Relating linear vibrational spectroscopy to condensed-phase hydrogen-bonded structures: Liquid-to-supercritical water.

The pressure and temperature-dependent linear absorption spectrum of partially deuterated water HOD dissolved in heavy water D(2)O was measured in the OH-stretching spectral region. The temperature was varied in the interval of 298 K</=T</=700 K while the density was changed within the range of 12 moll</=rho</=58 moll corresponding to the liquid and the supercritical phases of the fluid solution. The spectra were analyzed in terms of the temperature and density dependent frequency of maximal absorbance nu(max)(T,rho) and their full widths at half maximum Deltanu(T,rho). In parallel, molecular dynamics simulations of the fluid solution were carried out to obtain the average nearest neighbor O-O distance r(OO) ((1))(T,rho) and its dispersion Deltar(OO) ((1))(T,rho) at any state point (T,rho) for which an absorption spectrum was recorded. A correlation is presented between the experimental spectroscopic quantities nu(max)(T,rho) and Deltanu(T,rho) on the one hand and the local structural quantities r(OO) ((1))(T,rho) and Deltar(OO) ((1))(T,rho) on the other. This intuitive correlation can be used as a critical test for future perturbational simulations of the OH-stretching frequency shifts with hydrogen-bond geometry. Finally, a connection is made to the average hydrogen-bond connectivity in the fluid via the temperature and density dependent dielectric constant of water.