A DESCRIPTION OF THE EFFECTS OF SILYL VS ALKYL SUBSTITUENTS IN KETONES IN THE LIGHT OF O-17 NMR DATA AND WITH THE HELP OF QUALITATIVE MO THEORY

O-17 NMR spectra (natural abundance) for selected acylsilanes show high-frequency chemical shifts with respect to the corresponding ketones. An MO qualitative analysis clarifies the origin of the above experimental finding as being due to other well-known chemical and physical differences between the two types of compound. The importance of a relatively small HOMO-LUMO gap in acylsilanes is stressed. The alkyl and silyl substituents promote different electron distributions over the CO-sigma and pi-fragment orbitals which can ultimately contribute to the shift of the NMR resonances. The MO analysis addresses new viewpoints on acylsilanes: (i) the silyl groups, rather than strong sigma-donors should be better defined as weak sigma-acceptors; (ii) the carbonyl carbon atom is negatively charged, but the electron density is accumulated in the sigma-orbitals and not in the p(pi) one; (iii) the IR data are rationalized in terms of a weakened in plane component of the CO bond, while the pi(perpendicular-to), bond is unaffected; (iv) the carbonyl carbon atom keeps its electrophilic character in acylsilanes and it can exert nucleophilicity only upon departure of SiR3+, a good leaving group.