The conformations of allylic alcohols in solution from FT-i.r. OH stretching vibration measurements

• Published in: Spectrochimica acta. Part A: Molecular spectroscopy 1988, Vol.44 (12), p.1243-1249
• Main Authors: Bacon, J.F., van der Maas, J.H.
• Format: Article
• Language: English
• Subjects: Atomic and molecular physics; Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics); Exact sciences and technology; Molecular properties and interactions with photons; Physics; Properties of molecules and molecular ions
• ISSN: 0584-8539
• DOI: 10.1016/0584-8539(88)80164-8

The distribution of conformations of allylic alcohols in CCl 4 differs from that in the vapour phase and from ab initio calculations. FT-i.r. measurements of the OH stretching vibrations show two peak maxima which can be resolved by band splitting techniques. The predominant conformations of allylic alcohols are intramolecularly hydrogen bonded, with a conformation gauche ( G or G′) with respect to rotation about the CO bond and eclipsed ( E or E′) with respect to rotation about the CC bond. In contrast with vapour phase data and ab initio calculations, no other hydrogen bonded conformations have been identified. For primary alcohols, the shoulder to the higher wavenumber side can be unequivocally assigned to conformations in which the OH is free, with a trans ( T) conformation with respect to rotation about the CO bond. The secondary allylic alcohols exhibit no bands that can be attributed to free OH conformations except in the case of severe steric interaction in the eclipsed ( E) conformation. In accordance with our previous work on solvent interactions with the OH group, the contrast with the vapour state assignments are interpreted in terms of an interaction of the solvent with the OH proton which destabilizes the hydrogen bond.