Raman band intensities of thioethers and thiols

Abstract

Raman spectroscopy is potentially a useful technique for the analysis of thioethers and thiols because of the characteristic set of bands which appears in the C-S stretching region. A review of previous studies on Raman band intensity measurements is presented in which the experimental difficulties and influencing factors are given.An experimental system has been developed around a Coderg PHO Raman spectrometer for the measurement of absolute Raman band intensities. This includes the novel use of a Pockels cell for the accurate determination of depolarisation ratios. Carbon tetrachloride has been assessed as a suitable internal standard for use with this system.Raman band intensities are given for five thioethers and two thiols. Several bands are identified as being characteristic of sulphur-containing hydrocarbons. Analysis of bands in the C-S stretching region is accomplished by considering the intensity contributions provided by individual groups in the molecule, resulting in a successful prediction of the C-S stretching band intensities for t-butane thiol. In addition, temperature studies have been conducted for ethyl methyl sulphide in order to attempt calculation of the energy barrier between the two conformations. A computer programme has been developed for the theoretical calculation of Raman band frequencies and intensities. Molecular parameters have been derived for dimethyl sulphide, including an ab initio force field. Tests for transferabilityof these data to ethyl methyl sulphide and deuterated dimethyl sulphide have produced encouraging results. A study of infra-red intensities of thioethers has been carried out. This is an extension of some previous work in which the necessity to resolve individual bands is eliminated. For several discrete spectral regions quantification is made of the effects of a sulphur atom on C-H band intensities, thus allowing successful intensity predictions for other molecules studied.Suggestions for further work in this field are given.