Cation-π Interactions between Flame Chemi-ions and Aromatic Compounds

The cation-pi interactions in complexes that could be formed from the association of flame-generated chemi-ions with neutral aromatic molecules, including benzene, naphthalene, phenanthrene, and pyrene, were examined by electronic structure calculations using the MP2 and B3LYP methods with the 6-311++G(d,p) basis set. Single-point CCSD(T) calculations were also made on the MP2 geometries. The cations considered are CHO+, C3H3+, and H3O+. The purpose was to examine the possibility of chemi-ion enhanced aromatic bindings as a viable path to aromatic clustering at high temperatures or soot nucleation in flames. Although the binding energies of the complexes computed using the B3LYP and MP2 methods can be quite different, both suggest that the binding remains too weak for these complexes to survive at high temperatures for an appreciable period of time and, thus, to be related to aromatic clustering and soot nucleation in any way. Potential energy surfaces were examined for the reactions of benzene with all three cations and of pyrene with CHO+. The results show chemically activated proton transfer from CHO+ and H3O+ to benzene to be the dominant reaction path. Taken together, we conclude that chemi-ions play no role other than protonating aromatic molecules.