A comparative VCD study of methyl mandelate in methanol, dimethyl sulfoxide, and chloroform: explicit and implicit solvation models

Abstract

Vibrational absorption (VA) and vibrational circular dichroism (VCD) spectra of methyl mandelate, a prototype chiral molecule, in a series of organic solvents, namely methanol (MeOH-d(4)), dimethyl sulfoxide (DMSO-d(6)), and chloroform (CDCl(3)), have been measured in the finger print region from 1800 to 1150 cm(-1). Implicit solvation models in the form of polarizable continuum model and explicit solvation models have been employed independently and simultaneously. The goal is to evaluate their efficiencies in dealing with solvent effects in each solution and to establish a general strategy to adequately account for effects of solvents. Molecular dynamics (MD) simulation and radial distribution function analysis have been performed to aid the construction of the explicit solvation models. Initial geometry searches have been carried out at the B3LYP/6-31G(d) level for the methyl mandelate monomer and its explicit 1 : 1 and 1 : 2 solute-solvent hydrogen-bonded complexes. B3LYP/cc-pVTZ has been used for all the final geometry optimizations, the vibrational frequency, VA and VCD intensity, and optical rotation dispersion (ORD) calculations. The results show that inclusion of solvent explicitly and implicitly at the same time has significant impacts on the appearance of the VA and VCD spectra, and is crucial for reliable spectral assignments when solvents are capable of hydrogen-bonding interactions with solutes. When no strong solvent-solute hydrogen-bonding interactions in the case of chloroform are expected, the gas phase monomer model is adequate for spectral interpretation, while inclusion of implicit solvation improves the frequency agreement with experiment. ORD spectra of methyl mandelate in the aforementioned solvents at different concentrations under 5 excitation wavelengths have also been measured. The comparison between the calculated and the experimental ORD spectra supports the conclusions drawn from the VA and VCD investigations.

IR, Raman, and Vibrational Optical Activity Spectra of Methyl Glycidate in Chloroform and Water: The Clusters‐in‐a‐liquid Solvation Model

“…Several approaches have been used by researchers. One elaborative approach is to utilize classical MD or Car‐Parrinello MD (CPMD) to create a chiral solute in equilibrium with a box of water molecules ,. Very recently, the first fully ab initio MD calculation of dynamical VCD spectrum of propylene oxide in the liquid phase was reported, which reproduced much of the experimental data although the theoretical prediction of the induced solvent VCD signal was still buried in simulation noise .…”

“…These studies pointed out the importance of both explicit and implicit solute‐solvent interactions in modelling solvent effects, for example for solvents which can serve as a hydrogen (H)‐bond donor/acceptor to a solute. For many common solvents, such as chloroform and dimethyl sulfoxide (DMSO), one can adequately account for solvent effects by constructing small solute‐solvent H‐bonded complexes and placing them in a bulk solvent which is commonly modelled by the polarizable continuum model (PCM) ,,…”

“…For these reasons, some researchers advocate a molecular dynamics (MD) simulation approach and sample several thousand snap shots in order to catch such dynamics in modelling water solvent effects ,,. Some researchers use the combined explicit and implicit model discussed above and dubbed it the “ad hoc hydration” model since the selection of possible chiral solute‐water n complexes seemed random. These approaches will be further commented on in the main text.…”

Rotational spectroscopy of methyl benzoylformate and methyl mandelate: structure and internal dynamics of a model reactant and product of enantioselective reduction

“…c Percent abundance at 323 K, the source temperature, based on relative free energy. 21 While no corresponding study has been reported for MBF, for the sake of discussion, we tentatively assume that the aforementioned dihedral angles remain approximately the same for both MBF and MM in methanol solution as in the gas phase. five-membered ring is no longer nearly planar.…”

“…17 Le Barbu-Debus et al assigned their double-resonance IR/UV spectrum of MM in the gas phase to the latter conformer, containing an OHÁ Á ÁOQC intramolecular hydrogen bond, which they also calculated to be the minimum energy conformer. 21 Finally, Shi et al used MM as a representative species in their computational model of chirality recognition. 19 Furthermore, Nedić and Suhm characterized crystalline MM using Fourier-transform infrared microspectroscopy, 20 and Poopari et al characterized solvated MM using vibrational circular dichroism.…”