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.

Evidence for light-induced 13-cis , 14-s-cis isomerization in bacteriorhodopsin obtained by FTIR difference spectroscopy using isotopically labelled retinals

“…Starting with the force field as derived by Smith et al (1984), we fitted the frequencies obtained for BR, K and L for the unlabelled and 15-13C-labelled compounds in H20 and 2H20 by varying the diagonal force constants for stretching and bending vibrations. + 4.4 for linear polyens (Gavin and Rice, 1971). Bond orders were obtained using the relationship between diagonal stretching force constants (F..) and bond orders (P..) P.. = 6.4 Fi.…”

Analysis of the factors that influence the C=N stretching frequency of polyene Schiff bases. Implications for bacteriorhodopsin and rhodopsin

“…where KF is the scaled coupling constant, S1 and Sj are the scale factors for the diagonal force constants i and j, respectively, and K,j is the 2,985*Observed values are taken from Gavin and Rice (1971), and Pulay et al (1983) in parentheses. calculated coupling constant (Pulay et al, 1983).…”

“…This behavior has been qualitatively explained in terms of an increase in ir-electron delocalization as the length of the polyene increases; the decrease in "c-being due to the fact that increased delocalization reduces the bond order of the double bonds and therefore their force constants. With one exception (Lasaga et al, 1980) previous studies have not reproduced this trend in the vc.Jc (Gavin and Rice, 1971;Warshel and Karplus, 1972). (See Kakitani et al, 1983 for a discussion of this problem.)…”

A review on linear and non‐linear resonance Raman spectroscopy of the conjugated system polydiacetylene

“…The reason for the lowering of the chain mode wavenumbers is the increase of the electron delocalization in conjugated systems with longer chain length. 117 Differences in the wavenumbers of the modes Q 1 and Q 2 were also observed for several other PDA molecules. 59 Also small changes of the substituents lead to relatively great changes of the electronic and vibrational properties of these polymers.…”