Raman intensity and conjugation with participation of ordinary sigma-bonds

This review covers Raman investigations over a wide range, but focused on t he problem of conjugation with participation of ordinary sigma -bonds. The results show that sigma-sigma conjugation in a novel class of polymers, pol ydiakylmetallanes of the type [R2M](n) M = Si, Ge, Sn, studied recently, an d sigma-pi conjugation in some organometallic compounds of Si, Ge and Sn, s tudied long ago, both manifest themselves in the Raman spectra qualitativel y in a similar way to pi-pi conjugation in polyenes and lead to a significa nt increase in intensity of the lines, corresponding to stretching vibratio ns of the conjugated bonds, the effect being strongly dependent on mutual o rientation of the latter. It was found that pre-resonant enhancement of som e lines in the Raman spectra of polymetallanes is exhibited only by macromo lecules having a planar zig-zag all-anti conformation of the backbone. Less ordered conformations do not show this effect, in spite of the proximity o f the position of their lowest energy electronic absorption band nu (e) to the exciting line nu. Thus, proximity of nu. to nu in the resonance denomin ator is shown to be necessary but not sufficient for the pre-resonant incre ase in Raman intensity to occur. These experimental findings are in good ac cord with Shorygin's general views on the conjugation phenomenon and with h is semi-classical treatment of the Raman intensity problem, confirming an i mportant role of the derivative partial derivative nu (e)/partial derivativ eQ(i) that reflects the specificity of molecular electronic structure. Copy right (C) 2001 John Wiley & Sons, Ltd.