Origin of the cis effect-nonbonded intramolecular interactions:: quantum chemical studies on 1,2-dihaloethylene molecules

The cis effect present in 1,2-difluoroethylene and 1,2-dichloroethylene molecules have been studied using ab initio and density functional theory methods. The cis and trans conformers of these molecules are optimized at HF, MP2, B3LYP and B3PW91 levels of theory. The standard basis sets 6-31G(d) and 6-31 + G(d) have been used. Vibrational frequencies and thermodynamical quantities calculated at MP2/6-31G(d) level of theory are comparable with the previous experimental values. The C-13-NMR chemical shift values are calculated at B3PW91/6-311 + G(2d,p)//B3PW91/6-31G(d) level of theory. The chemical hardness and chemical potential values are calculated and the maximum hardness principle is examined for the cis effect. (C) 2002 Elsevier Science B.V. All rights reserved.