Gas-phase conformations of 3-buten-2-ol from density-functional theoretical results together with electron-diffraction and vibrational spectroscopic data

The conformational space of 3-buten-2-ol (BUO), governed by the torsion ang les phi (1)(O-C-C=C) and phi (2)(H-O-C-C(sp(2))), was searched, and nine en ergy minima were located, using the B3LYP/6-31**G approach. Then, geometrie s of these nine and the vibrational characteristics of the four lowest ener gy forms were calculated. Experimental evidence of the latter four conforme rs in the gas phase was obtained from IR frequencies, band intensities, and band shapes measured at room temperature. Analysis of electron-diffraction intensities also measured at room temperature gave the following abundanci es (with nomenclature pertinent to the S configuration): (+ac, -sc), 58%; ( sp, -sc) and (sp, +sc) together, 32%; (-ac, Ssc), 10%. The energy sequence as well as conformationally induced variations in bond lengths, valence ang les, and OH frequencies could be rationalized by attractive (hyper)conjugat ive and repulsive steric effects operative in BUG. The intermediacy of BUO between allyl alcohol and I-butene shows in the observation that C2-H, C2-O H, and C2-CH3 eclipse the C=C bond in the (+ac, xx), (sp, xx), and (-ac, xx ) conformers, respectively. In all four conformers, OH points toward C=C, i ndicative of an attractive intramolecular OH/pi (C=C) interaction. Experime ntal and calculated wavenumbers of the individual conformers could be match ed satisfactorily, after scaling the four B3LYP force fields using only thr ee transferable scale factors published by Rauhut and Pulay.