VIBRATIONAL PREDISSOCIATION IN HYDROGEN BONDED COMPLEXES
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Date
1978
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Ohio State University
Abstract
The Infrared spectra in the hydrogen stretching frequency region of hydrogen bonded molecules is characterized by a broad diffuse band shifted to lower frequency from the absorption of the uncompleted molecule. It has been $suggested^{1}$ that the broadness can, at Least in part, be explained by vibrational predissociation. This can occur because the energy in the vibrationally excited stretching mode $(\nu\sim 3400\; cm^{-1}=40\;kJ\; mol^{-1})$ exceeds that needed to rupture the hydrogen bond ($D\sim 20\; kJ\; mol^{-1}$). The transfer of energy within the molecule from the stretching mode to the hydrogen bond can result in spontaneous dissociation, $A-H^{*}\cdots B\stackrel{\tau}{\rightarrow} A-H +B$ characterized by a lifetime, $\tau$. It is our goal to calculate this lifetime. The calculation is performed using a modification of $recent^{2}$ theoretical models for discussing vibrational pre-dissociation in Van der Waals molecules. We show that with reasonable assumptions about the hydrogen bonded potential surfaces Lifetimes of $A-H^{*} \cdots B$ are often less than $10^{-12}$ sec and should produce easily observed spectroscopic broadening. Broadening of this magnitude has already been distinguished in gas phase hydrogen bonded $complexes^{3}$. Further experimental and theoretical studies can take advantage of vibrational predissociation in order to provide new insights into the nature of the hydrogen bond.
Description
$^{1}$B.I, Stepanov, Nature 157, 808 (1946). $^{2}$G. Ewingt Chem. Phys. (in press); J. A, Beswick and J. Jortner, Chem. Phys. Lettr. 49, 13 (1977). $^{3}$J. Bertie and D, J. Millen, J. Chem, Soc. 1965, 497.""
Author Institution: Department of Chemistry, Indian University
Author Institution: Department of Chemistry, Indian University