Hydrogen Bonding in Pyridine N-Oxide/Acid Systems: Proton Transfer and Fine Details Revealed by FTIR, NMR, and X-ray Diffraction

Abstract

The H-bonded complexes of pyridine N-oxide (PyO) with H(2)O, acetic, cyanoacetic, propiolic, tribromoacetic, trichloroacetic, trifluoroacetic, hydrochloric, and methanesulfonic acids have been studied by FTIR and NMR spectroscopy, X-ray diffraction, and quantum chemical DFT calculations. Correlations between vibrational frequencies of the NO stretching and PyO ring modes and geometric parameters of the H-bond have been established. FTIR experiments show and DFT calculations confirm that definite discontinuity is present in the vicinity of the midpoint in the proton transfer pathway. The established correlations significantly aid in the understanding of fine effects such as the isotope (deuteration) effect, crystal-to-solution transition, or criticality of aqueous solutions induced by ionic pairs. Geometric isotope effect in the ionic H-bond aggregate of PyO·H(D)Cl was found to be extraordinary large. Measured FTIR, CP/MAS, and high-resolution (13)C NMR spectra indicate that H-bond in the PyO·HCl complex in polar solvent can potentially be more ionic than in the crystal. Vibrational modes of ionic pairs originating via proton transfer in H-bond complexes can provide new information concerning the interionic interaction and its role in the phase separation and mezo-structuring processes. The results are compared to the relevant data for PyO·HCl complex in argon matrix.

Oxidation of 2-mercaptopyridine N-oxide upon iodine agent: structural and FT-IR studies on charge-assisted hydrogen bonds CAHB(+) and I…I halogen interactions in 2,2′-dithiobis(pyridine N-oxide) ionic cocrystal

“…This band can be assigned to the νNO mode perturbed by the hydrogen bond interaction, as indicated by X-ray studies. Analogous band shifts in FT-IR spectra were already observed during the proton transfer in pyridine N-oxide / acid systems [31]. In the title, cocrystal structure iodide anions I − are bound to iodine molecules I 2 forming I 3 − complex anions (compare Fig.…”

“…The distance between interacting oxygen atoms of donor and acceptor groups O1…O2 is equal 2.416(2) Å and belongs to shortest ones observed in N-oxide hydrogen bonds [27][28][29][30][31]. Other geometric parameters of the observed interactions are presented in the Table 2.…”

Protonation of Pyridine Monocarboxylate‐N‐Oxides – Determination of Thermodynamic, Absorbance and Ion Interaction Parameters

“…Most of the quantum chemical calculations reported in the literature focussed on evaluation of intra molecular hydrogen bonding, vibrational spectra and proton dynamics of picolinic acid‐N‐oxide . Nicotinic acid N‐oxide (NANO) and isonicotinic acid‐N‐oxide have been studied to determine the UV, NMR, IR and Raman spectra of their monomers and oligomers.…”

Proton transfer in a short hydrogen bond caused by solvation shell fluctuations: an ab initio MD and NMR/UV study of an (OHO)− bonded system

“…54,55 Band positions in optical spectroscopy could also be correlated with the hydrogen bond geometry. Previously it has been done for OH/NH stretching, 56,57 PyO vibrational bands of pyridine oxides, 58 ring modes of pyridines, 59,60 carbonyl stretch of carboxylic acids 23,24 and others.…”