SUPERSONIC JET STUDIES OF BENZYL ALCOHOLS - MINIMUM ENERGY CONFORMATIONS AND TORSIONAL MOTION

Supersonic jet mass resolved excitation spectroscopy is employed to determine the minimum energy conformations of benzyl alcohol and a series of nine methyl-, ethyl-, fluoro-, and aminobenzyl alcohols. The interpretation of the mass resolved excitation spectra of these molecules leads to the assignment of specific molecular geometries for each system. The minimum energy conformation of the -CH2O moiety is determined to be perpendicular relative to the plane of the aromatic ring, i.e., tau(C(ortho)-C(ipso)-C-alpha-O) = 90-degrees. The hydroxy proton in the sterically unencumbered benzyl alcohol points toward the benzene ring. The potential energy barrier for the low-frequency torsional motion of the hydroxymethyl group arises mainly from an internal hydrogen-bonding interaction between the OH group and pi-system of the ring. Using hindered rotor model calculations, the potential barrier to this torsional mode is determined to be V2 = -140 cm-1 for S0 and V2 = -330 cm-1 and V4 = -3 cm-1 for S1 with a CH2OH rotational constant of 0.52 cm-1 for both states. Similar potential barriers are observed for methyl-substituted benzyl alcohols. The potential energy barrier in S1 changes significantly, however, for fluoro- and amino-substituted benzyl alcohols, as these substituents interact strongly with the pi-electron system of the aromatic ring. For 2-fluorobenzyl alcohol, the nature of the low-frequency torsional mode changes to a combination of (O-H...F) hydrogen motion and -OH motion. The spectrum of benzyl fluoride is very similar to that found for benzyl alcohol, suggesting that the conformations of the two compounds are similar, e.g., tau(C(ortho)-C(ipso)-C-alpha-F) = 90-degrees.