Studies on the conformational equilibria of a series of dihalosubstituted cyclohexanes (I) and halocyclohexanones (II) are reported. Theoretical calculations using the B3LYP method and the 6-311 + g(d,p) basis set were applied to determine the energy differences (DeltaE) between the conformers of I and II, as well DeltaE between the conformers of monohalocyclohexanes, in order to obtain the intramolecular interaction energies, which govern the conformational equilibria of I and II. It is shown that while the intramolecular interactions for 1,3- and 1,4-disubstituted cyclohexanes, as well as for 3- and 4-halocyclohexanones, are due to classical effects (steric and electrostatic), for the 2-halocyclohexanones it becomes evident that orbital interactions between n(halo) and pi(CO)(*) drive the equilibria towards the axial conformers. Moreover, the gauche effect seems to be irrelevant for the trans-1,2-dihalocyclohexane equilibria, opposite to several reports from the literature on other trans-1,2-disubstituted cyclohexanes bearing electronegative substituents. (C) 2002 Elsevier Science B.V. All rights reserved.589147151