1H NMR studies of solvent effects on hydrogen bonding in some pyridine trifluoroacetates

Polar solvent fluctuations drive proton transfer in hydrogen bonded complexes of carboxylic acid with pyridines: NMR, IR and ab initio MD study

We study a series of intermolecular hydrogen-bonded 1 : 1 complexes formed by chloroacetic acid with 19 substituted pyridines and one aliphatic amine dissolved in CD₂Cl₂ at low temperature by ¹H and ¹³C NMR and FTIR spectroscopy. The hydrogen bond geometries in these complexes vary from molecular (O-HN) to zwitterionic (O^-H-N⁺) ones, while NMR spectra show the formation of short strong hydrogen bonds in intermediate cases. Analysis of C[double bond, length as m-dash]O stretching and asymmetric CO₂^- stretching bands in FTIR spectra reveal the presence of proton tautomerism. On the basis of these data, we construct the overall proton transfer pathway. In addition to that, we also study by use of ab initio molecular dynamics the complex formed by chloroacetic acid with 2-methylpyridine, surrounded by 71 CD₂Cl₂ molecules, revealing a dual-maximum distribution of hydrogen bond geometries in solution. The analysis of the calculated trajectory shows that the proton jumps between molecular and zwitterionic forms are indeed driven by dipole-dipole solvent-solute interactions, but the primary cause of the jumps is the formation/breaking of weak CHO bonds from solvent molecules to oxygen atoms of the carboxylate group.

Emission spectra studies on the hydrogen-bonding complexes between p-methoxy-2-styrylquinoline and acids

The ground and excited state interactions between p-methoxy-2-styrylquinoline (2-StQ-OMe) and different protic acids with various acid concentrations were studied in different media. Emission bands due to the hydrogen bonding complex, protonation complex and aggregation or solid complex were observed for the first time, in n-hexane. The different reaction pathways were controlled by the solvent polarity and the concentrations of the protic acid. The acidity of acid was also important to the acid-base complex.

A low-barrier hydrogen bond in the catalytic triad of serine proteases

Spectroscopic properties of chymotrypsin and model compounds indicate that a low-barrier hydrogen bond participates in the mechanism of serine protease action. A low-barrier hydrogen bond between N delta 1 of His57 and the beta-carboxyl group of Asp102 in chymotrypsin can facilitate the formation of the tetrahedral adduct, and the nuclear magnetic resonance properties of this proton indicate that it is a low-barrier hydrogen bond. These conclusions are supported by the chemical shift of this proton, the deuterium isotope effect on the chemical shift, and the properties of hydrogen-bonded model compounds in organic solvents, including the hydrogen bond in cis-urocanic acid, in which the imidazole ring is internally hydrogen-bonded to the carboxyl group.