1H NMR and DFT studies of steric effects on intermolecular C–H···O hydrogen bonding in solution

Abstract Significant changes in 1 H chemical shifts (Δ δ s) for H a and H b of l -2( S )-(1-imidazolyl)-3( S )-methylpentanol (IMP) from D 2 O to CDCl 3 have been observed. H a and H b of IMP experience deshielding effects in D 2 O with respect to in CDCl 3 . In order to assess the effects on Δ δ s, comparisons of 1 H chemical shifts ( δ s) with those for 2-(1-imidazolyl) ethanol (IE) and 1-methyl imidazole (MI) as well as density functional theory (DFT) calculations on IMP· n Sol hydrogen-bonded models were performed. By analyzing the values for Δ δ (IMP−MI) in both solvents and for structural parameters for IMP, it is indicative of that the main contribution to Δ δ s is most likely produced by an intermolecular C–H···O hydrogen bonding, which has been denoted as Δ δ (C–H···O). The steric effects arising from the C 2 H 5 CH(CH 3 )–CH–CH 2 OH group of IMP favor interactions of IMP molecule with D 2 O molecules over CDCl 3 . Moreover, Δ δ (C–H···O) is suggested to be rationalized by two effects: the electronic effects, Δ δ e (C–H···O), and the steric effects, Δ δ s (C–H···O). Experiments gave Δ δ s IMP ( C – H ⋯ O ) exp for H a and H b of IMP 0.354 and 0.396 ppm, respectively. Δ δ s IMP ( C – H ⋯ O ) cal for H a and H b of IMP are calculated to be 0.347 and 0.369 ppm, respectively, using the IMP·3Sol hydrogen-bonded models. The similarity of Δ δ s IMP ( C – H ⋯ O ) exp to Δ δ s IMP ( C – H ⋯ O ) cal encourages the use of IMP·3Sol hydrogen-bonded models as appropriate solution models for Δ δ s and the cause of Δδs attributing to an intermolecular C–H···O hydrogen bond produced by the steric effects of the C 2 H 5 CH(CH 3 )–CH–CH 2 OH group.