NMR and XRD Study of Hydrogen Bonding in Picolinic Acid N-Oxide in Crystalline State and Solutions: Media and Temperature Effects on Potential Energy Surface

Abstract

Solvent and temperature effects on H-bonding in crystalline picolinic acid N-oxide (PANO) and in solutions were studied by NMR (H MAS and H-C CP/MAS) and X-ray diffraction (XRD) methods. The single-crystal XRD experiments on β-polymorph were carried out at 105 and 299 K. C chemical shifts of PANO pyridine ring carbons were chosen as an effective diagnostic tool for the H-bond sensing. The crystal field in PANO forces the proton displacement from donor to acceptor atoms much stronger than the solvent reaction field, including that created by the most polar solvents. NMR and XRD data for crystalline PANO do not confirm any H-bond geometry changes in the studied temperature range. On the contrary, a considerable contraction of r(O-H) bond was observed for PANO in acetonitrile (ACN) solution upon heating. The relative contraction of r(O-H) bond with respect to R(O···O) perfectly fits the global dielectric scheme deduced for a vast set of common solvents and the dependence of the dielectric permittivity of ACN on temperature. The subtle H-bond changes can be explained by the temperature dependence of the shape of potential energy surface in the liquid state. Both factors, temperature and dielectric permittivity, are comparable in triggering this effect.

NMR Properties of the Cyanide Anion, a Quasisymmetric Two-Faced Hydrogen Bonding Acceptor

“…For polar solvents, a change in the values of the dielectric constant has a negligible effect. There are a number of approaches that can be used to properly simulate solvent effects [80][81][82][83][84][85]. None of these approaches are universal and effortlessly applicable to an arbitrary molecular system.…”

17O NMR and DFT study of hydrogen bonding: Proton sharing and incipient transfer

“…1. The sequence of subjects was approximately ordered increasing the available values of acid dissociation constants (pKa) as well as the FTIR frequencies of PyO ring modes and the 13 C chemical shifts of ring carbons [24][25][26]. It is clearly seen that the series splits into two parts -the neutral B…H-A bonds with r(O-H)/R(O…O) < 0.5 and the ionic pairs BH + …Awith r(O-H)/R(O…O) > 0.5 that result from the proton transfer (PT).…”

“…[26,46]. It is well-known that δ (OH) in strong H-bonds is high (∼14-20 ppm) [20,24,26]. If the moisture is present, due to the dynamic averaging with the signal of water molecules (∼5 ppm) the experimentally measured δ (OH) values can be shifted far up-field from the 'true' values.…”

“…Meanwhile, precise X-ray and neutron diffraction studies of representative N-oxide complexes with carboxylic acids and molecules with corresponding intramolecular H-bonds [21][22][23][24] have shown that the H-bonds are indeed extremely short, they are in the range between 2.41 and 2.44 Å, and that the protons may assume positions to either nearer acid oxygen or to N-oxygen, but in any case close to the middle of the O…O vector. 17 O liquid-state NMR is a useful high-resolution technique to solve structural problems for small organic molecules.…”

“…The predominant position of the proton was determined from the FTIR frequencies of protonation sensitive pyridine ring modes [25]. The 13 C chemical shifts of the ring carbon turned out to also be reliable indicators of the state of protonation [24,26].…”