SINGLE-BOND TORSIONAL POTENTIALS IN CONJUGATED SYSTEMS - A COMPARISONOF AB-INITIO AND DENSITY-FUNCTIONAL RESULTS

The fully relaxed single-bond torsional potentials in typical conjugat ed systems were evaluated with the aid of ab initio self-consistent-fi eld and Moller-Plesset second-order calculations and, additionally, wi th several recently developed variants of the density functional theor y. For this systematic investigation, 1,3-butadiene, styrene, biphenyl , 2,2'-bithiophene, 2,2'-bipyrrole and 2,2'-bifuran have been selected as model molecules. As representative examples for nonconjugated syst ems, the molecules n-butane and I-butene have been treated at the very same calculational levels. For all conjugated molecules, the electron correlation corrections to the self-consistent-field torsional potent ials, as obtained with the density functional methods, are dramaticall y different from those resulting from the more conventional Moller-Ple sset second-order approximation. For those cases where experimental da ta for torsional barriers are available, the self-consistent-field and the Moller-Plesset second-order results agree reasonably, whereas the density functional results consistently predict too large barriers. T his behavior is most probably caused by an overestimation of the stabi lity of the planar ct-systems by the density functional theory variant s in question.