Infrared spectra of 3-hydroxy-(1H)-pyridinium cation and 3-hydroxy-(1H)-pyridinyl radical isolated in solid para-hydrogen

Abstract

As pyridine and its derivatives are regarded as building blocks of nitrogen-containing polycyclic aromatic hydrocarbons, spectral identifications of their protonated and hydrogenated species are important. The infrared (IR) absorption spectra of the 3-hydroxy-(1H)-pyridinium cation, 3-CH(OH)NH, and the 3-hydroxy-(1H)-pyridinyl radical, 3-CH(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-CH(OH)N, and para-H to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-CH(OH)NH at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-CH(OH)NH at 3646.2, 3493.4, 3488.7, 1546.7, 1349.6, 1244.1, 1209.1, 1177.3, 979.8, and 685.2 cm and nine weaker ones increased. The intensities of lines of trans-3-CH(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-CH(OH)NH remained unchanged upon irradiation at 370, 450, and 520 nm. Observed vibrational wavenumbers and relative intensities of these species agree satisfactorily with the scaled harmonic vibrational wavenumbers and IR intensities predicted with the B3LYP/aug-cc-pVTZ method. The observed 3-CH(OH)NH cation and 3-CH(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

Infrared Spectra of Isomers of Protonated Aniline in Solid para-Hydrogen

“…In this section, we discuss the mechanism of formation of H + C 6 H 5 NH 2 isomers, including proton transfer from H 3 + to C 6 H 5 NH 2 and possible subsequent rearrangements through tunneling. The mechanism to form H + C 6 H 5 NH 2 is expected to be similar to those for the formation of protonated pyridine isomers , and polycyclic aromatic hydrocarbons. − ,,, During p -H 2 matrix deposition, H 2 molecules are ionized with electrons to produce H 2 + ; a subsequent rapid and exothermic proton transfer to a nearby H 2 molecule produces H and H 3 + . Because the proton affinity of aniline is much greater than that of H 2 , a proton transfer from H 3 + to aniline occurs readily to produce H + C 6 H 5 NH 2 : H 3 + + C 6 H 5 NH 2 → H 2 + H + C 6 H 5 NH 2 .…”

“…The red shift was slightly larger for amino-H + C 6 H 5 NH 2 (∼20 cm −1 ) compared with that for para-H + C 6 H 5 NH 2 (<10 cm −1 ). To consider the difference, we calculated the stabilization energy of forming a 1: is expected to be similar to those for the formation of protonated pyridine isomers 32,36 and polycyclic aromatic hydrocarbons. [29][30][31]34,35,55 + has yet to be established, the observation of protonated species supports its formation in electron-bombarded p-H 2 matrices.…”

“…Although this method lacks mass selection, the electron bombardment of a p-H2 matrix containing aromatic species has been proven to produce protonated and hydrogenated species with negligible fragmentations; methods to distinguish protonated and hydrogenated products have been established. [29][30][31][32][33][34][35][36] Because the spectral shift from the gaseous phase is expected to be small in solid p-H2, a comparison with quantum-chemically predicted IR spectra of potential carriers provides definitive assignments of these species. In this work, we applied this method to identify unambiguously the IR spectra of para-, ortho-, and amino-H + C6H5NH2.…”

“…Our method to produce protonated species in solid para -hydrogen ( p -H 2 ) by electron bombardment has several advantages to solve the problems discussed above. , The IR spectra of protonated species obtained in this method exhibit narrow lines, true relative IR intensities, and a wide spectral coverage, suitable to distinguish the spectra of ortho - and para -H + C 6 H 5 NH 2 . Although this method lacks mass selection, the electron bombardment of a p -H 2 matrix containing aromatic species has been proven to produce protonated and hydrogenated species with negligible fragmentations; methods to distinguish protonated and hydrogenated products have been established. − Because the spectral shift from the gaseous phase is expected to be small in solid p -H 2 , a comparison with quantum-chemically predicted IR spectra of potential carriers provides definitive assignments of these species. In this work, we applied this method to identify unambiguously the IR spectra of para -, ortho -, and amino -H + C 6 H 5 NH 2 .…”

Infrared Spectra of Monohydrogenated Aniline, ortho- and para-HC6H5NH2, Generated in Solid para-Hydrogen

“…For both methods, the hydrogenation typically stops at the first step because, with a limited amount of H atoms and with the mixing ratio of the parent much greater than that of the monohydrogenated radicals, further hydrogenation cannot compete with the first hydrogenation step. The electron bombardment method has been used to identify several hydrogenated aromatic species, including benzene derivatives, , heterocyclic compounds, , and polycyclic aromatic hydrocarbons. − …”