Electron Spin Resonance Study of the Pyramidal Geometry around the Tervalent Carbon Atom of the Bicyclo[2.1.1]hexan-5-yl Radical

The bicyclo[2.1.1]hexan-5-yl and the 5-deuteriobicyclo[2.1.1]hexan-5-yl radicals have been examined using ESR. A comparison of the spectra confirmed that geometry around C5 is pyramidal and showed highly stereoselective long-range spin-transfer from C5 to the endo proton on C6; a(H6endo)=28.22 and a(H6exo)=0.49 G. A potential curve for the out-of-plane bending motion of the C5–H5 bond, selected from the trial curves gave a reasonable simulation of the temperature dependence and isotope effects on a(H5) and a(H6endo). The curve has two minima with a relatively low barrier to inversion; the geometry of the C5–H5 bond bent in the exo direciton has been estimated as approximately 0.9 kcal/mol more stable than that with the C5–H5 bond located in the endo direction.