Raman intensity and conjugation with participation of ordinary σ‐bonds

Abstract

This review covers Raman investigations over a wide range, but focused on the problem of conjugation with participation of ordinary s-bonds. The results show that s-s conjugation in a novel class of polymers, polydiakylmetallanes of the type [R 2 M] n , M = Si, Ge, Sn, studied recently, and s-p conjugation in some organometallic compounds of Si, Ge and Sn, studied long ago, both manifest themselves in the Raman spectra qualitatively in a similar way to p-p conjugation in polyenes and lead to a significant increase in intensity of the lines, corresponding to stretching vibrations of the conjugated bonds, the effect being strongly dependent on mutual orientation of the latter. It was found that pre-resonant enhancement of some lines in the Raman spectra of polymetallanes is exhibited only by macromolecules having a planar zig-zag all-anti conformation of the backbone. Less ordered conformations do not show this effect, in spite of the proximity of the position of their lowest energy electronic absorption band n e to the exciting line n. Thus, proximity of n e to n in the resonance denominator is shown to be necessary but not sufficient for the pre-resonant increase in Raman intensity to occur. These experimental findings are in good accord with Shorygin's general views on the conjugation phenomenon and with his semi-classical treatment of the Raman intensity problem, confirming an important role of the derivative @n e /@Q i that reflects the specificity of molecular electronic structure.

Effects of volatile–char interactions on in-situ destruction of nascent tar during the pyrolysis and gasification of biomass. Part II. Roles of steam

“…7a). This could be taken as an indication of higher contents of O-containing groups on the char for the ''Reforming with char'' mode than those for the ''Pyrolysis'' and ''Reforming without char'' modes, because the presence of O-containing groups could increase the Raman area/intensity via a resonance effect with the aromatic rings [47]. In other words, additional O-containing groups were formed during the steamchar reactions.…”

Regularities and Peculiarities of Solid Polydialkylsilane Order-Disorder Transitions as Studied by Optical (UV, Raman and IR) Spectroscopy

“…350 nm as well as those with UV bands at ca. 375 nm both give Raman spectra enhanced by pre-resonance, in contrast to all the other modifications studied [11,19,[21][22][23].…”

“…The aim of this paper is to describe characteristic features of polydi-n-alkylsilane transitions based on generalization from the authors' experience [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] and literature data, as well as to demonstrate that the three types of ordering are not necessarily interrelated.…”

Characterization of Conjugation Effects in the Series of Quinoxaline-2-ones by Means of Vibrational Raman Spectroscopy

“…Such an approach based on the combinations of experimental and theoretical protocols, allows both a reliable assignment of the observed Raman bands and, more importantly, a semiquantitative evaluation of the conjugation strength within the molecular backbone. ,,− Indeed, the intensity of Raman bands is closely connected with the extent of the conjugation within a given molecule, and, as it will be briefly recalled in the following section, conjugated systems show a many-fold increase in the intensity of the polarized bands corresponding to the symmetric stretching vibrations of the bonds participating in the conjugation. Thus, the intensities of these enhanced Raman bands can be considered as observables providing an estimation of the strength and of the length of conjugation. ,− Moreover Raman spectroscopy provides an efficient tool tightly interconnecting conjugation effects with the inherent characteristics of the different excited electronic states, since the intensity of a selected Raman band formally depends on the properties, and especially the transition dipole moments, of all the electronic excited states. ,− Hence, in this work through the analysis and the identification of the most conjugation-sensitive Raman scattering bands in a series of novel Q-derivatives we provide a practical way to achieve a semiquantitative characterization of the conjugation length of these compounds, while identifying the effect of conjugation on the optical properties of the Q-derivatives. Our results and the combined use of Raman spectroscopy and molecular modeling and simulation represent a facile, yet rigorous way to assess the conjugation length that can be consistently used as a rational molecular design tool.…”