The cyclobutane molecule has been found by electron diffraction to have the following bond distances and bond angles: Csngbnd;C, 1.568plusmn;0.02A; Csngbnd;H, 1.098plusmn;0.04A; ang;HCH, 114plusmn;8deg;. On the average the ring is nonplanar, with dihedral angle 20deg; (+10deg;, minus;20deg;), but the equilibrium symmetry may be eitherD2d(puckered ring) orD4h(planar ring with low rigidity leading to large amplitude of outhyphen;ofhyphen;plane bending). This point is discussed in connection with earlier spectroscopic work. The long bond distances found in fourhyphen;membered rings are contrasted against the short distances in threehyphen;membered rings, and the strain energies, bond distances, and HCH angles of cycloalkanes are discussed in terms of modern valence concepts. It is suggested that the potential energy arising from a repulsion of the nonbonded carbon atoms may contribute significantly to the apparently anomalously high strain energy of cyclobutane. The repulsive force associated with such a potential is shown to account satisfactorily for the long Csngbnd;C distances.
展开▼
