}}^{ - 1} \) and \( \Delta \bar v = 3405\,{\text{cm}}^{ - 1} \) are nearly independent of pressure. The computer results also strengthen previous evidence indicating that the OD component which occurs at about \( \Delta \bar v = 2654\,{\text{cm}}^{ - 1} \) at 1 bar arises from broken O-D…O bonds, when it is understood that the severely deformed O-D…O bonds of ice VI give rise to intensity at a Raman shift of \( \Delta \bar v = 2617\,{\text{cm}}^{ - 1} \) , a difference of 37 cm-1 in the direction of stronger hydrogen-bonding.