Friday, August 14, 2009

Quantum decoherence in water

Water is an amazing substance which has many unique properties. Some of these properties are essential for the functionality of biomolecules.
Previously, I mentioned how it plays a key role in determining the spectral density which describes how electronic excitations in biomolecules decohere.
If one takes the simplest possible Onsager type continuum dielectric model where an electric dipole is placed inside a spherical cavity then the spectral density can be related to the frequency dependent dielectric function, epsilon(omega)For more on this see this review article.

The figure below [from a nice paper by Hsu, Song, and Marcus] shows a plot of -J(omega)/omega using the measured epsilon(omega) for liquid water at room temperature.

What is the origin of the large shoulder around 800 cm-1 (corresponding to a time scale of about 40 fsec)?

It is due to the "librational" motion of the water molecules. This is a rotational motion of an individual water molecule which is restricted by hydrogen bondint to four surrounding water molecules. A nice animation of the librational motion of water is here.

In heavy water, D2O, the librational frequency is decreased by a factor of about two.

The librational motion makes an important contribution to the heat capacity of water and ice, as described in the classic book of Eisenberg and Kauzmann.

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