How Short is the Strongest Hydrogen Bond in the Proton-Bound Homodimers of Pyridine Derivatives?

Hydrogen bond geometries in the proton-bound homodimers of ortho-unsubstituted and ortho-methylsubstituted pyridine derivatives in aprotic polar solution were estimated using experimental NMR data. Within the series of homodimers studied the hydrogen bond lengths depend on the proton affinity of pyridines andat least for the ortho-methylsubstituted pyridineson the p<i>K</i>_a of the conjugate acids in an approximately quadratic manner. The shortest possible hydrogen bond in the homodimers of ortho-unsubstituted pyridines is characterized by the N···N distance of 2.613 Å. Steric repulsion between the methyl groups of the ortho-methylsubstituted pyridines becomes operative at an N···N distance of ∼2.7 Å and limits the closest approach to 2.665 Å

Solvent and H/D Isotope Effects on the Proton Transfer Pathways in Heteroconjugated Hydrogen-Bonded Phenol-Carboxylic Acid Anions Observed by Combined UV–vis and NMR Spectroscopy

Heteroconjugated hydrogen-bonded anions A···H···X^– of phenols (AH) and carboxylic/inorganic acids (HX) dissolved in CD₂Cl₂ and CDF₃/CDF₂Cl have been studied by combined low-temperature UV–vis and ¹H/¹³C NMR spectroscopy (UVNMR). The systems constitute small molecular models of hydrogen-bonded cofactors in proteins such as the photoactive yellow protein (PYP). Thus, the phenols studied include the PYP cofactor 4-hydroxycinnamic acid methyl thioester, and the more acidic 4-nitrophenol and 2-chloro-4-nitrophenol which mimic electronically excited cofactor states. It is shown that the ¹³C chemical shifts of the phenolic residues of A···H···X^–, referenced to the corresponding values of A···H···A^–, constitute excellent probes for the average proton positions. These shifts correlate with those of the H-bonded protons, as well as with the H/D isotope effects on the ¹³C chemical shifts. A combined analysis of UV–vis and NMR data was employed to elucidate the proton transfer pathways in a qualitative way. Dual absorption bands of the phenolic moiety indicate a double-well situation for the shortest OHO hydrogen bonds studied. Surprisingly, when the solvent polarity is low the carboxylates are protonated whereas the proton shifts toward the phenolic oxygens when the polarity is increased. This finding indicates that because of stronger ion-dipole interactions small anions are stabilized at high solvent polarity and large anions exhibiting delocalized charges at low solvent polarities. It also explains the large acidity difference of phenols and carboxylic acids in water, and the observation that this difference is strongly reduced in the interior of proteins when both partners form mutual hydrogen bonds

NMR Studies of Solid Pentachlorophenol-4-Methylpyridine Complexes Exhibiting Strong OHN Hydrogen Bonds: Geometric H/D Isotope Effects and Hydrogen Bond Coupling Cause Isotopic Polymorphism

We have studied the hydrogen bond interactions of ¹⁵N labeled 4-methylpyridine (4-MP) with pentachlorophenol (PCP) in the solid state and in polar solution using various NMR techniques. Previous spectroscopic, X-ray, and neutron crystallographic studies showed that the triclinic 1:1 complex (4-MPPCP) exhibits the strongest known intermolecular OHN hydrogen bond in the solid state. By contrast, deuteration of the hydrogen bond gives rise to the formation of a monoclinic structure exhibiting a weaker hydrogen bond. By performing NMR experiments at different deuterium fractions and taking advantage of dipolar ¹H–¹⁵N recoupling under combined fast MAS and ¹H decoupling, we provide an explanation of the origin of the isotopic polymorphism of 4-MPPCP and improve previous chemical shift correlations for OHN hydrogen bonds. Because of anharmonic ground state vibrations, an ODN hydrogen bond in the triclinic form exhibits a shorter oxygen–hydron and a longer oxygen–nitrogen distance as compared to surrounding OHN hydrogen bonds, which also implies a reduction of the local dipole moment. The dipole–dipole interaction between adjacent coupled OHN hydrogen bonds which determines the structure of triclinic 4-MPPCP is then reduced by deuteration, and other interactions become dominant, leading to the monoclinic form. Finally, the observation of stronger OHN hydrogen bonds by ¹H NMR in polar solution as compared to the solid state is discussed

The Mobility of Water Molecules through Hydrated Pores

To achieve a more exact understanding of the water transport in natural channels, a series of non-natural structures have been developed. They have been studied by far-infrared spectroscopy, solid-state nuclear magnetic resonance, differential scanning calorimetry, thermogravimetric analysis, and variable-temperature powder X-ray diffraction to examine the behavior of water at the molecular level. Water in these predominantly nonpolar pores can be metastable, with filling and emptying occurring upon changes in solvent conditions. The water contained in these pores exhibits a dynamics that might be controlled, since it depends on the structural features of the monomers that form the pore “skeleton”. We have observed changes in the pore diameter depending on the selected isomer. This provokes at a given temperature differences in the arrangement and dynamics of the contained water. The water dynamics increases with both temperature and pore diameter in a process that is reversible over a temperature range specific for each structure. Beyond this particular temperature threshold, the pore water can be irreversibly evacuated, and at this point a decrease of the dynamics is observed. The slower dynamics of the remaining water in partially evacuated pores is probably due to the increased interaction with the inner-pore surface owing to a concomitant narrowing of the pore. These findings not only highlight the need for the presence of freely moving water inside the pore to sustain its permeability by water, but also point to the decrease in the dynamics of the remaining water in partially evacuated pores