A STUDY OF THE ELECTRONIC PROPERTIES OF SOME HETEROCYCLIC COMPOUNDS AND THE THERMAL DECOMPOSITION OF METHYLATED THIOBUTYROLACTONES
This dissertation could be broadly divided into two parts. The first part investigated the electronic structures of some biheterocyclic compounds and substituted thiophenes. The second discussed the pyrolyses of three methylated thiobutyrolactones. The electronic structures of some biheterocyclic compounds containing various combinations of selenophene, thiophene and furan were studied by the technique of ultraviolet photoelectron spectroscopy and from these studies, we want to predict the electrical conductivity of their corresponding polymers. From the photoelectron spectra, four bands corresponding to four ? molecular orbitals (MO) were located in the low energy region between 7 and 12eV. Careful examination of the Hel/Hell band intensity variations and band profiles showed that two of these orbitals were delocalized over two rings whereas the other two were predominantly localized on a single ring. From the ionization energies of the ? MOs and the energy gap between HOMO (highest occupied MO) and LUMO (lowest unoccupied MO), from UV-VIS spectra, the corresponding polymers of several biheterocyclic compounds were predicted to have slightly higher electrical conductivities than polythiophene. The electronic structures of several substituted thiophene compounds were investigated. From these studies we wish to deduce their relative suitability for forming biologically active complexes. It was found that, substituted thiophene compounds with higher (lower) HOMO ionization energies may be poorer (better) candidates for the formation of biologically active complexes. The thermal decomposition of three methylated thiobutyrolactones was investigated by the on-line detection of various pyrolysis products using ultraviolet photoelectron spectroscopy. The pyrolysis of the unmethylated analogue had been done previously, and it was of interest to us to determine the different degradation pathway, if any, of the methylated compounds. We discovered that butadiene, which was not produced in the thermal decomposition of the unsubstituted analogue, was a major product in the pyrolysis of the methylated thiolactones. To account for the production of butadiene, a new mode of decomposition was proposed. This reaction involved the simultaneous production of butadiene, carbon monoxide and hydrogen sulphide. Apparently, the latter compound was not solely produced from secondary reactions as previously anticipated. The experiments also suggested the possible existence of methylthietane and methylcyclopropane which were probably formed from the cyclization of biradical precursors. However, the amounts of decomposition products generated from further fragmentation of these cyclic intermediates indicated that these compounds may be produced in small quantities.
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