TETRAHEDRAL DISPLACEMENT - THE MOLECULAR MECHANISM BEHIND THE DEBYE RELAXATION IN WATER

The arguments for and against a single-molecule rotation mechanism for dielectric relaxation of water are surveyed. It is concluded that two distinct molecular mechanisms are operative in water. Single-molecule rotation is faster than the Debye relaxation time, tau(D), and posses ses a smaller activation energy, It governs the abnormally fast proton mobility in water. The temperature dependence of tau(D) agrees with t hat of water self-diffusion assuming a water hopping distance of 3.3 A , the separation between an occupied and unoccupied corners of a cube binding the pentawater tetrahedron. This slower translational mechanis m controls the ordinary transport phenomena in water. 'Tetrahedral dis placement'' correlates with two tetrahedral normal modes: the antisymm etric stretch in extended tetrahedral structures at low temperatures a nd a torsion mode in loosely bound tetrahedra at high temperatures. Th e temperature dependence of the 180 cm(-1) Raman band is in quantitati ve agreement with the activation energy for water reorientation and, i n the framework of a two-dimensional model, also explains the activati on energy for tau(D).