Change of molecular configuration in crystalline decanoic acid as studied by infrared spectra and normal coordinate analysis

Abstract

Infrared spectra of a well‐oriented crystalline film of decanoic acid were obtained in the range from room temperature to liquid‐helium temperature. Many absorption bands due to the carboxyl group and a band progression due to the CH₂ wagging modes appear as doublets. In each doublet, one component decreases in intensity with lowering temperature and disappears near liquid‐helium temperature (type I), while another component increases in intensity with lowering temperature (type II). It is confirmed by normal coordinate analysis that the type I and II bands are ascribed to fundamental vibrations of the cis and trans isomers, respectively. The latter isomer would be produced from the former by double proton transfer in the decanoic acid dimer. From the temperature dependence of band intensities, the enthalpy and entropy differences between the two isomers are evaluated to be about 240 cal⋅mol⁻¹ and 3.8 cal⋅mol⁻¹⋅deg⁻¹, respectively.