Reaction Pathways of Proton Transfer in Hydrogen-Bonded Phenol-Carboxylate Complexes Explored by Combined UV-Vis and NMR Spectroscopy

摘要

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 = l-~(13)C-2-chloro-4-nitrophenol, X~- = 15 carboxylic acid anions, 5 phenolates, Cl~-, Br~-, I~-, and BF_4~-). In this series, H is shifted from A to X, modeling the proton-transfer pathway. The ~1H and ~(13)C 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.