Structures of Norbornane and Norbornadiene as Determined by Gas Electron Diffraction

The structures of norbornane and norbornadiene in the gas phase have been investigated by electron diffraction. For the most probable models the rg bond lengths and the bond angles (based on the rα structure) with estimated limits of error are as follows: For norbornadiene: C1–C2=1.5354±0.007 Å, C2=C3=1.3432±0.003 Å, C1–C7=1.573±0.014 Å, ∠C1–C7–C4=94·1±3·0°, and θ (the dihedral angle between the C1–C2–C3–C4 and C4–C5–C6–C1 planes)=115·6±2·2°; For norbornane: C–C(average)=1.5488±0.003 Å, C1–C2=1.539±0.012 Å, C2–C3=1.557±0.025 Å, C1–C7=1.560±0.024 Å, ∠C1–C7–C4=93·1±1·7°, and θ=113·1±1·8°. The frame structures of these molecules are similar to each other except for the C2–C3 bond distances. All the C–C–C valence angles are appreciably smaller than the tetrahedral angle (∠C1–C7–C4 in particular). The C1–C7, bridge bonds appear to be longer than the normal C–C single bond. The above structures are compared critically with those reported so far by other investigators. A method for estimation of systematic errors in the structural parameters derived from a least-squares analysis with fixed mean vibrational amplitudes is discussed.