Reactions between atomic chlorine and pyridine in solid para-hydrogen: Infrared spectrum of the 1-chloropyridinyl (C ₅ H ₅ N-Cl) radical

With infrared absorption spectra we investigated the reaction between Cl atom and pyridine (C₅H₅N) in a para-hydrogen (p-H ₂) matrix. Pyridine and Cl₂ were co-deposited with p-H₂ at 3.2 K; a planar C₅H₅N-Cl₂ complex was identified from the observed infrared spectrum of the Cl ₂/C₅H₅N/p-H₂ matrix. Upon irradiation at 365 nm to generate Cl atom in situ and annealing at 5.1 K for 3 min to induce secondary reaction, the 1-chloropyridinyl radical (C ₅H₅N-Cl) was identified as the major product of the reaction Cl + C₅H₅N in solid p-H₂; absorption lines at 3075.9, 1449.7, 1200.6, 1148.8, 1069.3, 1017.4, 742.9, and 688.7 cm^-1 were observed. The assignments are based on comparison of observed vibrational wavenumbers and relative IR intensities with those predicted using the B3PW91/6-311++G(2d, 2p) method. The observation of the preferential addition of Cl to the N-site of pyridine to form C ₅H₅N-Cl radical but not 2-, 3-, or 4-chloropyridine (ClC₅H₅N) radicals is consistent with the reported theoretical prediction that formation of the former proceeds via a barrierless path.