Conformational behaviour of 1,2-dichloroethane and 1,2-dibromoethane: 1 H-NMR, IR, refractive index and theoretical studies.

Experimental and theoretical methods have been used to study the energy difference between the gauche and the trans conformational isomers of 1,2-dihaloethanes (XCH 2 CH 2 X, X  =  Cl and Br). Nuclear magnetic resonance (NMR) analysis, infrared (IR) spectra and refractive index of solutions of 1,2-dichloroethane (DCE) and 1,2-dibromoethane (DBE) in environments of varying polarity have been investigated. Molecular mechanics (MM2) calculations, semiempirical methods (AM1, PM3, MNDO, MNDO-d), ab initio methods (HF, MP2) and density functional theory (B3LYP, MPW1PW91) have been performed to study the relative energies of the two stable conformations of the 1,2-dihaloethanes. The solvent effect has been studied using the Onsager and IPCM models. The basis sets 6-31G(d), 6-31+G(d) and 6-311++G(d,p) were used in the ab initio and density functional theory (DFT) calculations. All calculations have been done on a PC, using facilities available from ChemOffice 2000 and Gaussian 98W. The results obtained show that there is a satisfactory correlation between experimental findings and theoretical predictions. [ABSTRACT FROM AUTHOR]