POTENTIALITIES OF SEMIEMPIRICAL CALCULATIONS (AMPAC AND INDO S) IN DETERMINING THE CONFORMATION AND ELECTRONIC-PROPERTIES OF 2,2'-BITHIOPHENE - A NEW JOINT EXPERIMENTAL AND THEORETICAL APPROACH/

A joint experimental and theoretical approach is proposed for the conf ormational analysis of 2,2'-bithiophene in nonrigid media. Semiempiric al calculations (AMPAC and INDO/S) are used to investigate the conform ational structure of 2,2'-bithiophene. The geometry of the molecule is optimized by the AMPAC method using the AM1 (Austin model 1) Hamilton ian. It is shown that AMPAC predicts the final geometry (bond lengths and bond angles) relatively close to the experimental values reported for this molecule even though these geometrical parameters are better reproduced as expected at the ab initio level. The AMPAC and INDO/S me thods are used to determine the torsional potential curve, within the rigid-rotor approximation, on the basis of the optimized geometry. The AMPAC calculations show two minima (theta = 150 degrees and theta = 3 0 degrees) and two maxima (theta = 90 degrees and theta = 0 degrees) r eproducing the ab initio calculations. The absolute minimum is located at theta = 150 degrees, and the rotation barrier over the perpendicul ar conformation is calculated to be 0.46 kcal/mol. The energy differen ce between the two minima is calculated to be 0.25 kcal/mol. On the ot her hand, the torsional potential curve determined by the INDO/S metho d shows that the syn (theta = 0 degrees) and anti (theta = 180 degrees ) conformers are quasi-isoenergetic and the most stabilized. The most destabilized conformation appears at theta = 90 degrees. But even if t he small. details of the potential curve are not reproduced by INDO/S, the barrier for the syn-anti conversion is calculated to be 5.7 kcal/ mol, and this compares very well with the rotation barrier determined experimentally by NMR measurements. Absorption spectra are obtained fo r 2,2'-bithiophene in the vapor phase, in a series of n-alkanes and in some solvents of various polarities. These spectroscopic measurements combined with the transition energies determined by INDO/S are used t o get deeper insights about changes in the molecular conformation upon the nature of the environment. In the vapor phase, it is shown that a mixture of conformers may coexist with theta = 140 degrees and theta = 40 degrees. This is confirmed experimentally by gas phase electron d iffraction measurements. By comparing the values of ground and excited state dipole moments together with the transition moment calculated b y INDO/S and determined by the solvatochromic shift method in nonpolar media, it is suggested that, following solute dipole-solvent polariza bility interactions, 2,2'-bithiophene be more twisted in solution comp ared to the gas phase. From INDO/S calculations and solvatochromic shi fts, it is not possible to distinguish between the anti-like and the s yn-like conformers of the 2,2'-bithiophene molecule.