Probing Enantiospecific Interactions at Chiral Solid-Liquid Interfaces by Absolute Configuration Modulation Infrared Spectroscopy Ronny Wirz, Thomas Bu ¨ rgi,* and Alfons Baiker Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH Ho ¨ nggerberg, CH-8093 Zu ¨ rich, Switzerland Received September 17, 2002. In Final Form: November 26, 2002 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)-R-methylbenzyl- carbamate] coated onto silica gel. D-Ethyl lactate interacts stronger with the chiral stationary phase. In particular the spectral shifts reveal a stronger N-H‚‚‚OdC 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)-R-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. Introduction Chiral interfaces are ubiquitous in nature and play an important role in technology, for example in separation processes. 1 Intrinsically chiral metal surfaces 2,3 and chiraly modified surfaces 4-7 have also gained considerable interest for applications in the field of heterogeneous catalysis. 8 Fundamental insight into processes occurring at such interfaces can benefit from techniques that give molecular level information on the interface on one hand and that selectively probe enantiospecificity on the other hand. The challenge for suitable experimental techniques is the combination of surface sensitivity with the selectivity to probe chirality. Many surface sensitive techniques can yield valuable information from chiral surfaces but do not probe selectively the chiral information. For example, adsorption of enantiomers on a chiral surface gives rise to diastereomeric interactions, which can be revealed by infrared spectroscopy. 9 A disadvantage of such “nonspe- cific” methods is that the spectral differences of the adsorbed enantiomers are usually small compared to the absolute signal from either one of the enantiomers. Furthermore, the small differences are buried in signals arising from nonspecifically adsorbed species. Nonlinear optical techniques, such as second harmonic generation- circular dichroism, are surface sensitive and may prove useful for studying chiral surfaces. 10 Here we present a method that selectively enhances difference signals arising from diastereomeric interactions of enantiomers with chiral solid-liquid interfaces. We use attenuated total reflection (ATR) 11 infrared spectros- copy to follow the adsorption of enantiomers at a chiral solid-liquid interface. Solutions of the two enantiomers are periodically admitted to the chiral solid surface in a flow-through cell. Time-resolved spectra are recorded and the signals subsequently demodulated by a digital phase- sensitive data analysis. The resulting spectra selectively reveal the enantiospecificity of the interaction between chiral molecule and chiral interface. The potential of the method is demonstrated by investigating adsorption of ethyl lactate on a chiral stationary phase (CSP), which is used in HPL chromatography. There is currently con- siderable interest in a more fundamental understanding of the chiral recognition between chiral selector and selectand, 12 which ultimately leads to separation of enantiomers. Absolute Configuration Modulation Infrared Spectroscopy Modulation spectroscopy is a sensitive technique for the investigation of reversible systems. 13-16 The method * Corresponding author: E-mail: buergi@tech.chem.ethz.ch Telephone: +41-1-632 22 67 Fax: +41-1-632 11 63. (1) Sheldon, R. A. Chirotechnology; Marcel Dekker: New York, 1993. (2) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483. (3) Attard, G. A. J. Phys. Chem. B 2001, 105, 3158. (4) Lorenzo, M. O.; Haq, S.; Bertrams, T.; Murray, P.; Raval, R.; Baddeley, C. J. J. Phys. Chem. B 1999, 103, 10661. (5) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature (London) 2000, 4004, 376. (6) Ferri, D.; Bu ¨ rgi, T. J. Am. Chem. Soc. 2001, 123, 12074. (7) Ferri, D.; Bu ¨ rgi, T.; Baiker, A. J. Phys. Chem. B 2001, 105, 3187. (8) Baiker, A.; Blaser, H. U. In Enantioselective Catalysts and Reactions. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno ¨zinger, H., Weitkamp, J., Eds.; VCH Publishers: Weinheim, Germany, 1997; Vol. 5, p 2422. (9) Gellman, A. G.; Horvath, J. D.; Buelow, M. T. J. Mol. Catal. A: Chem. 2001, 167, 3. (10) Kauranen, M.; Verbiest, T.; Maki, J. J.; Persoons, A. J. Chem. Phys. 1994, 101, 8193. (11) Harrick, N. J. Internal reflection spectroscopy; Interscience: New York, 1967. (12) Maier, N. M.; Schefzick, S.; Lombardo, G. M.; Feliz, M.; Rissanen, K.; Lindner, W.; Lipkowitz, K. B. J. Am. Chem. Soc. 2002, 124, 8611. (13) Mu ¨ ller, M.; Buchet, R.; Fringeli, U. P. J. Phys. Chem. 1996, 100, 10810. 785 Langmuir 2003, 19, 785-792 10.1021/la026568y CCC: $25.00 © 2003 American Chemical Society Published on Web 01/01/2003