1H NMR spectra of alkane-1,3-diols in benzene: GIAO/DFT shift calculations

Abstract

The proton nuclear magnetic resonance (NMR) spectra of propane-1,3-diol, 2-methylpropane-1,3-diol, 2,2-dimethylpropane-1,3-diol, butane-1,3-diol, 3-methylbutane-1,3-diol, pentane-2,4-diols (dl and meso), 2-methylpentane-2,4-diol and cyclohexane-1,3-diols (cis and trans) in benzene have been analysed. The conformer distribution and the NMR shifts of these diols have been computed on the basis of density functional theory, the solvent being included by means of the integral equation formalism phase continuum model (IEFPCM) implemented in Gaussian 09. Relative Gibbs energies of all conformers are calculated at the Perdew, Burke and Ernzerhof (PBE)0/6-311 + G(d,p) level, and NMR shifts by the gauge-including atomic orbital method with the PBE0/6-311 + G(d,p) geometry and the cc-pVTZ basis set. Vicinal coupling constants for 1,2- and 1,3-diols are rationalised in terms of relative conformer populations and geometries. The NMR shifts of hydrogen-bonded protons in individual conformers of alkane-1,n-diols show a very rough correlation with the OH⋯OH distances. The computed overall NMR shifts for CH protons in 1,2- and 1,3-diols are systematically high but correlate very well with the experimental values, with a gradient of 1.07 ± 0.01. Some values for nonequivalent methylene protons in 1,3-diols are reversed, calculation giving enhanced values for the proton anti to the COH bonds. Errors in the NMR shifts computed for the OH protons of nonsymmetrical diols appear to be related to relative populations of conformers where one or other of the OH groups is the donor. Some results based on the second-order Møller-Plesset approach, the Becke three-parameter Lee-Yang-Parr method and on the IEFPCM solvation model implemented in Gaussian 03 are included.