1H chemical shifts in NMR: Part 19. Carbonyl anisotropies and steric effects in aromatic aldehydes and ketones

Abstract

AbstractThe 1H chemical shifts of benzaldehyde, 2‐chloro‐, 2‐hydroxy‐ and 2‐methoxybenzaldehyde, acetophenone, 2‐methoxy‐ and 2‐hydroxyacetophenone, indanone, anthraquinone, fluorenone, anthrone, α‐tetralone, 2,4,6‐trimethylacetophenone, 9‐acetylanthracene, 9‐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 percentage of enol is linearly dependent on the Kamlett β hydrogen bonding parameter of the solvent, and not the solvent relative permittivity. 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‐ 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 Δχpar and Δχperp of 6.36 and −11.88 (10−30 cm3 molecule−1) 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 the 1H chemical shifts of these aromatic aldehydes and ketones. For the data set of 129 chemical shifts ranging from δ 2.5 to 11.5 the r.m.s. error of the observed vs calculated shifts was 0.094 ppm. The CO anisotropy and oxygen shielding differ appreciably from the corresponding values for the aliphatic aldehydes and ketones but are similar to the values for the CO group of amides, illustrating the effect of conjugation on these parameters. The model was used in the conformational analysis of some related compounds. In 2‐chlorobenzaldehyde the chemical shift calculations support a non‐planar molecule with the aldehyde–ring dihedral angle in the trans conformer of ca 25°. In the strained seven‐membered ring of benzosuberone, the model was used to test calculated geometries. The ab initio geometry at the B3LYP(6–31++G(d,p)) level gave the best agreement with the observed shifts. Copyright © 2002 John Wiley & Sons, Ltd.