Conformational properties of thiophene oligomers

The molecular geometries and the torsional potentials about the inter-ring C-C bond in alpha-oligothiophenes (alpha-nTh, n=2-4) have been calculated by means of conventional ab initio and density functional theory (DFT) calculations employing the hybrid B3LYP and BH&HLYP functionals. The position and the energetics of the critical points in the potential energy curve generated by rotation about the inter-ring C-C bond are shown to be dependent on the computational method. DFT calculations, in comparison with MP2 calculations, favour conjugative interactions, while steric and coulombic interactions are equally treated by both methods. On oligomerization the electron delocalisation increases slightly, the pi-charge being preferentially confined within the rings, although it is sufficient to move the molecular structure towards co-planarization and to increase the barrier through the perpendicular conformation. The IR and Raman spectra on the relevant rotamers of alpha-2Th have been computed at HF/6-31G* and B3LYP/6-31G* levels. The comparison with the experiment is excellent. It has been found that small twisting from the planar conformation has no apparent effects, while 90 degrees twisting and isomerization to the syn-gauche form produce significant frequency and intensity variations which could be useful probes in conformational studies. The simulated IR and Raman spectra of the alpha-2Th rotamers are consistent with a small pi-electron delocalisation between the rings.