This new paper is a very interesting approach to chiral separation, and I would like to go into detail about how it works. Unfortunately, section 2 of the Supplementary material goes into detail, and it’s titled “Three-level optical Bloch equations”, and I can just about follow it until I get to the part that says “The time evolution of the density matrix is then expressed by the Liouville equation“, and equally unfortunately, that’s at the end of the first paragraph. So someone with more physics than I have in my pockets will have to comment on the mechanism.
But the upshot is that they’re separating enantiomers by pulsing various polarized microwave bursts at them – you can promote either the R or S to a higher rotational state selectively. This sort of thing in general (enantioselectivity through electromagnetic means) has a long and not particularly successful history. It ranges from extremely tiny (but real) effects that are sometimes brought into origin-of-life debates (why did life on Earth settle on the enantiomeric series it did?), all the way to large, attention-getting, and completely unreproducible claims such as those from a group at Bonn in the 1990s, who published on enantioselective reactions in high magnetic fields until the work turned out to have been faked up by one of the coauthors (as I recall the story).
This work is certainly far more firmly grounded, and is a follow-up to the research program described in this review. Here’s the abstract:
We report the experimental demonstration of coherent enantiomer-selective enrichment of chiral molecules by employing a novel microwave five-pulse scheme. Our results show that enantiomers can be selectively transferred to a rotational level of choice by applying sequences of resonant microwave pulses in a phase- and polarization-controlled manner.
Unfortunately, I don’t have access to the full text, so I’m going to have to add more when I do. I did want to get this out there for comment, though, because it would seem to have implications for analytical chemistry (and perhaps possibly synthetic methods?) Look to the comments here for more from people who have better library access than I do at the moment, and better grasps of rotational spectroscopy as well. . .