GIAO/DFT calculations of relative conformer energies and 1H NMR shifts of unsaturated alcohols in benzene

Abstract

Density functional theory calculations were performed on alkenols and alkynols at the PBE1PBE/6‐311 + G(d,p) level with the inclusion of solvent (benzene) effects by the integral‐equation‐formalism polarizable continuum model (IEFPCM). For the smaller molecules, conformers in which the OH group is in the vicinity of the double or triple bond are preferred, but this preference falls as the alkyl chain is lengthened. The solvent effect on the relative Gibbs energies of different conformers is irregular, and in only two cases, 3‐buten‐1‐ol and 3‐butyn‐1‐ol, is there marked levelling. Gauge‐including atomic orbital calculations, based on the PBE1PBE/6‐311 + G(d,p) geometries but using a larger basis set, cc‐pVTZ, give nuclear magnetic resonance shifts for all the protons in each conformer. Overall shifts are calculated by weighing these according to the conformer population. Calculated values are well correlated with experimental data from high‐dilution spectra in the same solvent, ranging from about 0.4 to 5.8 ppm, with a slope of 1.09 ± 0.01. Calculations on some alcohols with hetero‐atom substituents confirm that these also prefer gauche (synclinal) conformers in the gas phase, with a less marked preference in benzene. The nuclear magnetic resonance shifts, however, are calculated to be on average over 0.8 ppm higher than observed. Copyright © 2011 John Wiley & Sons, Ltd.