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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.

Temperature and composition effects in Sunset Yellow FCF (SSY) aqueous solutions were studied by the H,N NMR as well as Raman spectroscopy passing through all phase transitions between isotropic phase (I) and chromonic phases-nematic (N) and columnar (M). It was shown that the tautomeric equilibrium in SSY is strongly shifted toward the hydrazone form. The corresponding equilibrium constant p K = 2.5 was deduced using the density functional theory solvent model density model. The dominance of the hydrazone form was confirmed experimentally using the long-range H-N correlation, widely known as heteronuclear multiple bond correlation. The peak found in the H NMR spectra at ca. 14.5 ppm can be attributed to the proton in the intramolecular N-H···O bond. The existence of this signal shows that (i) the growth of the SSY aggregates is accompanied by the segregation of water in the intercolumnar areas with no access for exchange with the N-H protons in the internal layers of the columnar stacks and that (ii) the lifetime of those aggregates is ≥10 s or even longer. The temperature dependences of HO chemical shift and Raman O-H stretching band shape show that water confined in the intercolumnar areas behaves as in the neat substance. When the sample is heated and the transition from M phase to N phase occurs, the molecular motion of water is seen to change in a manner similar to that when water is melting. The equilibration time for N + M→ M is very long because of slow supramolecular restructuring, i.e., the growing of columnar stacks and building of hexagonal arrays. If the sample is cooled down to the temperature below N → M transition relatively fast, the structural changes are behind, and the system falls into supercooled state. In this case, the system evolves via long-lasting self-assembling from the supercooled state to the equilibrium. This process affects the shape of the H NMR signal and is easy to monitor.

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