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.