Electronic Structure Studies of Pyrolytic Reactions of Hydrocarbons

The three energetic minima in the most favorable reaction path of the cyclization of 1-ethynylpyrene.

Research Objectives

The main objective of this project is the theoretical investigation of the possible reaction mechanisms involved in the pyrolytic formation of cyclopentafused polycyclic aromatic hydrocarbons (cp-PAHs) from ethynylsubstituted PAHs. Five main pathways were examined:

1. A direct insertion of the terminal C atom of the ethynyl substituent into an aromatic C-H bond of the neighboring ring, followed by a 1,2-hydrogen shift in the resulting carbene.

2. An acetylene-vinylidene rearrangement in the ethynyl substituent, followed by insertion of the resulting carbene into an aromatic C-H bond of the neighboring ring.

3. A homolytic fission of the terminal C-H bond of the ethynyl substituent, followed by an insertion of the resulting radical into an aromatic bond of the neighboring ring, and a final recombination with an H radical.

4. A homolytic fission of an aromatic C-H bond, followed by insertion of the resulting radical into the terminal C-H bond of the ethynyl substituent, and a final recombination with an H radical.

5. The addition of an H radical to the nonterminal C atom of the ethynyl substituent, followed by insertion of the resulting radical into an aromatic C-H bond of the neighboring ring, and final abstraction of an H radical.

Computational Approach

The density functional approach within the ab initio electronic structure model was used to evaluate the electronic wavefunctions. A BLYP functional in conjunction with a 6-311G** Gaussian basis set were used in all calculations.

Using the GAUSSIAN94 suite of programs, optimization and vibrational-frequency analysis were calculated for all the minima and transition states possibly involved in the reactions of ethynylsubstituted naphthalene, acenaphthylene, fluoranthene, and pyrene (1- and 4-substituted), yielding the corresponding cyclopentafused systems acenaphthylene, pyracylene, cyclopenta[c,d]fluoranthene, and cyclopenta[c,d]pyrene. The NERSC J-90 cluster was used in performing these calculations.

Accomplishments

Except for three transition states in Pathway 5, all relevant stationary points on the energy hypersurfaces of the above-mentioned reactions were successfully localized and characterized as minima or transition states. Pathway 2 was identified as the most important reaction channel for the pyrolytic buildup of the cp-PAHs under investigation. Changing the aromatic systems did not affect the overall picture of the energetic relation between different pathways. However, a general energetic upshift was observed for all cyclizations of acenaphthylene and fluoranthene compounds, due to an increased strain in these cyclopentafused systems.

Significance

In general, cp-PAHs are potent carcinogens and mutagens. They are formed during incomplete combustion of fossil fuels and other hydrocarbon material. Our identification of the exact mechanism that determines the formation of these compounds can help to effectively suppress or avoid their unwanted buildup.

Publications

J. Cioslowski, M. Schimeczek, P. Piskorz, and D. Moncrieff, "Thermally induced rearrangement of ethynylarenes to cyclopentafused polycyclic aromatic hydrocarbons: An electronic structure study," J. Am. Chem. Soc. (submitted, 1998).