Tetrahedral displacement: The molecular mechanism behind the Debye relaxation in water

Noam Agmon*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

202 Scopus citations

Abstract

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, τD, and possesses a smaller activation energy. It governs the abnormally fast proton mobility in water. The temperature dependence of τD 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 τD.

Original languageEnglish
Pages (from-to)1072-1080
Number of pages9
JournalJournal of Physical Chemistry
Volume100
Issue number3
DOIs
StatePublished - 18 Jan 1996

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