Intra- and intermolecular pi-type hydrogen bonding in aryl alcohols: UV and IR-UV ion dip spectroscopy

The structures of benzyl alcohol, its 1:1 water complex, and its dimer have been investigated by R?PI spectroscopy and IR-UV ion dip spectroscopy, com bined 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 CI-C, bond. but the extent of pi-type intramolecular H-bon ding is small. Analysis of its rotational band contours suggests the incide nce of vibronic coupling involving motion of the side chain and also lends to an estimate for the dihedral angle tau(1)(OCCC) lying in the range 35 de grees-60 degrees, in good agreement with the values (50 degrees-60 degrees) indicated by high-level ab initio calculations. The 1:1 water complex is a ssigned to a structure in which water binds as a proton acceptor to the alc ohol group, and as a weal; proton donor to the pi-system of the aromatic ri ng, The arrangement of H-bonds is similar within the dimer: the OH of one m olecule acts as both acceptor to the alcohol group and as donor to the pi-s ystem 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 expans ion: they have AGt and GGt conformations about their C-alpha-C-beta (anti/g auche), C-beta-C-gamma (anti/gauche), and C-gamma-O (trans/gauche) bonds th at do not involve any OH ...pi bond. The consequences of increasing chain l ength for the formation of OH ...pi bonds is discussed with reference to be nzyl alcohol, 3-phenylethanol, and 3-phenylpropanol. The short sidechain of benzyl alcohol permits only a very weak OH ...pi interaction. The extra me thylene units of 2-phenylethanol and 3-phenylpropanol provide enough flexib ility fur significant OH ...pi interactions to be possible, but only in 2-p henylethanol does this lead to a strong energetic preference for the H-bond ed conformer: the interaction energy gained via the intramolecular H-bond i n 3-phenylpropanol is negated by the strain induced in the aliphatic chain.