Structure and Intramolecular Motions in Bicyclo[2.2.2]octane as Studied by Gas Electron Diffraction

A least-squares analysis of the electron-diffraction intensities for bicyclo[2.2.2]octane in gas phase measured at 28°C has given the following structural parameters: rg(C–C) (average)=1.542±0.004Å, rg(C–H) (average)=1.107±0.009Å, rg(C1–C2)=1.538±0.015Å, rg(C2–C3)=1.552±0.029Å, ∠C1–C2–C3=109.7°±0.7°, and ∠H–C–H=110·1°±5·6°, where the uncertainties represent estimated limits of error. The potential function for the twisting motion around the D3h symmetry axis is found to have a broad minimum; in terms of the dihedral angle of torsion about the C2–C3 axis, φ, the potential has an rms angle of 12.0°±1.5° and a “classical turning point” of 21.5°±0.5° (7.2° and 12.8°, respectively, in terms of the dihedral angle of twist, τ). Best fit to the observed intensities has been achieved when a quartic function, V(φ)=k2φ2+k4φ4 with k2=−4.0 kcal/mol and k4=54.2 kcal/mol, is assumed. The potential function probably has a hump of the order of 100 cal/mol at the D3h conformation. Hence, this molecule may be regarded as having a “quasi-D3h structure”. The above features are consistent with those for the derivatives of this molecule in crystal phase studied by Dunitz and Ermer by X-ray diffraction, and also with semiempirical estimates based on the calculations of the intramolecular strain energy.