Probing enantiospecific interactions at chiral solid-liquid interfaces by absolute configuration modulation infrared spectroscopy

摘要

A method to selectively probe the different adsorption of enantiomers at chiral solid-liquid interfaces is presented, which combines attenuated total reflection infrared spectroscopy and modulation spectroscopy. The weak spectral changes upon adsorption of enantiomers at a chiral interface are followed in time, while periodically changing the absolute configuration of the admitted chiral molecule. A subsequent digital phase-sensitive data analysis reveals spectral differences arising due to the different diastereomeric interactions of the two enantiomers with the chiral interface. The main advantage of the method compared to conventional difference spectroscopy is the enhanced signal-to-noise ratio. The method is selective for differences in diastereomeric interactions of the enantiomers. Its potential is demonstrated by studying the adsorption of ethyl lactate on a chiral stationary phase, which is amylose tris[(S)-alpha-methylbenzylcarbamate] coated onto silica gel. D-Ethyl lactate interacts stronger with the chiral stationary phase. In particular the spectral shifts reveal a stronger N-H...O=C hydrogen bonding interaction between amide group of the chiral stationary phase and the ester group of the ethyl lactate. The spectra also indicate that one of the three (S)-alpha-methylbenzylcarbamate side chains of the amylose derivative is predominantly involved in the interaction with the ethyl lactate. Furthermore, the experimental observations indicate that more than one interaction mode is populated at room temperature and that interaction with the ethyl lactate may induce a conformational change of the amide group of the chiral stationary phase. [References: 21]