Structures, C–H and C–CH3 bond energies at borders of polycyclic aromatic hydrocarbons

Abstract

Quantum mechanical calculations based on density functional theory are employed to calculate C–H and C–CH₃ bond dissociation energies (D_e) at the “ zigzag” and “armchair ” borders of polycyclic aromatic hydrocarbons (PAHs), since these data are needed to model the pyrolysis of light hydrocarbons. Computed D_e for either process depend smoothly on the size and shape of the PAHs and scatter by less than or about 5 kJ mol⁻¹ for larger systems. Computed C–H dissociation energies on the zigzag or armchair border are also very similar, ≈480 and ≈477 kJ mol⁻¹, respectively. This implies that only a small fraction of C–H bonds are broken at 1400 K, a temperature that is typical for pyrolysis. The C–CH₃ bond dissociation energy is ≈428 kJ mol⁻¹ for the zigzag and ≈394 kJ mol⁻¹ for the armchair configuration. In the subsequent dissociation of two adjacent hydrogen atoms/methyl groups on an armchair border, the second step is energetically less demanding with reaction energies of ≈361 kJ mol⁻¹ for C–H and ≈287 kJ mol⁻¹ for C–CH₃ .