Vibrational optical activity spectroscopies, namely vibrational circular dichroism (VCD) and Raman optical activity (ROA), have been emerged in the past decade as powerful spectroscopic tools for stereochemical information of a wide range of chiral compounds in solution directly. More recently, their applications in unveiling solvent effects, especially those associated with water solvent, have been explored. In this review article, we first select a few examples to demonstrate the unique sensitivity of VCD spectral signatures to both bulk solvent effects and explicit hydrogen-bonding interactions in solution. Second, we discuss the induced solvent chirality, or chiral transfer, VCD spectral features observed in the water bending band region in detail. From these chirality transfer spectral data, the related conformer specific gas phase spectroscopic studies of small chiral hydration clusters, and the associated matrix isolation VCD experiments of hydrogen-bonded complexes in cold rare gas matrices, a general picture of solvation in aqueous solution emerges. In such an aqueous solution, some small chiral hydration clusters, rather than the chiral solutes themselves, are the dominant species and are the ones that contribute mainly to the experimentally observed VCD features. We then review a series of VCD studies of amino acids and their derivatives in aqueous solution under different pHs to emphasize the importance of the inclusion of the bulk solvent effects. These experimental data and the associated theoretical analyses are the foundation for the proposed “clusters-in-a-liquid” approach to account for solvent effects effectively. We present several approaches to identify and build such representative chiral hydration clusters. Recent studies which applied molecular dynamics simulations and the subsequent snapshot averaging approach to generate the ROA, VCD, electronic CD, and optical rotatory dispersion spectra are also reviewed. Challenges associated with the molecular dynamics snapshot approach are discussed and the successes of the seemingly random “ad hoc explicit solvation” reported before are also explained. To further test and improve the “clusters-in-a-liquid” model in practice, future work in terms of conformer specific gas phase spectroscopy of sequential solvation of a chiral solute, matrix isolation VCD measurements of small chiral hydration clusters, and more sophisticated models for the bulk solvent effects would be highly valuable.
Solvent effects, in particular those involving water as the solvent, are of significant interest to the chemistry and physics communities. IR, vibrational circular dichroism (VCD), Raman, and Raman optical activity (ROA) spectra of methyl glycidate in two very different solvents, namely CCl and water, have been measured experimentally and simulated theoretically. The observed spectra in CCl could be well modelled using the polarizable continuum model for the solvent, whereas the situation is much different in water. The experimental VCD spectrum of methyl glycidate in water reveals strong induced VCD signatures in the water bending region, indicating the presence of the relatively long-lived methyl glycidate-water complexes. We applied the clusters-in-a-liquid approach to identify the dominant methyl glycidate-water complexes which are the long-lived species responsible for all the spectra observed in water. We examined the influences of solvent dielectric environment and the hydrogen-bonding interactions on the conformational distribution of methyl glycidate. The geometry optimizations, frequency calculations, IR, VCD, Raman and ROA intensity calculations were performed at the B3LYP/6-311++G(2d,p) and aug-cc-pVTZ levels of theory with D3BJ dispersion correction. It is particularly satisfying to note that the clusters-in-a-liquid approach has captured all main experimental features in IR, VCD, Raman and ROA spectra of methyl glycidate in water.
The matrix isolation (MI) technique has been utilized with vibrational circular dichroism (VCD) spectroscopy to obtain MI-VCD spectra of lactic acid (LA) in cold argon matrices, in addition to their MI-IR spectra. The experiments have been done at three different deposition temperatures (10 K, 16 K and 24 K) under different Ar flow rates so that different degrees of LA self-aggregation occur. The structural and spectral investigations of the LA monomer and the larger (LA)2,3,4 aggregates have been undertaken at three levels of theory (B3LYP/6-311++G(2d,p), B3LYP-D3BJ/6-311++G(2d,p) and B3LYP-D3BJ/def2-TZVPD) to evaluate the effects of dispersion correction and basis sets on optimized structures, relative conformer energies, and IR/VCD spectral features. Interestingly, the relative conformer energies vary considerably with and without dispersion correction, especially when the molecule gets larger and when it is placed in solution. Such uncertainties in the relative energies and in the vibrational band positions and IR/VCD intensities highlight the challenges in interpreting experimental spectroscopic data, especially those obtained in solution. With the narrow MI-IR band width and highly characteristic MI-VCD spectral features and the trend observed at three temperatures, we have been able to correlate the spectral features confidently to those of the LA monomer and the larger (LA)2,3,4 aggregates, with the aid of theoretical modeling. Finally, by noting the similarity of MI-IR and especially MI-VCD features obtained at 24 K with those of the 0.2 M solution, and with the aid of spectral simulation at the B3LYP-D3BJ/def2-TZVPD level, a composition of LA aggregates dominated by the LA tetramer and trimer has been identified. This conclusion differs from the previous reports where the LA dimer was identified as the main species at even higher concentration in CDCl3. The present work showcases the power of MI-VCD spectroscopy in aiding solution spectral assignment and in providing insight into the complex self-aggregation behavior of LA in solution.
The infrared (IR) and vibrational circular dichroism (VCD) spectra of methyl β‐D‐glucopyranose in water were measured. Both implicit and explicit solvation models were utilized to explain the observed spectra. The vast body of existing experimental and theoretical data suggested that about eight explicit water molecules are needed to account for the solvent effects, supported by the current Quantum Cluster Growth (QCG) analysis. Extensive manual and systematic conformational searches of the molecular target and its water clusters were carried out by using a recently developed conformational searching tool, conformer‐rotamer ensemble sampling tool (CREST), and the microsolvation model in the associated QCG code. The Boltzmann averaged IR and VCD spectra of the methyl β‐D‐glucopyranose‐(water)n (n = 8) conformers in the PCM of water provide better agreement with the experimental ones than those with n = 0, 1, and 2. The explicit solvation with eight water molecules was shown to greatly modify the conformational preference of methyl β‐D‐glucopyranose from its monomeric form. Further analyses show that the result is consistent with the existence of long‐lived methyl β‐D‐glucopyranose monohydrates with the additional explicit water effects being accounted for with the quantum mechanical treatment of the other seven close‐by water molecules in the PCM of water.
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