A Theoretical Investigation of the Decomposition Mechanism of Pyridyl Radicals

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We carried out a detailed quantum mechanical study of the unimolecular decomposition mechanism of pyridine. The critical structures of all reasonable reaction pathways were optimized by density functional theory using the B3LYP functional and 6-3IG** basis set. Relative energies were evaluated based on single-point QCISD(T)/cc-pVDZ energies. In agreement with general belief and pervious theoretical studies, the calculated results indicate that C-H bond scission in pyridine preferentially produces the o-pyridyl radical. Also in agreement with the accepted mechanism, the calculations indicate that ring-opening via C-N bond cleavage in o-pyridyl radical is more favorable than C-C bond cleavage, as the former has a significantly lower activation barrier and the resulting open-chain cyano radical is more stable than other linear CsNFL; radicals. The calculated activation energy for the formation of cyanovinylacetylene + H from the open-chain cyano radical is the lowest, compared to the other channels considered. However, activation entropy favors C-C bond cleavage producing acetylene and cyanovinyl radical instead of cyanovinylacetylene and atomic hydrogen. On the basis of the calculated activation energies and activation entropies, transition state theory predicts that, in the temperature range of 1300-1800 K, the formation of acetylene + cyanovinyl radical from o-pyridyl radical is two to three times the rate of formation of cyanovinylacetylene + H. The calculations indicate that direct C-H bond scission from all three pyridyl radicals producing 2,3- and 3,4-pyridynes is also a favorable channel from energy consideration.