Conformational analysis of n-fluorine, 2,2′-X (n = 3 or 3′; X = bifuran, bithiophene, or thienylfuran): Ab initio, density functional, and multi-coefficient correlation results

Gas phase torsional potentials involving inter-ring rotation in 3 or 3′-fluorine, 2,2′-X (X = bithiophene, bifuran, or thienylfuran) have been examined using HF and MP2 ab initio methods, B3LYP and BBX density functional methods, and the MC3BBX multi-coefficient correlation method. Introduction of fluorine at the 3 or 3′ position is shown to affect the preferred conformation of each molecule analyzed. For 2,2′-bifuran and 2,2′-bithiophene, introduction of fluorine generally resulted in a flattening of the torsional energy profiles around syn and anti-planar conformations, as well as a reduction in relative energies between the two conformers. The anti conformer, however, remains slightly favored. For 2,2′-thienylfuran, fluorine shows a similar flattening of the energy profiles within planar regions as well as an observation of nearly isoenergetic syn and anti conformers. Pronounced effects are observed when fluorine substitution is made in the thienyl ring. Indeed, this is the only molecule whose energy profile displays a marked preference for the syn conformation. Lastly, we report that the MP2/6-31+G(d,p) model seems to yield results that are both qualitatively inaccurate and quantitatively erratic, and, by itself, should not be employed in the study of these molecules.