Molecular model of dielectric relaxation and the far-infrared isotopic effect in liquid H 2O and D 2O

Abstract

The wideband (dielectric + far-infrared) spectra of liquid H 2O and D 2O are described in terms of two potential wells, to which two types of molecular motion correspond with different values of a dipole moment. One well is narrow and steep with a flat bottom, flat brim and rectangular shape. Another well is concave and has a cosine squared (CS) shape. The steep rectangular (hat-like) well determines reorientation of dipoles; the concave well determines motions specific for the H-bonded water molecules. This type of motion, being connected to the far-infrared (FIR) translational band (near 200 cm −1), is more chaotic than reorientation, which is responsible for the intense librational band in the region 500 – 800 cm −1. The lifetime of the CS well is shorter than that of the hat well. Few mechanisms contributing to the dielectric/FIR spectra of liquid water are discussed.