Hydrogen bonding and the n → p* blue shift in a,b-unsaturated ketones. II. Franck-Condon factors and estimates of the effects of hydrogen bonding on CO bond lengths

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Abstract
Vibrational wave functions and Franck-Condon factors are calculated for the n → π* transition in α,β-unsaturated ketones by approximating vibrations of the chromophore by those of a diatomic moiety (CO). Comparison of the results with recent c.d. and u.v. absorption measurements indicates that the principal effect of hydrogen bonding on the n → π* system of these ketones arises from a differential lengthening of the CO bond in the excited electronic state as compared with the ground electronic state. It is these bond length changes which cause (through the Franck-Condon factors) the redistribution of intensity between vibrational sub-bands which is such a striking feature of recent highly resolved c.d. spectra.

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