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 co nformational behavior of 2,2'-bithiophene. Fully optimized torsional p otentials 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 nonbondin g interactions and are compared with gas-phase and solid-state experim ental data. The reliability of a recent electron diffraction determina tion of the molecular structure of 2,2'-bithiophene is discussed in li ght of MP2 calculations, which provide a more delocalized structure th an HF calculations. Very flat 4-fold potentials where minima correspon d 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 o f conformations experimentally observed for thiophene oligomers. A tor sional angle of about 147 degrees is predicted at the HF level for the most stable s-trans-gauche conformer in agreement with electron diffr action data. The inclusion of the electron correlation at the MP2 leve l comparatively destabilizes the planar s-trans conformer and reduces the torsional angle for the s-trans-gauche minimum to 142.2 degrees. A dditional MP2 calculations using the 6-31G* basis set affect the rela tive conformational energies by less than 0.2 kJ/mol. The torsional po tentials are finally analyzed in terms of a Fourier decomposition trun cated to the sixth term.