Evidence of strong O–H⋯C interactions involving apical pyramidane carbon atoms as hydrogen atom acceptors

Using high-level quantum chemical calculations, we predicted a strong O-H⋯C interaction between the apical carbon atoms of pyramidane and its derivatives and water molecules. Analysis of calculated electrostatic potential maps showed that there are areas of strong negative potential above apical carbon atoms in all studied structures. The results of quantum chemical calculations showed that the O-H⋯C interaction between the hydrogen atom of water and the apical carbon atom of pyramidane derivatives with four -CH₃ substituents is unexpectedly strong, Δ<i>E</i>_CCSD(T)/CBS = -7.43 kcal mol^-1. The strong hydrogen bonds were also predicted in the case of unsubstituted pyramidane (Δ<i>E</i>_CCSD(T)/CBS = -6.41 kcal mol^-1) and pyramidane with four -OH substituents (Δ<i>E</i>_CCSD(T)/CBS = -5.87 kcal mol^-1). Although there are not many crystal structures of pyramidane-like molecules, we extracted examples of pyramidal-shaped molecules with apical carbon atoms from the Cambridge Structural Database and analyzed their hydrogen-bonding patterns. Analysis of crystal structures confirmed the existence of short non-covalent contacts between apical carbon atoms and neighboring hydrogen atoms.