1H Chemical Shifts in NMR. Part 191. Carbonyl Anisotropies and Steric Effects in aromatic aldehydes and ketones.

anthranaldehyde and benzosuberone were obtained and completely assigned in CDCl3 and DMSO solution. In anthrone a keto-enol tautomerism (anthrone/9-hydroxyanthracene) was observed by NMR in hydrogen bonding solvents but not chloroform. The % of enol is linearly dependant on the Kamlett β hydrogen bonding parameter of the solvent, not the solvent relative permitivity. The chemical shift data allowed the determination of the carbonyl substituent chemical shifts (SCS) in these molecules. These were analysed in terms of the carbonyl electric field, magnetic anisotropy and steric effects for long-range protons together with a model (CHARGE7) for the calculation of the two-bond and three bond effects. The SCS of the carbonyl bond was reproduced with an asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond with values of ∆χ parll and ∆χ perp of 6.36 and –11.88 (10-30 cm3/molecule) plus a steric term from the oxygen atom and the C=O electric field effect. The short range effects of the carbonyl group on the aldehyde proton were modelled using the appropriate β functions in the CHARGE routine. For the 9-substituted anthracenes the Hückel π calculation was modified to account for the 1H chemical shifts of the H-10 protons. This model gave a comprehensive calculation of th