A Theoretical Investigation of the Decomposition Mechanism of Pyridyl Radicals

Abstract

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 6-31G** 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 C 5 NH 4 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.

High-resolution photoelectron spectroscopy of the pyridinide isomers

“…(4) to determine an appropriate value of Z 2 . This gives parameter values of Z 2 = 1.45 (8), γ 1 = 0.03 (2), and γ 2 = 0.68 (13) for the ortho isomer. The extracted value of Z 2 as well as the previously set value of Z 1 gives Hanstorp coefficients of A 2 = 0.5 and B 2 = 2.1 eV −1 .…”

“…1. [9][10][11][12][13] Due to the prevalence of pyridine and the products of its decomposition in a wide range of chemical environments, spectroscopic characterization of the pyridyl radicals is of considerable interest. In this work, high-resolution anion photoelectron spectroscopy (PES) of cryogenically cooled pyridinide anions is used to obtain isomer-specific spectra that reflect the vibronic structure of the neutral pyridyl radicals, providing quantitative and qualitative insight into these species.…”

A Theoretical Investigation on the Reaction Mechanism of the C8H10+· Side‐Chain Decomposition Processes

“…8 10 C H +• is an ideal model compound for studying the complex decomposition processes of alkylbenzene cations, and a theoretical study on the reaction mechanism of this unique radical can help us better understand the decomposition processes. Liu et al 5 mentioned that in the open-ring decomposition system of pyridyl radicals the thermal decomposition of aromatic heterocyclic radicals could be interpreted as a chain reaction initiated by C-H bond scission, so maybe the side chain degradation processes of these activated ethylbenzene cations also begin with the leaving of hydrogen atoms. In this paper, a theoretical investigation has been carried out to elucidate the reaction mechanism.…”

Reaction Mechanism of the Multi‐channel Decomposition Reactions of 1‐Pentenyl Free Radicals

“…When used as energy sources, they can produce free radicals and precursors of polycyclic aromatic hydrocarbons. These radicals may give rise to negative effects on our circumstances, and some in the form of aromatic heterocycles 2 like pyridyl radicals 3,4 even pollute atmosphere. Successive pyrolytic degradation of large radical structures can lead to smaller unsaturated fragments via isomerization and bond cleavage.…”