Theoretical investigations of the γ-gauche effect on the 13C chemical shifts produced by oxygen atoms at the γ position by quantum chemistry calculations

The γ-gauche effect on 13C chemical shifts that is produced by the O atoms located at the γ positions has been evaluated by quantum chemistry calculations based on the GAIO–CHF procedure. The γ-gauche effects produced by the O and Cl atoms in n-propanol and n-propyl chloride are found to be, respectively, +1.4 and −0.7 ppm, whereas that due to the C atom in n-butane is −3.0 ppm in good agreement of the values previously calculated. The apparent cause of such a difference in the γ-gauche effect is mainly relatively higher shielding of the CH3 carbon in the trans conformation for the n-propanol and n-propyl chloride. Extending the n-propanol chain at both ends causes no significant change in the γ-gauche effect produced by the O atom. In 2-butanol and 2-methyl-2-butanol as examples of secondarily and tertiarily substituted compounds, the γ-gauche effects produced by the γ-OH groups are estimated to be −7 to −9 ppm. In addition, the γ-gauche effect due to the C atom is found to increase in n-butane, secondary, and tertiary butanols in this order. The γ-gauche effect produced by the O atom in hydroxyethylcyclohexane is as negligibly small as −0.7 ppm, whereas that produced by the C atom in ethylcyclohexane is about −5 ppm. These results suggest that the γ-gauche effect, including downfield shift, produced by the O atom in a compound greatly depends on its chemical structure, whereas upfield shifts of −3 to −7 ppm are induced in all examined compounds as the γ-gauche effect due to the C atom.