A theoretical investigation of the decomposition mechanism of pyridyl radicals

We carried out a detailed quantum mechanical study of the unimolecular deco mposition mechanism of pyridine. The critical structures of all reasonable reaction pathways were optimized by density functional theory using the B3L YP functional and 6-31G** basis set. Relative energies were evaluated based on single-point QCISD-(T)/cc-pVDZ energies. In agreement with general beli ef 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 t hat ring-opening via C-N bond cleavage in o-pyridyl radical is more favorab le than C-C bond cleavage, as the former has a significantly lower activati on barrier and the resulting open-chain cyano radical is more stable than o ther linear C5NH4 radicals. The calculated activation energy for the format ion of cyanovinylacetylene + H from the open-chain cyano radical is the low est, compared to the other channels considered. However, activation entropy favors C-C bond cleavage producing acetylene and cyanovinyl radical instea d of cyanovinylacetylene and atomic hydrogen. On the basis of the calculate d activation energies and activation entropies, transition state theory pre dicts that, in the temperature range of 1300-1800 K, the formation of acety lene + cyanovinyl radical from o-pyridyl radical is two to three times the rate of formation of cyanovinylacetylene + H. The calculations indicate tha t direct C-H bond scission from all three pyridyl radicals producing 2,3- a nd 3,3-pyridynes is also a favorable channel from energy consideration.