Pyrolysis mechanisms are studied using the ab initio molecular dynamics at constant temperature T = 3000 K with the Langevin thermostat. Total simulation time is equal to 2 ps, whereas the time step is equal to 0.1 fs. Ten independent simulations were performed for each system. Decomposition paths adopted from the molecular dynamics were optimized in the frame of nudged elastic band (NEB) technique as it was implemented in TeraChem. Configurations with the highest energies on the NEB paths were additionally optimized using the Hessian-based local algorithm for the transition state search as it was implemented in GAMESS. Hessian calculations and an intrinsic reaction coordinate approach are carried out to confirm the transition state configuration. To take into account the influence of vibrational energies on pyrolysis mechanisms, we consider the Gibbs energies G for reactants, products and transition state configurations. Taking into account the ideal gas approximation, we evaluate Gibbs energy as

\(G=E+E_{vib}+RT-TS_{vib}\),

where E and E_vib are the ground state and vibrational energies, respectively, S_vib is the vibrational entropy, R = 8.31 J/(mol·K) is the gaseous constant, T is the temperature.

Electron affinities (EA) and ionization potentials (IA) of considered compounds are calculated from their HOMO and LUMO energies in accordance with the Koopmans’ theorem for the closed-shell molecular systems.\cite{Koopmans_1934} Descriptors of reactivity, such as chemical hardness (η) and softness (S), the chemical potential (μ), electronegativity (χ) and electrophilicity index (ω), are determined by the same way as it was made in Ref. \cite{Gimaldinova_2018}. To calculate the ultraviolet (UV) spectra, all geometries were reoptimized with the CAM-B3LYP functional,\cite{Yanai_2004} which took into account the long-range Coulomb correlations. Twenty excited states were found using the same CAM-B3LYP approach in the frame of Tamm-Dancoff time-dependent density functional technique implemented in TeraChem.\cite{Isborn_2011}