Reaction pathways of proton transfer in hydrogen-bonded phenol-carboxylate complexes explored by combined UV-Vis and NMR spectroscopy

Combined low-temperature NMR/UV–vis spectroscopy (UVNMR), where optical and NMR spectra are measured in the NMR spectrometer under the same conditions, has been set up and applied to the study of H-bonded anions A··H··X– (AH = 1-13C-2-chloro-4-nitrophenol, X– = 15 carboxylic acid anions, 5 phenolates, Cl–, Br–, I–, and BF4–). In this series, H is shifted from A to X, modeling the proton-transfer pathway. The 1H and 13C chemical shifts and the H/D isotope effects on the latter provide information about averaged H-bond geometries. At the same time, red shifts of the π–π* UV–vis absorption bands are observed which correlate with the averaged H-bond geometries. However, on the UV–vis time scale, different tautomeric states and solvent configurations are in slow exchange. The combined data sets indicate that the proton transfer starts with a H-bond compression and a displacement of the proton toward the H-bond center, involving single-well configurations A–H···X–. In the strong H-bond regime, coexisting tautomers A··H···X– and A–···H··X are observed by UV. Their geometries and statistical weights change continuously when the basicity of X– is increased. Finally, again a series of single-well structures of the type A–···H–X is observed. Interestingly, the UV–vis absorption bands are broadened inhomogeneously because of a distribution of H-bond geometries arising from different solvent configurations.