Reactions between atomic chlorine and pyridine in solid para-hydrogen: infrared spectrum of the 1-chloropyridinyl (C5H5N-Cl) radical.

With infrared absorption spectra we investigated the reaction between Cl atom and pyridine (C(5)H(5)N) in a para-hydrogen (p-H(2)) matrix. Pyridine and Cl(2) were co-deposited with p-H(2) at 3.2 K; a planar C(5)H(5)N-Cl(2) complex was identified from the observed infrared spectrum of the Cl(2)/C(5)H(5)N/p-H(2) 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(5)H(5)N-Cl) was identified as the major product of the reaction Cl + C(5)H(5)N in solid p-H(2); 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(5)H(5)N-Cl radical but not 2-, 3-, or 4-chloropyridine (ClC(5)H(5)N) radicals is consistent with the reported theoretical prediction that formation of the former proceeds via a barrierless path.