Toward high-level theoretical studies on the reaction kinetics of PAHs growth based on HACA pathway: An ONIOM[G3(MP2,CC)//B3LYP:DFT] method developed

Abstract Polycyclic aromatic hydrocarbons (PAHs) as precursors of soot, have been one of intensive concerns of human health and environmental contamination. G3(MP2,CC)//B3LYP theory has been widely used for prediction of PAHs growth. It is, however, a computational challenge for studying fairly large PAHs formation using this method. Herein, an ONIOM[G3(MP2,CC)//B3LYP:DFT] method, a two-layer composite ab inito molecular-orbital theory, was developed where the high layer of the real reaction systems is treated with G3(MP2,CC)//B3LYP and low layer is dealt with hybrid density functional B3LYP method. To evaluate the performance of the ONIOM method on PAHs growth, the potential energy surfaces (PESs) of naphthalene growth obtained from ONIOM method were compared with that from G3(MP2,CC)//B3LYP based on hydrogen-abstraction/C2H2-addition (HACA) mechanism. It is found that the results of ONIOM method are in good agreement with the results from G3(MP2,CC)//B3LYP method with mean energy error of −0.24 kcal/mol. After calculating the PESs of anthracene, phenanthrene, pyrene, and coronene growth based on HACA mechanism by ONIOM method, it is found that the computational costs of ONIOM method decrease significantly compared with G3(MP2,CC)//B3LYP method as the numbers of aromatic rings in PAHs increases. At the same time, ONIOM method maintains accurate prediction of PAHs growth. At last, time-dependent RRKM-based master equations using the obtained energy from ONIOM method were solved to calculate the pressure- and temperature-dependent rate coefficients of PAHs formations. Armed with this developed ONIOM method, more kinetics studies of PAHs growth may be carried out.