Correlation of Pi-Electron Density with Vibrational Frequencies of Linear Polyenes

Abstract

Distributions of fundamental vibrational frequencies for linear polyenes are estimated from a transferable set of force constants. Particular attention is focused on reproducing observed experimental spectra in the carbon—carbon double‐bond stretching region, 1400–1650 cm⁻¹. Stretching force constants for carbon—carbon bonds are deduced from fractional pi‐bond orders, whereas interaction constants for these stretching motions are inferred from the mutual polarizabilities of the bonds. Calculated distributions for vibrational frequencies are in agreement with experimental data only if long‐range interaction constants are employed. The results obtained in the present study are compared to results obtained in previous studies where the long‐range interaction constants were assumed to be zero. Predictions of the vibrational spectra of the first excited electronic states of linear polyenes are provided along with a discussion of the types of vibrational frequencies which are expected to undergo significant changes on electronic excitation.