Among the many physicochemical properties that distinguish chiral molecules, perhaps none has had as profound and sustained an impact on chemistry as the characteristic interactions taking place with polarized light. Both the resonant (circular-differential absorption: circular dichroism or CD) and the non-resonant (circular-differential retardation: circular birefringence or CB) components of this optical activity long have been used to quantify enantiomeric purity and determine stereochemical configuration; however, the fruitful application of such spectral methods often can be hampered by extrinsic perturbations incurred from the environment (<i>e.g.</i>, solvation), which potentially can alter the magnitude and even the sign of attendant chiroptical phenomena. The present work has exploited the ultrasensitive techniques of cavity ring-down polarimetry (CRDP) to interrogate intrinsic properties in the rarefied vapor phase, thereby affording an incisive counterpoint to conventional solution-phase polarimetric measurements by discriminating and accentuating the roles of specific and/or non-specific solute-solvent coupling. Emphasis has been placed on unraveling the nature of dispersive signatures (CB), with linear-response calculations built upon density-functional and coupled-cluster frameworks being enlisted to elucidate the electronic and structural origins of innate (isolated-molecule) rotatory powers, as well as their mediation by the surroundings. Pronounced effects arising from nuclear degrees of freedom (<i>viz.</i>, small-amplitude vibrational and large-amplitude conformational dynamics) will be highlighted, especially for chiral species that combine putatively small values of purely electronic response with unexpectedly strong solvation processes. Styrene oxide and related aryl-oxirane derivatives will be shown to represent particularly challenging examples of such systems in which striking solvent-dependent variations of optical rotatory dispersion profiles (that affect both magnitude and sign) can be correlated to subtle intramolecular displacements of the phenyl and epoxide moieties.

Unraveling intrinsic chiroptical properties: Cavity ring-down polarimetry of small organic molecules

255th National Meeting (2018)

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