State-Specific Enrichment of Chiral Conformers with Microwave Spectroscopy

An interesting class of molecules is that in which the molecules do not possess a stereogenic center but can become chiral because of their spatial arrangement. These molecules can be seen as chiral conformers, whose two nonsuperimposable forms can interconvert from one to another by rotations about single bonds. Here, we show that an initially racemic mixture of chiral conformers, such as a sample of cyclohexylmethanol, C₇H₁₄O, can be enantiomerically enriched by performing the enantioselective process of coherent population transfer between rotational levels. By first performing a population transfer cycle, followed by a three-wave mixing experiment, we show that an enantiomeric excess in a rotational level of choice can be achieved. This represents the first experimental demonstration of such an effect in a chiral pair of conformers, and it showcases the broad applicability of three-wave mixing not only for analytical applications but also to a wide scope of experiments of fundamental interest.