Proton chemical shifts in NMR. Part 13.1 Proton chemical shifts in ketones and the magnetic anisotropy and electric field effect of the carbonyl group

Abstract

The proton resonance spectra of a variety of cyclic ketones including 2-tert-butylcyclohexanone, 4-tert-butylcyclohexanone, fenchone, trans-1-decalone, androstan-3-one, androstan-17-one, androstane-3,17-dione and androstane-3,11,17-trione were obtained and completely assigned. These data together with previous literature data allowed the determination of the carbonyl substituent chemical shifts (SCS) in a variety of cyclic molecules. These SCS were analysed in terms of the carbonyl electric field, magnetic anisotropy and steric effect for long-range protons together with a model (CHARGE6) for the calculation of the two-bond and three-bond effects.

The anisotropic effect of the carbonyl bond was found to be well reproduced with an asymmetric magnetic anisotropy acting at the carbon atom with values of Δχ_parl and Δχ_perp of 17.1 and 3.2 (10^–30 cm³ molecule^–1). This together with the electric field effect of the carbonyl group gave good agreement with the observed proton shifts without the need to invoke any steric effects. The short range effects of the carbonyl group (i.e. HCCO) were modelled by a cos θ function which was found to be dependant on the ring size of the cyclic ketone via the CC(O)C bond angle.

This model gives the first comprehensive calculation of the SCS of the carbonyl group. For the data set of ca. 200 proton chemical shifts spanning ca. 2 ppm the rms error of the observed vs. calculated shifts was 0.11 ppm.