Here’s a weird one for the Strange Things to Do With Chirality file. A multi-center team (Hebrew Univ., Weizmann Inst., Univ. Modena, and Pitt) report that electron spin, of all things, can be a chiral reagent. Some readers will sit up at that phrase, and others will (understandably) wonder what I’m talking about. Perhaps that happens most mornings around here, but I’m pretty sure about this instance.
I Am Not a Physicist, but here’s the brief background. Even us knuckle-dragging synthetic organic types are aware that electrons come in “spin-up” and “spin-down” forms (we had to fill out those electron configuration diagrams for test questions back in first-year chemistry, for one thing). But we certainly don’t spend much time thinking about them. What that means, though, is that electrons have a type of intrinsic angular momentum, a built-in property that’s quantized (they can’t spin a bit faster or a bit slower) and cannot be shed or cancelled out. And since electrons have linear momentum as well, they end up with helicity. If the spin and the linear momentum are in the same direction, that’s right-handed/positive helicity, and if they’re opposite each other (no other angles are possible, quantization and all), then it’s left-handed/negative. You can sort of imagine them corkscrewing through space, although the standard warning applies: if you think of elementary particles as little round physical objects, spinning on their axes and caroming along like ping-pong balls, you will come to grief because the reality is a lot stranger and very hard (perhaps even impossible, for humans) to have a really accurate mental picture of.
There’s been a lot of work put into exploiting an electron’s spin to go along with its charge (which we exploit constantly, every time we turn on an electric switch) which defines the field of “spintronics”. As for chemistry, it’s been shown that chiral molecules can act as “spin filters” in electrochemistry, which is the background of this latest paper. In it, it’s further demonstrated that such spin-polarized electrons can do chiral chemistry. There are key background experiments with association of chirally biased molecules on electrode-surface monolayers, but I wanted to highlight this part: an enantioselective reduction which indeed seems to be determined purely by the spin of the electrons coming from the anode.
The setup is an electrode that has nanometer-scale layers of nickel and gold, and has a strong permanent magnet next to it which can be flipped up (parallel) or down (antiparallel) normal to the surface. Charged particles with spin have a magnetic moment, so this will affect them. One of the great triumphs of quantum electrodynamics, QED, is the prediction of the electron’s magnetic moment (well, its g-factor constant) to an insane degree of accuracy – quantum mechanics as a whole is unnervingly robust, but this part is especially covered with a solid layer of armor.
Shown at rate are data from the reduction of camphorsulfonic acid to the corresponding borneol. You can see that the cyclic voltammetry curves are different when you start with either the R and S enantiomers and run the magnet up/magnet down experiments. But since I realize that most people’s eyes glaze over at the sight of CV plots, check out the next two. Panel D is a circular dichroism experiment – the straight-across black line is the CD spectrum of racemic CSA, and by gosh, it’s flat, as it should be. The blue and red lines are measured after the electrochemical reduction, though, magnet-up and magnet-down, and it clearly seems to have produced an enantiomeric excess by decomposing one component of the racemic mixture more than the other. And indeed, panel E shows this as a function of time. The longer you run the current, the more chiral the resulting solution becomes, and the sign depends perfectly on how you have the magnet oriented to the electrode surface.
The enantiomeric excesses themselves are not huge – they get up to around 11% e.e. – but that’s way more than you need to prove an effect. As the authors note, if you used a more magnetic layer on the electrode, you’d presumably see an even more pronounced effect. You don’t get chiral resolution out of nothing, and the only thing chiral in this whole setup is the helicity of the electron spins. We’re going to have to add another one to the list of unusual ways to influence chirality, and it will be quite interesting to see if something like this can be turned into preparative chemistry!