Vibrational predissociation in hydrogen bonded complexes

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Abstract
A vibrationally excited hydrogen bonded complex, A–H*⋅⋅B, is metastable since the vibrational energy of the A–H* chemical bond exceeds that needed to sever the hydrogen bond. After a time the excited complex spontaneously breaks up and fragments A–H and B are produced. We show that this vibrational predissociation process is inefficient when there is unfavorable Franck–Condon overlap between the vibrational wave functions which characterize the bound complex A–H*⋅⋅⋅B and the wave functions which describe the fragments A–H+B. We also show that the vibrational predissociation process is most efficient when the fragments are produced in rotationally or vibrationally excited states. Analytical expressions are presented which allow numbers for the lifetime of A–H*⋅⋅⋅B to be easily obtained with a hand electronic calculator. These lifetimes are exceedingly sensitive to the parameters which characterize the multidimensional potential surface of the hydrogen bonded complex. Since the surface is poorly understood, the resulting lifetimes have only a qualitative significance. Sample calculations of vibrational predissociation of HF*⋅⋅⋅HF and its isotopes together with figures of potential surfaces, wave functions, and coupling terms serve to illustrate the ideas of this paper.

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