17O, 13C and 1H NMR and IR spectral study of crowded ketones: possible intramolecular C—H···O interactions

Unusual behaviour was observed in the study of the ¹⁷O, ¹³C and ¹H NMR and IR spectra of crowded (1‐adamantyl)alkyl ketones. As the size of the alkyl substituent is increased, abnormal upfield chemical shifts in the ¹³C NMR and downfield shifts in the ¹⁷O NMR of the carbonyl group, as well as downfield shifts in the ¹H NMR of the adamantyl γ'‐protons, are found. In the IR spectrum, the ν_C?O stretching frequencies of the ketones with bulky substituents show considerable red shifts. Correlation of the NMR shifts with the number of γ‐carbon atoms of the alkyl substituents and comparison with the IR results indicated that there is an intramolecular through‐space CH···O interaction in crowded ketones. This was supported by the results of ab initio calculations. Copyright © 2002 John Wiley & Sons, Ltd.