Intra- and Intermolecular π-Type Hydrogen Bonding in Aryl Alcohols: UV and IR−UV Ion Dip Spectroscopy

Abstract

The structures of benzyl alcohol, its 1:1 water complex, and its dimer have been investigated by R2PI spectroscopy and IR−UV ion dip spectroscopy, combined with ab initio computation. The sole molecular conformer observed in the jet has a gauche arrangement of the gauche arrangement of the OH group relative to the C1−Cα bond, but the extent of π-type intramolecular H-bonding is small. Analysis of its rotational band contours suggests the incidence of vibronic coupling involving motion of the side chain and also leads to an estimate for the dihedral angle τ1(OCCC) lying in the range 35°−60°, in good agreement with the values (50°−60°) indicated by high-level ab initio calculations. The 1:1 water complex is assigned to a structure in which water binds as a proton acceptor to the alcohol group, and as a weak proton donor to the π-system of the aromatic ring. The arrangement of H-bonds is similar within the dimer: the OH of one molecule acts as both acceptor to the alcohol group and as donor to the π-system of the other molecule. Reexamination of published UV band contour and IR/UV ion dip spectroscopic data on 3-phenylpropanol provides unambiguous assignments for the two conformers most heavily populated in the jet expansion: they have AGt and GGt conformations about their Cα−Cβ (anti/gauche), Cβ−Cγ (anti/gauche), and Cγ−O (trans/gauche) bonds that do not involve any OH···π bond. The consequences of increasing chain length for the formation of OH···π bonds is discussed with reference to benzyl alcohol, 2-phenylethanol, and 3-phenylpropanol. The short side-chain of benzyl alcohol permits only a very weak OH···π interaction. The extra methylene units of 2-phenylethanol and 3-phenylpropanol provide enough flexibility for significant OH···π interactions to be possible, but only in 2-phenylethanol does this lead to a strong energetic preference for the H-bonded conformer: the interaction energy gained via the intramolecular H-bond in 3-phenylpropanol is negated by the strain induced in the aliphatic chain.