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, τ_D, and possesses a smaller activation energy. It governs the abnormally fast proton mobility in water. The temperature dependence of τ_D agrees with that of water self-diffusion assuming a water hopping distance of 3.3 Å, the separation between an occupied and unoccupied corners of a cube binding the pentawater tetrahedron. This slower translational mechanism controls the ordinary transport phenomena in water. "Tetrahedral displacement" correlates with two tetrahedral normal modes: the antisymmetric stretch in extended tetrahedral structures at low temperatures and a torsion mode in loosely bound tetrahedra at high temperatures. The temperature dependence of the 180 cm^-1 Raman band is in quantitative agreement with the activation energy for water reorientation and, in the framework of a two-dimensional model, also explains the activation energy for τ_D.