Nuclear Quadrupole Coupling Constant and Conformational Studies*

A molecular conformation study on three 3-monosubstituted derivatives o f propene has been carried out by evaluating the EFG at the site o f the halogen nucleus using a semi-empirical LCAO-MO-SCF method. Calculations are performed on several conformers at INDO and CNDO level, using “sp” and “spd” valence basis sets, with Slater type orbitals as the basis functions. The results obtained for various conformers indicate that the nuclear quadrupole coupling constant is practically independent o f the orientation o f the CH 2X group (X = Cl, Br and I) (Figure 1). The mean value o f e2 Qqz:/h (averaged over all conformers) in each system matches favourably with the experimental value, despite the severe approximations that are inherent in the semi-empirical theory as well as in its