Vibrational spectroscopy of some organic compounds

Abstract

In the work reported in this thesis, Raman spectroscopy in conjunction with infra red spectroscopy and other physical techniques such as D.S.C and X-ray diffraction has been used to study the structure of organic compounds and polymers. 

Characteristic Raman frequencies have been deduced for a variety of molecular backbones and groups belonging to substituted benzenes. These have been shown to be particularly useful where polar substituents such as carbonyl and nitro groups are involved. Nevertheless, the joint use of Raman and IR frequencies usually provides much more information than either Raman or IR. 

Considerable expertise has been developed in the synthesis of a large number of polymers and their subsequent handling and processing. The observation of correlation band splitting and longitudinal acoustic modes which occur at very low frequencies in the Raman spectrum have been particularly emphasised because of their significance to the interpretation of polymer structure. Conclusions reached from the analysis of vibrational spectra of polyethers and polythioethers of the series [(CH₂)_m-Y]_n (m = 1-6,10; Y = 0 or S), may be summarised as follows: A careful analysis of the molecular vibrations of these polymers has been carried out before looking for correlation band splitting in their spectra. Assignments for the observed bands have been made on the basis of reported calculations or on tentative basis. Correlation band splitting has been observed in the spectra of polyethylene oxide, polydecamethylene oxide and polythicethers with m = 2-6 and 10. It has been shown, however, that its occurrence in the spectra of polydecamethylene oxide and polythioethers with m ≥ 4 is dependent upon the mode of crystallization and annealing. These observations have been applied to deduce molecular and crystal structures of these polymers. 

The longitudinal acoustic nodes observed at very low frequencies in the Raman spectra of polyethers with m = 4, 6 and 10 and polythioethers (with m 2-6 and 10) indicate that the crystallization of these polymers is predominantly lamellar. Crystallization from solution result in very thin lamellae which thicken on annealing just below T_m. Melt crystallization, however, gives rise to well developed lamellae which thicken further on annealing. Although the application of Raman technique to this field is limited because of experimental difficulties, it offers a number of advantages over other techniques such as low angle X-ray diffraction electron microscopy etc. 

It is concluded that Raman spectroscopy is a powerful analytical technique which when applied in conjunction with other techniques, yields very much valuable information about the structure of organic compounds and polymers.

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