In the search for an accurate yet inexpensive method to predict thermodynamic properties of large hydrocarbon molecules, we have developed an automatic and adaptive distance-based group contribution (DBGC) method. The method characterizes the group interaction within a molecule with an exponential decay function of the group-to-group distance, defined as the number of bonds between the groups. A database containing the molecular bonding information and the standard enthalpy of formation (Hf,298K) for alkanes, alkenes, and their radicals at the M06-2X/def2-TZVP//B3LYP/6-31G(d) level of theory was constructed. Multiple linear regression (MLR) and artificial neural network (ANN) fitting were used to obtain the contributions from individual groups and group interactions for further predictions. Compared with the conventional group additivity (GA) method, the DBGC method predicts Hf,298K for alkanes more accurately using the same training sets. Particularly for some highly branched large hydrocarbons, the discrepancy with the literature data is smaller for the DBGC method than the conventional GA method. When extended to other molecular classes, including alkenes and radicals, the overall accuracy level of this new method is still satisfactory.
Using density functional theory, a possible pathway of soot surface growth is studied in the low-temperature, postflame region in which spin-triplet polycyclic aromatic hydrocarbon (PAH) molecules with a small singlet-triplet energy gap react with unsaturated aliphatics such as acetylene via the carbon-addition-hydrogen-migration (CAHM) reaction. Results show that a PAH-core-aliphatic-shell structure is formed and the mass growth rate of this triplet soot surface growth reaction is one order of magnitude larger than that of the surface hydrogen-abstraction-carbon-addition (HACA) reaction at temperatures below 1500 K.
In this study, we examined the influence of an embedded five-membered ring on the thermal decomposition of graphene oxyradicals. Their decomposition potential energy surfaces were explored at the B3LYP/6-311g(d,p) level. The temperature and pressure dependence of the rate coefficients was computed by master equation modeling. The results suggest that the embedded five-membered ring leads to a generally slower decomposition rate for CO elimination than that of graphene oxyradicals with only six-membered rings, but the impact of the embedded five-membered ring diminishes when it is two layers away from the edge. Well-behaved first-order kinetics was demonstrated at 1500 K, but collisional relaxation was incomplete on the dissociation timescale at higher temperatures. The ways of determining the effective rate coefficient were discussed and the influence of the uncertainty in rate constants on the predictions of species profiles was also estimated by performing kinetic modeling.
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