Can Small Polyaromatics Describe Their Larger Counterparts for Local Reactions? A Computational Study on the H-Abstraction Reaction by an H-Atom from Polyaromatics

Abstract

Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semiempirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/ GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system.

Detailed kinetic study of hydrogen abstraction reactions of triphenylene, benzo[e]pyrene, dibenzo[fg,op]naphtacene, and coronene by H atoms

“…However, the present work and literature data given in Ref. 8 in the case of pyrene, benzo(e)pyrene, and coronene species allowed us to conclude that this process is also insensitive to aromatic number rings. These observations, therefore, make it possible to affirm that the propagation processes involving H atoms taking place within the PAH submechanism are not sensitive to the size of the PAHs.…”

“…The obtained results are in good correlation with the H‐abstraction reactions by H atom of C 16 H 10 isomers (pyrene, fluoranthene, aceanthrylene, and acephenanthrylene) 7 and more larger PAHs studied by Yönder et al 8 . using the density functional theory (DFT) method and highly correlated ab initio wave functions within the UCCSD(T)‐F12 approach.…”

“…The corresponding rate constant is lower in the case of aceanthrylene and acephenanthrylene in which the K‐region duo corresponds to the five‐membered rings. In this case, the result of the present work shows that the conclusions of Yönder et al 8 . are then valid only for PAH structures with six‐carbon cycles.…”

An automatized workflow from molecular dynamic simulation to quantum chemical methods to identify elementary reactions and compute reaction constants

“…The reaction rate constants are calculated using transition state theory (TST). The partition functions needed for this are calculated using a modified version of the quasi‐rigid rotor and harmonic oscillator (qRRHO) approximation 38,39 . Using the ideal gas law as the equation of state, the site specific reaction rate constants (excluding the symmetry numbers) can be calculated at constant pressure within TST according to where is the gas constant, Boltzmann's constant, the molar volume, the temperature, Planck's constant, and is the partition function which includes translational, vibrational, and rotational contributions.…”

“…In all cases we started from a PAH, where already one H atom was abstracted to enhance reactivity. The H abstraction process was not studied in detail here since we already investigated it in a recent publication 38 …”