Energy transfer in intermolecular collisions of polycyclic aromatic hydrocarbons with bath gases He and Ar

Theoretical Studies on Lennard-Jones Parameters of Benzene and Polycyclic Aromatics Hydrocarbons

Lennard-Jones (L-J) parameters, i.e. collision diameter and well depth, of benzene and polycyclic aromatic hydrocarbons (PAHs) interacting with bath gases helium and nitrogen are studied theoretically in this work. The...

Predicting third-body collision efficiencies for water and other polyatomic baths

Low-pressure-limit microcanonical (collisional activation) and thermal rate constants are predicted using a combination of automated ab initio potential energy surface construction, classical trajectories, transition state theory, and a detailed kinetic...

Spiers Memorial Lecture: theory of unimolecular reactions

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our...

Hydrogen abstraction/addition reactions in soot surface growth

The hydrogen abstraction (HB) and addition reactions (HD) by H radicals are examined on a series of polycyclic aromatic hydrocarbon (PAH) monomers and models of quasi-surfaces using quasi-classical trajectory (QCT) method. QCT results reproduce the rate constants of HB reactions on PAH monomers from density functional theory (DFT) in the range of 1500-2700 K. The PAH size has a minor impact on the rates of HB reactions, especially at temperatures beyond 2100 K. In contrast, HD reactions have a clear size dependence, and a larger PAH yields a higher rate. It was also found that the preferred reaction pathway changes from HB to HD reactions at ∼1900 K. The rates of surface HB and HD reactions exceed those in the gas phase by nearly one factor of magnitude. Further analysis of the detailed trajectory of the QCT method reveals that about 50% of surface reactions can be attributed to the events of surface diffusion, which depends on the local energy transfer in gas-surface interactions. However, this phenomenon is not preferred in PAH monomers, as expected. Our finding here questions the treatment of the surface reactions of soot as the product of the first collision between the gaseous species and particle surface. The surface diffusion-induced reactions should be accounted for in the rates of the surface HB and HD reactions. The rate constants of HB and HD reactions on each reactive site (surface zig-zag, surface free-edge and pocket free-edge sites) were calculated by QCT method, and are recommended for the further development of surface chemistry models in soot formation.

Collision Efficiency Parameter Influence on Pressure-Dependent Rate Constant Calculations Using the SS-QRRK Theory

The system-specific quantum Rice-Ramsperger-Kassel (SS-QRRK) theory (J. Am. Chem. Soc. 2016, 138, 2690) is suitable to determine rate constants below the high-pressure limit. Its current implementation allows incorporating variational effects, multidimensional tunneling, and multistructural torsional anharmonicity in rate constant calculations. Master equation solvers offer a more rigorous approach to compute pressure-dependent rate constants, but several implementations available in the literature do not incorporate the aforementioned effects. However, the SS-QRRK theory coupled with a formulation of the modified strong collision model underestimates the value of unimolecular pressure-dependent rate constants in the high-temperature regime for reactions involving large molecules. This underestimation is a consequence of the definition for collision efficiency, which is part of the energy transfer model. Selection of the energy transfer model and its parameters constitutes a common issue in pressure-dependent calculations. To overcome this underestimation problem, we evaluated and implemented in a bespoke Python code two alternative definitions for the collision efficiency using the SS-QRRK theory and tested their performance by comparing the pressure-dependent rate constants with the Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) results. The modeled systems were the tautomerization of propen-2-ol and the decomposition of 1-propyl, 1-butyl, and 1-pentyl radicals. One of the tested definitions, which Dean et al. explicitly derived (Z. Phys. Chem. 2000, 214, 1533), corrected the underestimation of the pressure-dependent rate constants and, in addition, qualitatively reproduced the trend of RRKM/ME data. Therefore, the used SS-QRRK theory with accurate definitions for the collision efficiency can yield results that are in agreement with those from more sophisticated methodologies such as RRKM/ME.