Concerted motion in bicyclobutane

Abstract

In bicyclobutane, the bridgehead substituents move inward as the bridge bond lengthens, and vice versa. The effect of BH2, H, F, and OH as bridgehead substituents was investigated using quantum-mechanical calculations. The degree of invertedness of the bridgehead carbon was found to depend on the electronegativity of the substituent. It becomes more pronounced with electropositive substituents which, at the same time, also cause lengthening of the central bond (1.43 Å for F to 1.55 Å for BH2). The oscillation phenomenon was found to be common to all the substituents regardless of their electronegativity. Substituent effects on the structure, energy, and electronics of the system are discussed.Key words: bicyclobutanes, oscillation, geometry distortion.