DFT investigation on heats of formation and group interactions of polyisocyanoadamantanes with respect to isocyano substituents

Heats of formation (HOFs) and strengths of group interactions for 19 polyisocyanoadamantanes were obtained by using the density functional theory (DFT). The adamantane skeleton was chosen for a reference compound in the process of designing isodesmic reactions. It was found that the HOF increases 213-252 kJ/mol for each additional number of the isocyano group being added to the adamantane skeleton. The HOFs slightly deviate from the group additivity with respect to the isocyano group. The distance between isocyano groups influences the values of HOFs. The disproportionation energies of neighbor isocyano groups in polyisocyanoadamantanes are in the range of 10.3-15.8 kJ/mol, which are also slightly related to the substituent numbers. The average interaction energy between nearest neighbor -NC group in decaisocyanoadamantane is 10.34 kJ/mol at the B3LYP/6-311G∗∗ level. The relative stability related to the number of isocyano groups of the title compounds was evaluated based on the calculated HOFs, the energy gaps between the frontier orbitals, the bond orders of the C-NC and C-C bonds, and bond dissociation energies.