nu(CO) frequencies and intensities of some alpha-heterosubstituted carbonyl compounds [XCH(2)C(O)Y: X = F, OMe, NR(2), Cl, Br, SEt, or I; and Y = Me, Ph, SR, OMe or NEt(2)], together with molecular mechanics calculations indicated the existence of cis-gauche rotational isomerism. In solvents of low polarity the gauche rotamers predominate over the cis ones, except for the fluoro and methoxy derivatives of acetophenone and methyl acetate series. The progressive increase in the gauche rotamer population in each series, on going from the fluoro to the iodo derivative has been mainly ascribed to the increasing contribution of the pi*(CO)/sigma(C-X) and pi*(CO)/n(X) orbital interactions. The carbonyl frequency shifts of the cis rotamers are interpreted as being due to the substituent field and inductive effects, while the corresponding shifts of the gauche rotamers have been attributed to an interplay of inductive and hyperconjugative effects. The carbonyl shifts induced by inductive (Delta nu(I)), field (Delta nu(F)) and hyperconjugative effects (Delta nu(H)) were estimated separately for the acetone derivatives. The larger negative carbonyl gauche shifts (Delta nu(g)) along with a higher non-additivity effect (Delta delta) of the alpha-methylene carbon chemical shifts can be associated with stronger pi*(CO)/sigma(C-X) and pi*(CO)/n(X) orbital interactions. The higher stabilization of the gauche rotamers for the alpha-akylthio carbonyl compounds, the larger nu(CO) gauche shifts and the lower non-additivity effect for the alpha-methylene carbon have been interpreted as being due to the simultaneous occurrence of pi*(CO)/sigma(C-S) and pi(CO)/sigma*(C-S) orbital interactions. The progressive bathochromic shifts of n(O) --> pi*(CO) transition for the alpha-heterosubstituted ketones, on going from the fluorine to the iodine substituent, were mainly ascribed to a contribution of the hyperconjugative interaction (pi*(CO)/sigma*(C-X)). Ab initio calculations, and photoelectron and electron transmission spectroscopies have supported, in general, the mentioned orbital interactions.15115159