Strain in bicyclobutane: on the interpretation of quantitative data

In small rings, strain energy is believed to be dominated by bond angles (B aeyer strain). However, it was found (Dill et al.) that, in bicyclobutane, bridgehead substitution effects on strain energy range from +10 to -30 kcal . Ab initio computation at the 6-31G and 6-31+G* levels have shown that the deformation caused by the bridgehead substitution is minimal and so are it s energetic consequences.