Torsional Potentials of Perfluoro-1,3-butadiene and Perfluoro-1,3,5-hexatriene:  A Comparison of ab Initio and Density Functional Results

The torsional potentials of perfluoro-1,3-butadiene and perfluoro-1,3,5-hexatriene were investigated systematically with the aid of ab initio self-consistent field (SCF) and Møller−Plesset second-order (MP2) methods and with several density functional theory (DFT) variants applying large basis sets and complete geometry optimizations. Significant quantitative differences exist between the results of DFT and MP2 calculations concerning the relative energies of the syn-gauche, minimum and the planar anti- and syn-saddle points in perfluoro-1,3-butadiene. This behavior persists in perfluoro-1,3,5-hexatriene. For both molecules, the qualitative predictions of all the methods applied, SCF, MP2 and DFT, are similar, provided sufficiently large basis sets are used. For perfluoro-1,3,5-hexatriene at least three distinct minima are observed, two of them corresponding to the lower lying energetically near-degenerate syn-g + g + - and syn-g + g - -conformations, respectively, and one to a higher-lying syn-g + /anti-g - -conformation. With MP2 a fourth minimum, syn-g + /anti-g + , separated only by a very small barrier from the syn-g + /anti-g - -conformation, is predicted as well. Regularities in the calculated equilibrium structures of longer perfluorinated oligoenes up to C10F12 are pointed out.