An infrared study of zinc, cadmium, and lead salts of some fatty acids

The IR spectra between 1800 and 700 cm−1 of zinc, cadmium and lead carboxylates of even numbered chain length from C6 to C18 inclusive, have been measured as a function of temperature from ambient temperature to a few degrees above the melting point. From the COO− stretching frequencies it is deduced that the coordination of the carboxylate group to the metal ions is unsymmetric chelating bidentate. The only significant change observed prior to the transition point is a decrease in the intensity and a broadening of the progression of the narrow bands between 1350 and 1150 cm− and the band at ca. 720 cm−, most of which virtually disappear in the melt. Above the melting point there is an abrupt change in the shape of the COO − stretching bands. A new band at 1634 cm− is observed in zinc salts. This band has been attributed to the chelating bidentate structure acquiring some monodentate character as the temperature increases. It is found that the spectral changes due to thermal effects are reversible.