Reaction Kinetics of a Selected Number of Elementary Processes Involved in the Thermal Decomposition of 9-Methylphenanthrene Using Density Functional Theory

Thermochemistry and Kinetics of Hydrogen Abstraction by Methyl Radical from Polycyclic Aromatic Hydrocarbons

Thermodynamic and kinetic properties relating to hydrogen abstraction by methyl radical from various sites in polycyclic aromatic hydrocarbons (PAHs) have been investigated. The reaction enthalpies (298 K), barriers (0 K), and activation energies and pre-exponential factors (700-1100 K), have been calculated by means of density functional theory, specifically with B3-LYP/6-311G(d,p) geometries, followed by BMK/6-311+G(3df,2p) single-point energy calculations. For uncongested sites in the PAHs, a reasonable correlation is obtained between reactivities (as characterized by the reaction barriers) and reaction enthalpies. This is reflected in a Bell-Evans-Polanyi (BEP) relationship. However, for congested sites, abstraction is accompanied both by lower reaction enthalpies (due to relief of steric strain) and also by reduced reactivities (due to significantly increased steric hindrance effects in the transition structures), so that the BEP relationship does not hold. In addition, the reaction enthalpies and kinetic parameters for the series of linear acenes indicate that abstraction is more difficult from the central rings.

Hydrocarbon Bond Dissociation Enthalpies: From Substituted Aromatics to Large Polyaromatics

Hydrocarbon-bond dissociation enthalpies (BDE) at 298 K are calculated for a set of hydrocarbons. An efficient method for calculating the BDE values is derived on the basis of a comparative study with experimental data. The methods considered are based on density functional theory (DFT) including the B3LYP, MPW1PW91, B3P86, B3PW91, MPW1P86, KMLYP, MPW1K and BMK functionals. The commonly known sequence for radical stability is quantified on the basis of BDE values. The recommended procedure is extrapolated to substituted aromatics and large polyaromatic hydrocarbons (PAHs) to obtain insight into the factors that govern the stability of the radicals. Furthermore it is shown that BDEs are also good reactivity descriptors for subsequent additions involving the formed radicals. Linear correlations, similar to classical Evans-Polanyi-Semenov plots, between the BDE and the reaction barriers for addition reactions with ethene, ethyne, propene, propyne and butadiene are found, as the exothermicity is primarily determined by the stability of the originating reactant radical.

Growing Graphene Sheets from Reactions with Methyl Radicals: A Quantum Chemical Study

Hydrogen abstraction reactions by methyl radicals on the zigzag and armchair edges of perylene are studied by density functional theory (DFT) to explore various growth pathways that seem to be in line with experimental observations. The DFT approach is validated by comparing the results obtained from calculations with six different functionals with those obtained from correlated ab initio methods, thereby emphasizing the calculation of reaction barriers. A useful compromise between accuracy and computational cost is provided by DFT, and possible pathways are studied in detail at this level of calculation. Our computational study is carried out by ordering, as a first step, all of the isomers that arise from the abstraction of one or two H atoms from 1,12-dimethyl-1,12-dihydroperylene and 3,4-dimethyl-3,4-dihydroperylene with respect to their energies. Subsequently, only those pathways that connect low-energy isomers are investigated. The calculations reveal that the selected pathways are favored thermodynamically, and also that the reaction barriers are somewhat higher than the energy locally available for the respective reaction. Notably, in the case of 3,4-dimethyl-3,4-dihydroperylene, the first two reaction steps have no or only a very low reaction barrier. The final conclusion of our study is that a cascade of reactions is possible that leads to the growth of a graphene sheet on a graphite surface.

Computational Studies on the Cyclization of Polycyclic Aromatic Hydrocarbons in the Synthesis of Curved Aromatic Derivatives

Computational studies on the cyclization reactions of some polycyclic aromatic hydrocarbons (PAHs) were performed at the DFT level. Compounds C26H14 and C24H14, which show the connectivity of C60 fullerene fragments, were chosen as suitable models to study the formation of curved derivatives by six- or five-membered ring formation, upon oxidation to their radical cations. Four possible pathways for the cyclization process were considered: a) initial C-C bond formation to afford a curved derivative, followed by dehydrogenation; b) homolytic C-H cleavage prior to cyclization; c) initial concerted H2 elimination and subsequent cyclization; and d) deprotonation of the radical cations prior to cyclization. Computed reaction and activation energies for these reactions show that direct cyclization from radical cations (pathway a) is the lowest-energy mechanism. The formation of five-membered rings is somewhat more favourable than benzannulation. After new cycle formation, homolytic C-H dissociation to afford the corresponding cations is the most favourable process. These cations react with H* without barrier to give H2* Intermediate deprotonations are strongly disfavoured. The relatively low activation energies compared with carbon cage rearrangements suggest that ionization of PAHs can be used for the tailored preparation of nonplanar derivatives from suitable precursors.