Thursday, August 4, 2011

Spectroscopy of a strong Hydrogen bond

The paper Fundamental Excitations of the Shared Proton in the H5O2+ and H3O2- complexes contains the graph below. It shows the frequency dependence of the Infra-red absorption intensity of the complex H3O2-, which can be viewed as a water molecule hydrogen bonded to a hydroxide anion (OH)-.
The authors state:
The H3O2-·Ar spectrum is presented in Figure 2B, and it is dominated by a very strong band at 697 cm-1, far below the bands displayed by the cationic system. To put the intensity of the 697 cm-1 feature in context, this band exhibits a transition moment that is approximately 1000 times larger than that associated with excitation of the free OH stretches in this complex. 

This is fascinating as it suggests to me something fundamentally new is happening. However, it is not clear to me what physics could give such a large effect  (n.b. a thousand fold increase in transition moment means a million fold increase in intensity!). A previous post considered the empirical correlation between hydrogen bonding energy and increase in intensity absorption. That will certainly not give an effect of this magnitude.

Some of the authors have a recent theory paper which compares to the above experimental results. However, it appears to me that it only discusses mode frequencies and not intensities. 

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