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Raman spectra of 1,3-dithiole-2-thione and a series of related compounds were recorded on a fourier transform Raman spectrometer using near-infrared laser excitation. The spectra proved, with limited exception, to be of high quality. With the help of measured depolarisation ratios it has proved possible to make assignments for the stronger peaks in the spectrum of 1,3-dithiole-2-thione, thereby resolving ambiguities and disagreements in the results of previous workers. These assignments have been used to aid in the interpretation of the very characteristic spectra of substituted thiones. The value of these spectra for analytical purposes is discussed.
THE USE of near-infrared (NIR) laser excitation is, in many cases, the only way to obtain good quality Raman spectra of organic species that are highly conjugated, coloured, or have impurities present after work-up. When an NIR laser source is used in concert with a multiplexing spectrometer such as a Fourier transform spectrometer a powerful tool becomes available to the organic chemist; NIR Fourier transform (PT)-Raman spectroscopy is a technique that affords routine and straightforward collection of vibrational spectra, and provides data that is complementary to that obtained from Fourier transform infrared (FT IR) spectroscopy. NIR FT-Raman spectra have been collected from such diverse organic and bio-organic systems as polymers [l], peptides [2], high explosives and narcotics [3], paint systems [4] and catalysts [5].
The series of compounds studied here are based on the 1,3-dithiole-2-thione ring structure (Fig. 1).
The ten thiones in this study are important precursors in the synthesis of a number of tetrathiafulvalene (TIP) derivatives. TTFs are organic Jr-donors, and their cation radical salts are of considerable interest because of their ability to form molecular conductors and superconductors [6-g]. The NIR PI-Raman spectrum of one such TTF is discussed later.
NIR FT-Raman spectroscopy has several specific advantages over both conventional Raman and FI'IR spectroscopy of thiones and their derivatives. Conventional Raman spectroscopy normally uses a visible wavelength laser as the excitation source. With the exception of C, a dark red liquid, the compounds studied were coloured crystalline solids. Coloured samples normally absorb visible laser wavelengths to some extent; this can lead to fluorescence or sample pyrolysis, both of which render the vibrational spectrum difficult, if not impossible, to observe. NIR excitation at a wavelength of 1064 nm gives a photon energy too low to excite fluorescence in the majority of molecules, since most do not have electronic absorbtion bands in the near-infrared. This is the major advantage of NER excitation, which is realised in this work by using an Fourier transform spectrometer. FTIR spectroscopy generally requires the samples in mull form or pressed into potassium bromide (KBr) discs; the compounds in this study do not mull well, and good quality spectra from KBr discs are difficult to obtain. In comparison, the spectra of A-J obtained by the FT-Raman technique are generally of high quality.
GAYATHRI DEVI et al. [9], and IQBAL and OWEN [lo], have undertaken vibrational analyses of A. The former reports infrared spectra and the results of a normal-coordinate analysis, whilst the latter provides infrared and Raman data, together with normal