Theoretical Aspects of the Vibrational Spectra

6) THEORETICAL ASPECTS OF THE VIBRATIONAL SPECTRA OF POLYCONJU6ATED MOLECULES. 

A unifying theory which explained some of the observations listed in section 5 of this chaper has been first presented by Horo-vitz [80,93] and his collaborators who have treated mainly the case of trans polyacetylene. This theory has been the basis for the qualitative understanding of the spectra of many more complex polyconjugated 

For a long time X remained a useful, but exhotic, parameter which could hardly be understood in molecular terms. More recently it has been shown [14-17] that can be related to some chemical force constants which reflect the state of conjugation of the molecule. 

In section 3.3 we have shown that the Raman spectra of Polycon-jugated materials arise from resonance scattering of the totally symmetric modes a few of which are selectively enhanced if they contain a contribution by the coordinate which describes the changes of geometry from the ground to the excited electronic state. 

As clearly described in paper I and in refs.3 and 14-17 SI has been defined empirically and rigorously (from MO calculations on the excited state of short oligomers [94]) for polyacetylene. These concepts have been extrapolated for the case of polyaromatic systems thus making possible to defineas a linear combination of the chemical internal coordinates R associated with the stretching of the C-C and C=C bonds. 

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