Polyenes Raman spectra

Resonance Raman spectroscopy has been applied to studies of polyenes for the following reasons. The Raman spectrum of a sample can be obtained even at a dilute concentration by the enhancement of scattering intensity, when the excitation laser wavelength is within an electronic absorption band of the sample. Raman spectra can give information about the location of dipole forbidden transitions, vibronic activity and structures of electronically excited states. A brief summary of vibronic theory of resonance Raman scattering is described here. 

Parker, S.F. Tavender, S.M. Dixon, N.M. Herman, H.K. Williams, K.P.J. 8c Maddams, W.F. Raman Spectrum of /3-Carotene Using Laser Lines from Green (514.5 nm) to Near-Inlfared (1064 nm) Implications for the Characterization of Conjugated Polyenes Appl. Spectrosc. 1999, 53, 86-91. 

Williams el al. [421] have assessed the potential of UVRRS as a general analytical tool, both to distinguish between molecules with similar electronic absorptions, and to wavelength tune the laser to enhance selectively the Raman spectrum of individual components in a complex mixture. Raman spectroscopy provides a method whereby the distribution of polyene sequence lengths can be monitored. Not surprisingly, resonance Raman is also a major tool for characterising conducting polymers [422]. 

Equations (15) and (16) define the so-called effective conjugation coordinate of ECC theory. The important conclusion that can be derived from Eqs. (11) and (17) is that only normal modes that contain an oscillation of the dimerization amplitude ( R vibration) can have relevant Raman cross sections. Then the two relevant lines of the Raman spectrum of polyenes are necessarily assigned to normal modes that involve a large contribution by the fl oscillation (FI modes in the ECC theory). More precisely, the treatment of the dynamical problem of polyenes in terms of ECC theory assigns the two strong Raman lines to two different combinations (in-phase and out-of-phase) of the R oscillation with C—H wagging vibration. 

No resonance Raman spectra are observed for uncolored PVC films. The polyene chains, of varying chain length, have a broad electronic absorption in the visible region which supports a strong resonance Raman enhancement of vibrations associated with the — C—C—C— backbone [64]. The resonance Raman spectrum of degraded... 

It has already been pointed out that a modulation of the structure is related to a modulation of the intensity pattern. Starting from a centrosymmetric polyene structure (alternated), it is known that modes have selectively enhanced Raman intensities and vanishing infrared activities. As soon as electrical symmetry is broken, a variation of the dimerization is induced, thus determining an enhancement of the Raman intensity and the simultaneous activation of the modes in the infrared spectrum. This means that the molecule becomes /3-active and its NLO response increases as the degree of alternation decreases. Thus 3 reaches a maximum value after which the Raman intensity starts decreasing... 

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