Structure of tricyclo[3, 3, 0, 02,6]-octane as determined by electron diffraction

Abstract

Tricyclo[3,3,0,0^2,6]-octane (C₈H₁₂) was prepared from cis, cis-1,5-cyclo-octadiene by mercury-sensitized photoisomerization. Sectored differaction photographs were taken with the sample at 0°C. With 70 kV electrons the recorded patterns covered the angular range q= 6–140 Å^–1. The interatomic distances and bond angles in this highly-strained, rigidly-fused ring system were determined by resolution of the refined radial distribution curve, followed by a least-squares fitting of the calculated molecular intensity pattern to that observed. Convergence was obtained for a structure with D_2d symmetry, in which the highly puckered four-member ring has (C—C)= 1.558 ± 0.003 Å with a dihedral angle of 127°. The sides of the five-member rings are 1.569 ± 0.005 Å while their bases are 1.516 ± 0.010 Å; (C—H)_av= 1.106 ± 0.003 Å. The root-mean-square amplitudes were lower than for less rigidly bound hydrocarbons. Distortions of the four- and five-member rings in this octane are compared with rings in related molecules.