A joint experimental-computational approach for characterizing distinctive chiroptical signatures for mandelic acid and a small family of derivatives are described herein, using circularly polarized light to afford a unique strategy in enantioselective photochemistry. Our collaborative effort has shown the strengths and limitations of contemporary computational approaches using time-dependent density functional theory (TDDFT) to model electronic circular dichroism (ECD) spectra in parallel with experimental methods obtained using synchrotron radiation electronic circular dichroism. Our computational ECD spectra, performed over the wavelength region of 160 to 260 nm, were obtained at the TD-wB97X-D/6-311++G//B3LYP/6-31+G theory level following thermal averaging over relevant molecular conformations and torsion angles. Solvent effects were also included using a polarizable continuum model. Notably, only significant neutral and anionic thermal populations (≥ 0.01) were included, as the relative weights of either form were determined based on the equilibrium concentrations of the acids and their conjugate bases according to their pKa values. Where explicit use of R enantiomers to calculate our ECD spectra afforded computational efficiencies, as the ECD spectra of R and S enantiomers are identical to within their sign. In our study, compelling agreement was found with three of the four molecules analyzed (mandelic acid, 2-phenylpropionic acid, and methyl mandelate), and the resulting ECD spectra provided valuable descriptions of major electronic transitions band positions. With the last molecule, benzoin, less compelling agreement was found as the combination of level of theory used and increased structural complexity (due to extended aromatic conjugation systems) was theorized to be limiting factors in accurately predicting major band positions with good agreement. Our efforts have demonstrated unique predictive tools for the determination of particular wavelength(s) at which this family of small chiral organic molecules will absorb into the vacuum ultra-violet wavelength regions. This joint experimental-computational study serves as a foundation for further chiroptical signature research for non-planar small molecules.

Joint experimental-computational approachs to predicting electronic circular dichroism of mandelic acid and other small chiral organic molecules

ACS Spring Meeting 2021

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