Infrared spectra of the 1-pyridinium (C ₅ H ₅ NH ⁺ ) cation and pyridinyl (C ₅ H ₅ NH and 4-C ₅ H ₆ N) radicals isolated in solid para-hydrogen

Protonated pyridine and its neutral counterparts (C ₅ H ₆ N) are important intermediates in organic and biological reactions and in the atmosphere. We have recorded the IR absorption spectra of the 1-pyridinium (C ₅ H ₅ NH ⁺ ) cation, 1-pyridinyl (C ₅ H ₅ NH), and 4-pyridinyl (4-C ₅ H ₆ N) produced on electron bombardment during matrix deposition of a mixture of pyridine (C ₅ H ₅ N) and p-H ₂ at 3.2 K; all spectra were previously unreported. The IR features of C ₅ H ₅ NH ⁺ diminished in intensity after the matrix was maintained in darkness for 15 h, whereas those of C ₅ H ₅ NH and 4-C ₅ H ₆ N radicals increased. Irradiation of this matrix with light at 365 nm diminished lines of C ₅ H ₅ NH ⁺ and C ₅ H ₅ NH but enhanced lines of 4-C ₅ H ₆ N slightly, whereas irradiation at 405 nm diminished lines of 4-C ₅ H ₆ N significantly. Observed wavenumbers and relative intensities of these species agree satisfactorily with the anharmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/6-31++G(d,p) method. Assignments of C ₅ H ₅ NH and 4-C ₅ H ₆ N radicals were further supported by the observation of similar spectra when a Cl ₂ /C ₅ H ₅ N/p-H ₂ matrix was irradiated first at 365 nm and then with IR light to generate H atoms to induce the H + C ₅ H ₅ N reaction.