Theoretical evaluation of N-alkylcarbazoles potential in hydrogen release

Alkyl chain effect (ethyl, propyl and butyl) on the dehydrogenation mechanism of H₁₂-N-alkylcarbazoles has been investigated theoretically under various different conditions. Gibbs energies of activation of about 107.88 kcal mol^?1 have been determined as the least energy barriers among the studied dehydrogenation processes for dehydrogenation of H₁₂-N-ethylcarbazole to H₄-N-ethylcarbazole in decalin and 57.44 kcal mol^?1 for dehydrogenation of H₁₂-N-propylcarbazole to H₈-N-propylcarbazole under the experimental conditions. Kinetic and thermodynamic studies have shown that the route of H₄-N-alkylcarbazoles formation passes through a higher barrier than that of the H₈-N-alkylcarbazoles. Natural bond orbital (NBO) analysis showed a decrease in electron transfer between π_C–C and σ*_C–H at the center of the reaction. The electron density of the C–H bonds of the transition states was evaluated as evidence of hydrogen release via quantum theory of atoms in the molecules (QTAIM) procedure. Based on this analysis, a change in the nature of C–H bonds was confirmed from covalence to electrostatic interactions during the reaction.