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Analytical Chemistry

The Enantiomers Did What, Now?

In today’s episode of “Fun With Chirality”, we have a look at phenomenon that could be very useful, come out of the blue, and which the very authors who report it have no explanation for. This is from a new paper in Angewandte Chemie from a team in Germany (TU-München) who have been looking at the behavior of enantiomers when adsorbed onto a glass surface (in this case, plain old borosilicate BK7, nothing fancy).

To their no doubt great surprise, they found that shining circularly polarized laser light onto such a surface causes selective desorption of enantiomers. They were able to coat the glass with various thicknesses of a racemic mixture of BINOL (1,1′-bi-2-naphthol, a classic chiral ligand) by evaporating it in vacuo, and hit this with laser pulses out at 600 and 650nm (which is outside the absorption range of the compound). Measurements of optical activity taken by second harmonic generation circular dichroism (SHG-CD, a very sensitive technique) showed that the remaining film (at the irradiated spot) gradually became more optically active as repeated circularly polarized laser pulses hit it – and this activity was invariably in the same direction for each handedness of the light pulses. This happened at both wavelengths, but linearly polarized light did not show the effect at all. They have, they say, no choice but to conclude that the circularly polarized laser pulses are selectively desorbing one enantiomer over the other, and I certainly don’t see any other explanation for the data, either.

But how that’s happening is another question. Laser desorption is supposed to be pretty much a pure thermal process. The energy of the incident photons is coupled right into vibrational modes, and the molecule just shakes itself right off the surface. And while that’s surely a big part of the story, there must be some quantum mechanical component if circular polarization makes such a difference, and the authors sort of throw their hands up at this point. They have a model of the overall process (that is, desorption versus heating of the surrounding molecules in the film) if you stipulate such a QM contribution, but where that comes from has yet to be put on a sound basis.

Interestingly, this model fits the observed data the best if there is a rapid and quite selective clearance (90% ee) of one enantiomer from the upper layers of the film as the first laser pulses hit. That, the authors say, is probably an overestimate, and experiments with slightly enantioenriched films suggest a lower bound of 20% ee for the process, so it’s probably somewhere between those two (which admittedly doesn’t tell you much beyond what you could have guessed while standing next to the instrument with a puzzled expression!) But the fact that it happens at all is extraordinary – this could lead to completely new ways to analyze and perhaps even purify optically active compounds.

You’re probably wondering (I was) what happens when you made a film of enantiomerically pure BINOL and try zapping that. Unfortunately, the pure enantiomers crystallize out in structured optically active layers when you form films of them, as opposed to the far more amorphous-looking racemic films (an effect which had already been described in the literature). And this leads to very nonlinear and hard-to-reproduce anisotropy in the films – they have larger-scale structures in them which are optically active themselves, and the data go all over the place. No doubt there will be other odd effects that show up as this phenomenon is further explored, and there are a lot of experiment that immediately suggest themselves that I’m sure the authors are banging away on right now. But if there’s a method to separate and characterize enantiomers that doesn’t rely on chiral reagents but only on polarized laser light, a lot of us would like to hear more about it!

 

20 comments on “The Enantiomers Did What, Now?”

  1. Per-Ola Norrby says:

    Sounds like you’re exciting selectively in the overtones of the VCD spectrum? How high can vibrational overtones be?

    1. Anon PChemist says:

      Vibrational overtones of small, gas-phase molecules like HCl, HCN and water vapor have been measured at wavelengths below 600 nm using extremely sensitive absorption methods. These overtones are very weak – a rough rule of thumb for simple R-H stretches is that the first overtone is a factor of 100 less intense than the fundamental and each additional overtone is a factor of 10 weaker. For something like BINOL, for an O-H stretch, the 5th overtone would be very roughly near 600 nm, so the absorption cross section would be tiny…

  2. Bob Seevers says:

    In these days of legalized marijuana, pretty high.

    1. John Wayne says:

      LOL

  3. Jake O says:

    I would be very interested to see what, if any, change occurs when a molecule with point chirality instead of axial chirality was used… More studies, please!

