Ability of strained C atoms to act as an electron donor

Abstract

There is a great deal of strain within the propellane and pyramidane hydrocarbon molecules. Quantum chemical calculations evaluate how this strain affects the ability of the bridgehead C atom to act as an electron donor in hydrogen, halogen, chalcogen, pnicogen, and tetrel bonds, despite the absence of a formal C lone pair or CC multiple bond. The strain induces the formation of a substantial region of negative electrostatic potential on this C atom which can attract the σ-hole of an electrophile. Each such molecule also contains an occupied molecular orbital that can be described as either a C lone pair or C–C bond, which is spatially disposed to align with, and transfer charge to, a σ* antibonding orbital of an approaching Lewis acid. The degree of strain within the hydrocarbon is closely correlated with the magnitude of the negative electrostatic potential, which is in turn connected with the strength of the ensuing bond. Tetrel bonds are strongest, followed by halogen, both of which contain a significant degree of covalency.