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Fourier transform infrared (FT-IR) spectra were measured of the sodium, zinc, and calcium metal salts of stearic acid. The normal mode analysis of sodium decanoate provides guidance on the group frequency-based band assignments for these metal stearates. The principal absorption bands are assigned to the vibrations of specific molecular groups, carboxylate and methylene (CH 2 ), and to a lesser extent methyl species. Highly characteristic group frequencies without significant mechanical coupling are observed above 1500 cm –1 . Below 1500 cm –1 , the vibrational modes of the carboxylate and alkane chain exhibit mechanical coupling. The prominent carboxylate stretching vibrations differ based on the metal, metal coordination structure, and metal-stearate polymorphic form. The nature of the mechanical coupling of the CH 2 wag is discussed in order to understand the empirical correlation between the number of observed infrared bands (1360–1160 cm –1 spectral region) and the number of carbons in the n-alkane chain.
Larkin et al. (Wed,) studied this question.