Substituent effects in cubane and hypercubane: a DFT and QTAIM study

Using the quantum theory of atoms in molecules in combination with density functional theory, we investigate the charge densities of different cubane and hypercubane derivatives in which all the terminal hydrogen atoms of the title hydrocarbons are replaced by an equal number of substituents (F, Cl, Br, CH₃, and NO₂). The analysis of the charge densities of the bond, ring, and cage critical points indicates that these substituents have the ability to alter the charge density of the cubane skeleton, thereby enhancing the strain of its carbon–carbon bonds. Also, the change in the charge density as a function of the expansion and contraction of the cubane cage indicates that the persubstituted derivatives respond differently to the deformation of their carbon cages. A linear correlation between the one-bond nuclear spin–spin coupling constant ¹J(¹³C¹³C) and the charge densities calculated at the bond critical points suggests the possibility of employing this NMR parameter for studying the effects of substituents in cage hydrocarbons.