Conformational properties of benzyl alcohol in dilute solution

Abstract

STO-3G MO computations, with substantial geometry optimization, are reported for benzyl alcohol. They are performed at 15° intervals for the angle by which the C—O bond twists out of the plane of the phenyl group, as well as for 30° intervals for the HCOH dihedral angle. The ensuing conformational energies are used to estimate expectation values of these two angles as well as of functions of these angles. The latter are compared with those obtained from long-range proton–proton coupling constants for dilute solutions in CS2 and acetone-d6. At infinite dilution in CS2 these coupling constants give a distribution for motion about the [Formula: see text] bond that happens to agree with the theoretical results for the free molecule. The latter are used to derive the dihedral angle dependence of 3J(HCOH), the coupling constant between the hydroxyl and methylene protons. This dependence is compared with the observed coupling constants in benzyl alcohol in the gas phase at 422 K and with those measured for the 2,6-difluoro and 2,6-dichloro derivatives of benzyl alcohol. Infinite dilution shifts are reported for the latter two compounds and for α,α-dimethylbenzyl alcohol. The first self-association complex of any significance for benzyl alcohol in CS2 at 300 K is most likely a trimer. Between zero and 2.5 mol% in CS2, all five proton chemical shifts of this molecule are linearly proportional to one another, four of them becoming less shielded at decreasing concentrations. Some nuclear magnetic resonance data are also reported for 3,5-dichlorobenzyl alcohol. Keywords: conformations of benzyl alcohol, 1H NMR of benzyl alcohol, STO-3G MO calculations of benzyl alcohol.