Estimation method for the thermochemical properties of polycyclic aromatic molecules

Abstract

Polycyclic aromatic molecules, including polycyclic aromatic hydrocarbons (PAHs) have attracted considerable attention in the past few decades. They are formed during the incomplete combustion of hydrocarbon fuels and are precursors of soot. Some PAHs are known carcinogens, and control of their emissions is an important issue. These molecules are found in many materials, including coal, fuel oils, lubricants, and carbon black. They are also implicated in the formation of fullerenes, one of the most. chemically versatile class of molecules known. Clearly, models that provide predictive capability for their formation and growth are highly desirable. Thlermochemical properties of the species in the model are often the most important parameter, particularly for high temperature processes such as the formation of PAH and other aromatic molecules. Thermodynamic consistency requires that reverse rate constants be calculated from the forward rate constants and from the equilibrium constants. The later are obtained from the thermochemical properties of reactants and products. The predictive ability of current kinetic models is significantly limited by the scarcity of available thermochemical data.

(cont.) In this work we present the development of a Bond-Centered Group Additivity method for the estimation of the thermochemical properties of polycyclic aromatic molecules, including PAHs, molecules with the furan substructure, molecules with triple bonds, substituted PAHs, and radicals. This method is based on thermochemical values of about two hundred polycyclic aromatic molecules and radicals obtained from quantum chemical calculations at the B3LYP/6-31G(d) level. A consistent set of homodesmic reactions has been developed to accurately calculate the heat of formation from the absolute energy. The entropies calculated from the B3LYP/6-31G(d) vibrational frequencies are shown to be at least as reliable as the few available experimental values. This new Bond-Centered Group Additivity method predicts the thermochemistry of C₆₀ and C₇₀ fullerenes, as well as smaller aromatic molecules, with accuracy comparable to both experiments and the best quantum calculations. This Bond-Centered Group Additivity method is shown to extrapolate reasonably to infinite graphene sheets.

(cont.) The Bond-Centered Group Additivity method has been implemented into a computer code within the automatic Reaction Mechanism Generation software (RMG) developed in our group. The database has been organized as a tree structure, making its maintenance and possible extension very straightforward. This computer code allows the fast and easy use of this estimation method by non-expert users. Moreover, since it is incorporated into RMG, it will allow users to generate reaction mechanisms that include aromatic molecules whose thermochemical properties are calculated using the Bond-Centered Group Additivity method. Exploratory equilibrium studies were performed (l. Equilibrium concentrations of individual species depend strongly on the thermochemistry of the individual species, emphasizing the importance of consistent thermochemistry for all the species involved in the calculations. Equilibrium calculations can provide many interesting insights into the relationship between PAH and fullerenes in combustion.