CONCERTED MOTION IN BICYCLOBUTANE

In bicyclobutane, the bridgehead substituents move inward as the bridg e bond lengthens, and vice versa. The effect of BH2, H, F, and OH as b ridgehead substituents was investigated using quantum-mechanical calcu lations. The degree of invertedness of the bridgehead carbon was found to depend on the electronegativity of the substituent. It becomes mor e pronounced with electropositive substituents which, at the same time , also cause lengthening of the central bond (1.43 Angstrom for F to 1 .55 Angstrom for BH2). The oscillation phenomenon was found to be comm on to all the substituents regardless of their electronegativity. Subs tituent effects on the structure, energy, and electronics of the syste m are discussed.