  4. Isidore says:

    I wonder if it will work on quartz, this way one could use the surface of the CD spectrophotometer’s quartz cell to deposit the film.

  5. ScientistSailor says:

    Let’s see this independently reproduced before we get too excited (pardon the pun) about it…

  6. Mark says:

    Does this mean that someone could build an enantiomer-determining MALDI mass spec?

    1. Isidore says:

      Sure, all you’d need to do is spray on the matrix after depleting one of the enantiomers. Of course your matrix would have to absorb at the wavelength of the laser. Or you can have two lasers, one with polarized light for depleting the enantiomer of choice, which will not have to be done in vacuum, and a second for laser desorption that works with the standard MALDI matrix compounds. I imagine someone is building such an instrument at this very moment.

  7. Anonymous says:

    I haven’t read the Angew reference yet, but Derek’s summary had me thinking back to work on “cold pyrolysis” using lasers and the use of silicon compounds as “sensitizers.” I found a few articles (which may not be the correct ones, but I don’t have a lot of time to dig deeper). Google-scholar “Keehn laser” for some lead references. They were definitely seeing non-thermal reactions at laser wavelengths where you wouldn’t expect to see absorption. They were using SiF4 and other silanes which may have been acting as sensitizers.

    Which leads me to wonder if the current reaction is being mediated by glass transitions in the glass (Si-O), transferring energy to the BINOL. Can amorphous glass be “locally chiral”? Someone else asked about using quartz, but alpha-quartz and beta-quartz are, themselves, chiral crystals. I wonder if they could also try other achiral substrates (such as IR windows Zn-Se, CaF2, KBr, …).

  8. Peter Kenny says:

    I wonder how evenly the enantiomers are distributed on the surface. If the cover is uneven then a small effect may be getting amplified? Looks very interesting!

    1. MTK says:

      I was thinking the same thing. Localized areas of enrichment which upon exposure to polarized light result in ee enrichment due to symmetry effects like Dreiding and others proposed in chromatography back in the 80’s.

      (of course, I really didn’t understand it then, and still don’t understand it now)

  9. Uncle Al says:

    Given a closely spaced stack of microscope cover slips, or microporous glass (otherwise on the way to Vycor 7913), or unclad fiberoptic (with the light internal, re frustrated total internal reflectance probes). Is that preparative optical enantiomer separation?

  10. JimM says:

    Well, if an enantiomer absorbs circularly polarized photons all polarized with the same handedness, it must experience a torque; and since the molecule itself has handedness (as screws do), perhaps it simply ‘unscrews’ itself out of the layer to the surface and then preferentially evaporates from the surface if it hasn’t already gained enough energy to escape the surface.

  11. Scott says:

    The surest sign of a major discovery is the entire research team saying, “Huh, *that’s* weird…”

    Now comes the really, *really* hard part: figuring out *why* it does that.

  12. John Wayne says:

    I really want to be able to purify compounds with a laser tube. Also, lasers can do *everything*

  13. Anonymous says:

    John Wayne: Does this method count?
    https://www.youtube.com/watch?v=DoQwKe0lggw
    JB: “Do you expect me to talk?”
    GF: “No, Mr. Bond. I expect you to purify my compounds for me … or else.”

  14. Chris Phoenix says:

    “the pure enantiomers crystallize out in structured optically active layers when you form films of them”

    So, the loose molecules aren’t expected to interact with the laser, but the crystal would, except that the racemic mixture doesn’t form crystals?

    By analogy with water forming temporary ice-like nanoscale domains above melting point, perhaps a racemic mixture is forming a lot of temporary nano-enantio-crystalloids even in its “liquid” state.

    Testable prediction: The selective desorption effect is temperature-dependent, according to the thermodynamics of temporary-crystalloid formation (first intuitive guess: the effect is weaker at higher temperatures that would tend to disrupt temporary structures).

    1. Chris Phoenix says:

      …Obvious second testable prediction: This effect won’t be seen in chiral chemicals where the pure enantiomer doesn’t form optically active crystals. Which would probably imply that it’s not broadly useful.

      1. Me says:

        Was thinking the same.

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