A theoretical investigation of the torsional potential in 3,3′-dimethyl-2,2′-bithiophene and 3,4′-dimethyl-2,2′-bithiophene:: A comparison between HF, MP2, and DFT theory

The torsional potential curve Of 3,3'-dimethyl-2,2'-bithiophene has been investigated at the MP2 and DFT (B3LYP and BLYP) levels with the 6-31G* basis. The DFT (BSLYP) method has also been used to compute the torsional curve for 3,4'-dimethyl-2,2'-bithiophene. In contrast with previous HF computations, our results indicate for both isomers the existence of a syn-gauche and an anti-gauche conformational minimum. This finding is in agreement with X-ray structural investigations and UV data. Our computations point out the importance of including the correlation energy contributions to study the conformational properties of oligo and polythiophenes and indicate that the DFT approach (in particular the hybrid BSLYP functional) provides a description of these systems which is in satisfactory agreement with the experimental evidence.