Abstract
The dispersion of the H-bond pair volume ΔV over the decoupled OD and coupled OH-stretching contours from HDO in H2O was determined from Raman intensities at pressures to 9700 bar at 301 K. The dispersion of ΔV was determined from −RT[∂ ln(Ii/IREF)/∂P]T versus ω (in cm−1), where i refers to ω’s over the stretching contours and IREF refers to the reference intensity at the isosbestic frequency. The maximum H-bond pair volume (defined for breakage) is 1.4±0.1 cm3/mol H-bond, which corresponds to the volume difference between a large dispersion maximum at 2675 cm−1 (near the OD stretch ω of HDO in dense supercritical water) and a large, broad minimum centered near 2375 cm−1 (just below the OD stretch ω of HDO in lda ice). The average 〈ΔV〉 is 0.71±0.10 cm3/mol H-bond. Other minima near 2625 cm−1 (OD) and 3550 cm−1 (OH) refers to bent H-bonds whose angles are ≈150 deg. Isothermal pressurization of water lowers the molal volume by decreasing the concentration of long, weak H-bonds, and increasing the concentrations of bent H-bonds and short, strong, linear H-bonds. Such bending, shortening, and strengthening produces freezing to ice VI near 10 kbar at 301 K. The isobaric temperature derivative of the maximum H-bond volume is (∂ΔV/∂T)P⩽(2–5)×10−3 cm3/deg mol H-bond. The OH enthalpy dispersion curve for saturated NaBF4 in water, yields a large maximum at 3530–3540 cm−1 indicating that BF4− interacts preferentially with the dangling or “free” OH groups of water thus producing weak, strongly bent H-bonds having angles similar to those of the 3550 cm−1 high-pressure H-bond bending feature.
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@article{Walrafen_2004, title={Raman H-bond pair volume for water}, volume={121}, ISSN={1089-7690}, url={http://dx.doi.org/10.1063/1.1767992}, DOI={10.1063/1.1767992}, number={6}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Walrafen, George E.}, year={2004}, month=aug, pages={2729–2736} }