INTERNAL-ROTATION IN 1,2-DI-(P-BROMOPHENYL)ETHANE - INFRARED-SPECTRA AND NORMAL-COORDINATE CALCULATIONS

Infrared (4000-250 cm(-1)) spectra of 1,2-di-(p-bromophenyl)ethane (DP BPE) in crystalline phase, liquid and solutions have been investigated . Molecular mechanics study was performed in order to reveal stable co nformations of DPBPE. Six unequivalent minima on the potential energy surface were found, and the normal coordinate analysis of these rotati onal isomers has been carried out. The conformer which is present in t he most stable crystalline modification is shown to have the p-bromoph enyl groups in trans positions, the phenyl rings being nearly orthogon al to the C-Ph-C-C'-C'(Ph) plane. The experimental spectra in combinat ion with the normal coordinate analysis show the evidence of at least three more stable conformers in the liquid and solutions. Rotations ar ound both C-Ph-C-aliph and C-aliph-C'(aliph) bonds were found to be re sponsible for the appearance of these conformers. The approximate assi gnment of the IR bands to the conformers is given. (C) 1997 Elsevier S cience B.V.