Infrared examination of rotational isomerism in alkyl furan-2-carboxylates : determination of thermodynamic parameters from infrared data

Abstract

Solutions of 66 alkyl furan-2-carboxylates in carbon tetrachloride and in acetonitrile have been examined at high dispersion in the i.r. CO region. In carbon tetrachloride one ester shows four bands, eight show single bands, and 57 show well separated doublets. With one exception, the doublets are the consequence of rotational isomerism between syn-s-trans- and anti-s-trans-forms. The higher wavenumber component of a doublet arises from the form with the higher dipole moment and the higher enthalpy: this form is very probably the syn-s-trans-rotamer. The failure of attempts to obtain reliable free energy and entropy differences reveals the shortcomings of standard approaches to the determination of energy differences between rotational isomers from i.r. data.