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Tetrahedral Displacement: The Molecular Mechanism behind the Debye Relaxation Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity,
 

Summary: Tetrahedral Displacement: The Molecular Mechanism behind the Debye Relaxation
in Water
Noam Agmon
Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew UniVersity,
Jerusalem 91904, Israel
ReceiVed: June 13, 1995; In Final Form: October 4, 1995X
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, D, and possesses a smaller activation energy. It governs
the abnormally fast proton mobility in water. The temperature dependence of D agrees with that of water
self-diffusion assuming a water hopping distance of 3.3 , 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 D.
Important information on solvent response times comes from
dielectric relaxation measurements.1 Many dynamical processes,

  

Source: Agmon, Noam - Institute of Chemistry, Hebrew University of Jerusalem

 

Collections: Chemistry