Dependence of hydrocarbon sigma CC bond strength on bond angles: The concepts of “inverted”, “direct” and “superdirect” bonds

The bond energy (BE) of CC in CH3-CH3 with respect to geometry frozen fragments follows a sigmoidal increase as function the θ = HCC pyramidalization angle. Using dynamic orbital forces BE index, same behaviour unique 〈θ〉 parameter, mean angle substituents, is found for 24 single bonds various hydrocarbons. Thus parameter appears straightforward and robust index geometrical constraints which can either strengthen or weaken bond. This way, sigma be easily classified into weak “inverted” <90° (eg. [1 1 1]propellane bicyclobutane), “direct” (or “normal”) 90° < 120° ethane), strong “superdirect” > tetrahedryl-tetrahedrane butadiyne).