Investigation of barriers to conformational interchange in oligothiophenes and oligo(thienyl)furans

The conformational analysis of unsubstituted and alkyl-substituted oligothiophenes and oligo(thienyl)furans by dynamic NMR (DNMR) spectroscopy and ab initio calculations is described. The DNMR studies of oligothiophenes indicate that the conformational exchange is fast even on the (13)C chemical shift scale and that the barriers are too low to be measured by this technique. Ab initio 6-31G** and MP2/6-31G** calculations are in agreement with these experimental findings. Moreover, it is found that the torsional barriers do not change significantly as the chain length is increased. Torsional parameter sets for the MM2 force field a-re derived for substituted oligothiophenes and oligo(thienyl)furans using the ab initio torsional energy profiles for the appropriate dimers, with the expectation that these parameter sets will be transferable to larger oligomers.