A MODEL POTENTIAL FOR THE INTERNAL-ROTATION OF NEIGHBORING RINGS OF BITHIOPHENE AND BIPYRROLE

Based upon ab initio Hartree-Fock calculations we propose a model pote ntial for simplifying the study of the internal rotation of neighbouri ng rings. The quality of the proposed analytic potential was numerical ly verified in the case of bithiophene and bipyrrole by determining th e deviations of the predicted energy values along the torsional angle with respect to 19 independent reference values. It is found that our model potential describes correctly the torsional motion: the average deviations of the predicted energies along the torsional angle with re spect to the ab initio reference energies are 0.20 and 0.22 kJ mol(-1) for bithiophene and bipyrrole, respectively. In both molecules, when going from a coplanar to a perpendicular conformation, two stable isom ers have been detected. These isomers are separated by a barrier heigh t of about 4 kJ mol(-1). Full optimization of the structural parameter s along the torsional angle shows that they remain almost constant alo ng the isomerization process.