Conformations of Unsaturated and Aromatic Alcohols; Intramolecular Hydrogen Bonding in 2-Phenylethanol.

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Abstract
The conformational composition of 2-phenylethanol has been investigated by 1H NMR and IR spectroscopy of solutions at low concentration in CCl4 and CFCl3. The relative energies of the stable conformations have been estimated by molecular mechanics calculations (MMPMI in the dipole-dipole electrostatic mode and with charge-charge interaction) and by semi-empirical (MNDO) and ab initio (STO-3G and 4-31G) molecular orbital calculations. Comparison of the results from the calculations with the experimental data shows the MMPMI with charge-charge interaction to represent the best model for the conformational composition. The major conformation of 2-phenylethanol (ca. 45%) has a gauche relationship both around the CH2CH2 and the CH2OH bonds, with the hydroxy proton pointing towards the phenyl ring (conformation C). This conformation is stabilised to the extent of ca. 1.2 kcal mol-1 over the other ones. By comparison with the IR spectra in the OH region and the energetics of other unsaturated alcohols, the conformational stability of C is found to be caused by an intramolecular hydrogen bond between the hydroxy group and the .pi.-electrons of the phenyl ring.

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