AB-INITIO DETERMINATION OF THE GEOMETRIC STRUCTURE AND INTERNAL-ROTATION POTENTIAL OF 2,2'-BITHIOPHENE

We report a detailed ab initio study of the molecular structure and conformational behavior of 2,2'-bithiophene. Fully optimized torsional potentials keeping planar the thiophene rings; are calculated at the HF/3-21G*, HF/6-31G*, and MP2/6-31G* computational levels. The optimized geometries are analyzed in terms of conjugative effects and nonbonding interactions and are compared with gas-phase and solid-state experimental data. The reliability of a recent electron diffraction determination of the molecular structure of 2,2'-bithiophene is discussed in light of MP2 calculations, which provide a more delocalized structure than HF calculations. Very flat 4-fold potentials where minima correspond to s-cis- and s-trans-gauche structures are obtained at both the HF and MP2 levels. The flatness of the potentials justifies the variety of conformations experimentally observed for thiophene oligomers. A torsional angle of about 147 degrees is predicted at the HF level for the most stable s-trans-gauche conformer in agreement with electron diffraction data. The inclusion of the electron correlation at the MP2 level comparatively destabilizes the planar s-trans conformer and reduces the torsional angle for the s-trans-gauche minimum to 142.2 degrees. Additional MP2 calculations using the 6-31G** basis set affect the relative conformational energies by less than 0.2 kJ/mol. The torsional potentials are finally analyzed in terms of a Fourier decomposition truncated to the sixth term.