HF/3-21G*ab initio calculations on methoxy-substituted bithiophenes

We report, for the first time, the lowest energy structures as well as a de tailed conformational analysis of several bithiophenes substituted with met hoxy and methyl groups in various positions. For each molecule, geometrical parameters and potential energy surfaces were obtained from HF/3-21G* ab i nitio calculations. The effect of the position of substituents on structura l parameters and on torsional potentials is examined. These calculations gi ve results, which are in good agreement with available experimental data. F or oligomers having methoxy groups in positions 3,3' and 3,4', planar anti conformations are well stabilized with large rotational barriers at 90 degr ees. This phenomenon was interpreted in terms of charge transfer in the thi ophene units induced by the methoxy groups in the 3- and/or 3'-positions an d delocalization throughout the molecular frame. For 4,4'-methoxy-2,2'-bith iophene, the alkoxy groups do not show any influence on the electronic stru cture of the molecule and do not create steric hindrance such that the most stable conformer is very similar to that of the unsubstituted bithiophene molecule. In contrast, it is found that the most stable conformer of 3,4'-d imethoxy-4,3'-dimethyl-2,2'-bithiophene is much twisted owing to the presen ce of the methyl group in the 3'-position. This work also reports on theore tical results performed at lower levels (AM1 and PM3) as these levels of ca lculation are often the only one practically available for large substitute d oligothiophenes. It is shown that these calculations fail to predict accu rate conformations of the molecules studied here mainly because of the meth oxy group substituent. A ZINDO/S analysis performed on the geometries obtai ned by the various theoretical methods employed is also reported. It is obs erved that, for the same torsional angle, the ZINDO/S method is very sensit ive to the starting geometry. (C) 1999 Elsevier Science B.V. All rights res erved.