Raman Spectra from Saturated Water Vapor to the Supercritical Fluid

Raman OH stretching peak frequencies from saturated water vapor decrease by about 20−30 cm-1 as monomers (OH peak frequency, 3657 cm-1) are replaced by dimers (diagnostic frequency ≤ 3628−3638 cm-1) with temperature rise from 298 K to T c = 647 K. Dimerization increases with temperature in the saturated vapor because of the increase in the pressure/density, and this pressure/density driven dimerization extends into the supercritical fluid until additional polymerization produces trimers, etc., at higher densities. Increasing H-bonding (dimerization) in the saturated vapor equals the decreasing, mainly T-driven, liquid H-bonding when T rises to T c, where the H-bonding is roughly 17%. This means that the mole fraction of (H2O)2 molecules X A is ≈0.7, and the mole fraction of H2O molecules X B is ≈0.3, at the critical point. H-bonding in water at the critical point is not abnormal, because it is essentially independent of the correlation length divergence. The H-bond ΔH of the dimer is discussed.