TEMPERATURE-DEPENDENCE OF THE VIBRATIONAL-SPECTRA OF THALLIUM(I) ALKANOATES

IR spectroscopy has been used to model the stepwise melting process of nine Tl(I) n-alkanoate salts from the solid phase into the isotropic liquid. This study has provided an explanation of the thermal effect observed by differential scanning and adiabatic calorimetry as an enhancement in the normal sigmate shape heat-capacity morphology. Infrared spectra show that these salts exist at low temperature as crystals with a subcell different from orthorhombic or monoclinic formation (no factor group splitting observed). In the lowest solid-to-solid phase transitions, the alkyl chains remain, mainly, in a totally trans-planar conformation, but the concentration of nonplanar conformers increases continuously as the temperature rises, and at a particular value (different for each compound), the CH, wagging progression bands disappear. The temperature range at which this chain ''melting'' takes place coincides with the final steps of the calorimetric enhancement of the heat capacity. Changes in wavenumbers of some characteristic bands are observed in the infrared spectra at several phase transitions found by calorimetry. Because of its enhanced sensitivity to conformational order, Raman spectroscopy also was used for several alkanoate salts.