Ab initio structures for 90°-twisted s-trans-1,3-butadiene and cyclooctatetraene:: The naked sp2-sp2 bond

The bond length of a carbon-carbon sp(2)-sp(2) a-bond without the perturbing effects of pi-interactions has been estimated by high level calculations on two prototypical systems: a 90 degrees-twisted form of butadiene and the tub conformer, of cyclooctatetraene. The former system yields a value of 1.4818 angstrom, considerably longer than previous estimates. The corresponding bond length in the latter is slightly shorter due to some pi-electron delocalization.