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May 24, 2013
A New Way to Determine Chirality
There's a new paper out today in Nature on a very unusual way to determine the chirality of organic molecules. It uses an exotic effect of microwave spectroscopy, and I will immediately confess that the physics is (as of this morning, anyway) outside my range.
This is going to be one of those posts that comes across as gibberish to the non-chemists in the audience. Chirality seems to be a concept that confuses people pretty rapidly, even though the examples of right and left shoes or gloves (or right and left-handed screw threads) are familiar from everyday objects, and exactly the same principles apply to molecules. But the further you dig into the concept, the trickier it gets, and when you start dragging the physics of it in, you start shedding your audience quickly. Get a dozen chemists together and ask them how, exactly, chiral compounds rotate plane-polarized light and see how that goes. (I wouldn't distinguish myself by the clarity of my explanation, either).
But this paper is something else again. Here, see how you do:
Here we extend this class of approaches by carrying out nonlinear resonant phase-sensitive microwave spectroscopy of gas phase samples in the presence of an adiabatically switched non-resonant orthogonal electric field; we use this technique to map the enantiomer-dependent sign of an electric dipole Rabi frequency onto the phase of emitted microwave radiation.
The best I can do with this is that the two enantiomers have the same dipole moment, but that the electric field interacts with them in a manner that gives different signs. This shows up in the phase of the emitted microwaves, and (as long as the sample is cooled down, to cut back on the possible rotational states), it seems to give a very clear signal. This is a completely different way to determine chirality from the existing polarized-light ones, or the use of anomalous dispersion in X-ray data (although that one can be tricky).
Here's a rundown on this new paper from Chemistry World. My guess is that this is going to be one of those techniques that will be used rarely, but when it comes up it'll be because nothing else will work at all. I also wonder if, possibly, the effect might be noticed on molecules in interstellar space under the right conditions, giving us a read on chirality from a distance?
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