DAVID FELLER, Department of Chemistry, Washington State University, Pullman, WA 99164-4630; NORMAN C. CRAIG AND ALBERT R. MATLIN, Department of Chemistry and Biochemistry, Oberlin College, Oberlin, OH 44074.
The bond length of a carbon-carbon sp2-sp2 -bond without the perturbing effects of -electron interactions has been estimated by high-level ab initio calculations [CCSD(T)/cc-pVNQ with CBS extrapolation] on two prototypical systems: a 90\circ-twisted form of butadiene and the tub conformer of cyclooctatetraene. The butadiene system yields a value of 1.4818 Å, considerably longer than previous estimates. The corresponding bond length in cyclooctatetraene is slightly shorter due to some -electron delocalization. Ab initio results for related equilibrium structures of other rotamers/transition states of butadiene along the internal rotation coordinate (C-C bond) will be reported and compared with an experimental potential function.