Chemical applications of Fourier-transform Raman spectroscopy

Abstract

Details are presented of five studies in the application of Fourier transform (FT) Raman spectroscopy. The studies encompass a diverse range of chemical species, and indicate the wide applicability of FT-Raman methods.

A series of coloured sulphur-containing organic compounds based on the thione ring structure were analysed. Characteristic group frequencies were determined, enabling the thione ring structure to be easily identified. Spectral changes on reaction of thiones or related oxones to yield tetrathiafulvalenes (TTFs) were discovered, whilst the differentiation of thione and oxone species was shown to be straightforward.

The novel allenylketenimine-tetracyanoethylene (TCNE) system was investigated, in order to provide the first vibrational assignment of these compounds. Characteristic frequencies of the underlying cyclic structure were determined. The extremely weak IR absorption of the nitrile (-CN) group was noted, contrasted against the reliable Raman signature, and discussed.

The first study of the hydrated y-alumina surface was undertaken in a combined FTIR/FT-Raman study, that unambiguously determined the surface to comprise the bayeritic polymorph of aluminium hydroxide. The temperature and pH-dependency of the formation of the hydroxide were studied. The role of phenylphosphate in the suppression of hydration was discussed.

Near-IR excitation was shown to produce anomalous luminescence from a series of calcium silicate cement minerals. Investigations using near-IR diffuse reflectance suggested a connection with the presence of metal ions impurities (especially manganese) and calcium hydroxide. Calcium oxide and hydroxide were shown to display similar anomalous behaviour. The application of conventional Raman spectroscopy in this area was appraised as an alternative, and shown to be successful.

A method for obtaining rotational and vibrational spectra of gases on a commercial FT-Raman spectrometer using multiple-reflection cells was designed, developed, and appraised. The technique was then applied to a previously-intractable system (NO2/N2O4) to record the first vapour-phase spectrum of the dimer. In addition, the data suggests that the NO minority species scattering may be strongly-enhanced, and that NO? may be unenhanced at this wavelength.

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