Infrared spectral consequences of conformational heterogeneity in saturated alcohols

Abstract

The best explanation for the doublet character of IR OH stretching absorptions of many saturated alcohols is conformational heterogeneity. According to this theory each C—O rotational isomer (rotamer) gives rise to a symmetrical OH absorption. Therefore, as predicted, molecules with only one minimum energy rotamer have symmetrical OH spectra. When the absorption frequencies of two or more rotamers are different, the experimentally observed summation spectra are ordinarily asymmetrical. The main class of compounds studied was cyclohexanol derivatives. Asymmetrical spectra were decomposed by digital computer into two symmetrical component bands, assumed to have Lorentzian band shapes, and these bands were remarkably constant for a given rotamer type and did not depend on whether the alcohols were secondary or tertiary. Absorption bands near 3624 cm−1 in tertiary axial cyclohexanols were assigned to a type of rotamer in which the OH bond lies over the cyclohexane ring. To rationalize the symmetrical OH absorptions of secondary axial cyclohexanol, it was previously suggested that this type of rotamer was absent due to steric repulsion of the OH hydrogen with the 3,5-diaxial hydrogens of the ring. We favor an alternative explanation: that the rotamers with the OH bond over the ring and with the OH bond pointing away from the ring are both present, but have about the same adsorption frequency.