RESUMEN
Scanning tunneling microscopy (STM) data for α-ketoester/1-(1-naphthyl)ethylamine complexes on Pt(111) reveal a tumbling motion that couples two neighboring binding states. The interconversion, resulting in prochiral inversion of the α-ketoester, occurs in single complexes without breaking them apart. This is a surprising observation because the overall motion requires rotation of the α-ketoester away from the surface without branching exclusively into diffusion away from the complex or desorption. The multi-step interconversion is rationalized in terms of sequences of bound states that combine transient H-bond interactions with the chiral molecule and weakened adsorption interactions with the metal. The observation of tumbling in single long-lived complexes is of relevance to self-assembly and directed molecular motion on surfaces, to ligand-controlled surface reactions, and most directly to stereocontrol in asymmetric heterogeneous catalysis.
RESUMEN
We report in this work detailed measurements of the chiral and achiral sum-frequency vibrational spectra in the C-H stretching vibration region (2800-3050 cm(-1)) of the air/liquid interfaces of R-(+)-limonene and S-(-)-limonene, using the recently developed high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS). The achiral SFG spectra of R-limonene and S-limonene, as well as the RS racemic mixture (50/50 equal amount mixture), show that the corresponding molecular groups of the R and S enantiomers are with the same interfacial orientations. The interference chiral SFG spectra of the limonene enantiomers exhibit a spectral signature from the chiral response of the Cα-H stretching mode, and a spectral signature from the prochiral response of the CH(2) asymmetric stretching mode, respectively. The chiral spectral feature of the Cα-H stretching mode changes sign from R-(+)-limonene to S-(-)-limonene surfaces, and disappears for the RS racemic mixture surface. While the prochiral spectral feature of the CH(2) asymmetric stretching mode is the same for R-(+)-limonene and S-(-)-limonene surfaces, and also surprisingly remains the same for the RS racemic mixture surface. Therefore, the structures of the R-(+)-limonene and the S-(-)-limonene at the liquid interfaces are nevertheless not mirror images to each other, even though the corresponding groups have the same tilt angle from the interfacial normal, i.e., the R-(+)-limonene and the S-(-)-limonene at the surface are diastereomeric instead of enantiomeric. These results provide detailed information in understanding the structure and chirality of molecular interfaces and demonstrate the sensitivity and potential of SFG-VS as a unique spectroscopic tool for chirality characterization and chiral recognition at the molecular interface.