Intrinsic Chirality and Prochirality at Air/R‐(+)‐ and S‐(–)‐Limonene Interfaces: Spectral Signatures With Interference Chiral Sum‐Frequency Generation Vibrational Spectroscopy

Abstract

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.

Sum frequency generation vibrational spectroscopy (SFG-VS) for complex molecular surfaces and interfaces: Spectral lineshape measurement and analysis plus some controversial issues

“…The results helped to develop the concept and experimental methodology to study intrinsic chirality and prochirality of chiral molecules on surfaces. It was also found that the two limonene enantiomers are actually not mirror image to each other on the liquid surfaces [90]. Figure 5 HR-BB-SFG-VS resolves two closely overlapping peaks in the 2920 cm -1 peak of the air/DMSO interface.…”

“…One recent experiment with the HR-BB-SFG-VS clearly showed that the chiral and achiral SFG-VS spectra of the R-(+)-limonene and S-(-)-limonene liquid surfaces [90] are both significantly different from the chiral spectra in the limonene bulk liquid [210]. This study not only provided a clear demonstration of the surface-selectivity of the chiral SFG-VS, but also presented a detailed analysis of the intrinsic chirality and orientational (prochiral) chirality contributions in the observed SFG signal [90].The relative phases of each spectral peaks from the R-(+)-limonene and S-(-)-limonene liquid surfaces can be obtained by fitting the HR-BB-SFG-VS [90]. These results are consistent with the direct phase-resolved measurement [207] and are with much better spectral accuracy and better resolved peaks.…”

“…There have been techniques based on the interference between the chiral and achiral terms for chiral SFG-VS measurement of chirality on molecular surfaces and interfaces, such as the PMP polarization method [31,90,210,211], and the twin polarization angle (TPA) method [87], etc. Recently, Petersen at al.…”

“…Like the discussion above, careful examination to separate the surface and bulk contributions to the chiral SFG-VS is the first step for interpretation and analysis of the chiral SFG-VS data from the chiral liquid surface/interface. When the liquid phase itself is not chiral or can only generate insignificant amount of chiral SFG-VS signal, then the chiral signal can only come from the intrinsic chiral and prochiral contributions from the molecular surface/interface [9,90,208,213].…”

“…Previous quantitative SFG-VS studies have focused on the polarization dependence that can provide symmetry properties of the vibrational transition of molecular groups for spectral assignment [21,[80][81][82][83][84], and for molecular orientation analysis [17,31,82,85,86]. Polarization dependence in SFG-VS was also used to study chiral structures and interfaces at molecular and biological surfaces and interfaces [27,30,31,[87][88][89][90].…”