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

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-C₅H₄(OH)NH⁺, and the 3-hydroxy-(1H)-pyridinyl radical, 3-C₅H₄(OH)NH, produced on electron bombardment during deposition of a mixture of 3-hydroxypyridine, 3-C₅H₄(OH)N, and para-H₂ to form a matrix at 3.2 K were recorded. Intense IR absorption lines of trans-3-C₅H₄(OH)NH⁺ at 3594.4, 3380.0, 1610.6, 1562.2, 1319.4, 1193.8, 1167.5, and 780.4 cm^-1 and eleven weaker ones decreased in intensity after the matrix was maintained in darkness for 20 h, whereas lines of trans-3-C₅H₄(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^-1 and nine weaker ones increased. The intensities of lines of trans-3-C₅H₄(OH)NH decreased upon irradiation at 520 nm and diminished nearly completely upon irradiation at 450 nm, whereas those of trans-3-C₅H₄(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-C₅H₄(OH)NH⁺ cation and 3-C₅H₄(OH)NH radical are predicted to be the most stable species among all possible isomers by quantum-chemical calculations.

The photohydration of N-alkylpyridinium salts: theory and experiment

Bicyclic aziridines formed by the irradiation of pyridinium salts in basic solution have recently been recognized to have great synthetic potential. We have undertaken a joint computational and experimental investigation of the mechanism of this photoreaction. We have computationally determined the structures and relative energies of the relevant stationary points on the lowest potential energy surface (PES) of the pyridinium and methylpyridinium ions. Two important intermediates are shown to be bound minima on the ground-state PES: azoniabenzvalene and a 6-aza[3.1.0]bicyclic ion with an exo-oriented substituent (analogous to prefulvene). We advance a mechanism which involves initial formation of this exo-bicyclic ion, followed by nitrogen migration around the ring via the azoniabenzvalene intermediate. Thus, the barrier separating the two intermediates is the factor that determines the degree of scrambling observed in the photoproducts when the carbon atoms are labeled with deuterium or substituted with additional methyl groups. For N-methylpyridinium, the exo-methyl bicyclic ion was computed to be approximately 1 kcalmol(-1) lower in energy than N-methyl-azoniabenzvalene. The transition state was computed to lie several kcal mol(-1) above the exo-methyl bicyclic ion (+8.4kcalmol(-1), 6-31G* RHF; +3.7kcalmol(-1), 6-31G* B3LYP), but still well below the energy available from the 254 nm excitation of the N-methylpyridinium ion. The computed relative energies correspond splendidly with several experimental findings which include the preference for exo products, the results of deuterium labeling, and the impact of additional substituent methyl groups on the product distribution.

Trends in Inversion Barriers of Group 15 Compounds. 3. Are Fluorinated Pyridone Derivatives Planar or Nonplanar?

Fluorinated compounds of 4-pyridone are studied using the semiempirical PM3 method, and the ab initio HF and MP2 methods. The perfluorinated derivative of 4-pyridone is predicted to have a nonplanar ring structure with the fluorine ligand at the nitrogen atom lying above the pyridine ring. The inversion barrier for the pentafluoro-4-pyridone is predicted to be 26 kJ/mol similar to that found for NH(3). This distortion corresponds to a static second-order Jahn-Teller effect and is expected to be experimentally detectable at low temperatures. N-Fluoro-4-pyridone is predicted to be nonplanar and has a small inversion barrier of 0.2 kJ/mol at the MP2 level. However, the maximum point of this barrier lies below the lowest zero-point out-of-plane inversion vibrational mode ((1)/(2) 84 cm(-1) identical with 0.5 kJ/mol). This corresponds to a dynamic Jahn-Teller effect and thus is experimentally difficult to verify. The MP2 calculations indicate that at least one fluorine atom is required at the ring nitrogen in order to achieve nonplanarity. Schleyer's negative-independent chemical shift method (NICS) is applied, and the results are used to discuss aromaticity in fluorinated pyridones. The NICS values show that succesive fluorination increases aromaticity. The vibrational spectra of all fluorinated pyridone derivatives are predicted. The vibrational spectrum of 4-pyridone is discussed in detail using a normal-mode analysis defined within a set of nonredundant internal coordinates.