STRUCTURAL DISTORTIONS IN DIIODINE-SUBSTITUTED UNSATURATED-HYDROCARBONS

1,10-Diiodophenanthrene, prepared for the first time by reacting I-2 w ith dilithiophenanthrene, has a twist angle omega(I ... C-C ... I)=63 degrees between the two iodine centers and a distance d(I ... I) = 3.6 1 Angstrom, which amounts to only 84% of the sum of van der Waals radi i, 2r(I)(vdW) = 2 x 2.15 = 4.30 Angstrom. Based on an extensive litera ture search for distortions of carbon skeletons by repulsion between o verlapping iodine substituents, the low-temperature structures of 2,2' -diiodobiphenyl to = 85 degrees, d(I ... I) = 4.35 Angstrom, 101% of 2 r(I)(vdW)) and 1,8-diiodonaphthalene (omega = 2 degrees to 17 degrees, d(I ... I) = 3.51-3.54 Angstrom, 82% of 2r(I)(vdW)) have also been de termined. Density functional B3LYP calculations with 6-31G* basis set s and 31G effective pseudopotentials for iodine provide information o n an unexpectedly balanced charge distribution, leading to estimates o f about 30 kJ mol(-1) for the I/I repulsion and of about 10 kJ mol(-1) for the reduced pi delocalization in the extremely twisted skeleton o f 1,10-diiodophenanthrene.