22
$\begingroup$

I'm wondering if someone can give me a physically motivated reason for why chiral molecules interact with left and right circularly polarized light (LCP) (RCP) differently. Particularly, a quantum mechanical view of what's happening here.

I understand why it is that a molecule has to be chiral in order for it to manifest circular dichroism (my use of terminology may not be perfect here so I apologize). That is, both the electric and magnetic dipole operators must be non-zero and there's an argument here on the basis of point groups.

Nothing about that, however, is particularly illuminating when it comes to understanding why I should expect a molecule to interact differently with LCP and RCP light.

I don't know what else I can even say. A good answer would address the different behavior of both the light and the molecule in these different situations, as I imagine that's a necessary piece to the puzzle.

Also, if one has some references (a review or textbook chapter etc.) that would be appreciated.

Improve this question
$\endgroup$
7
  • 2
    $\begingroup$ see chemistry.stackexchange.com/questions/1012/… $\endgroup$
    – Mithoron
    Nov 30, 2016 at 16:37
  • $\begingroup$ also chemistry.stackexchange.com/questions/6271/… $\endgroup$
    – Mithoron
    Nov 30, 2016 at 16:38
  • 2
    $\begingroup$ Chiral molecules interact differently with chiral light, just like a screw (chiral) interacts differently if you twist it clockwise or counterclockwise with a screwdriver. $\endgroup$
    – jjgoings
    Dec 1, 2016 at 18:58
  • $\begingroup$ Hmm... Perhaps I need to do some reading before I can really give words to the thing that's bothering me cause it might just be based on simple misunderstandings. @jjgoings do you know of any references that discuss this on a simple level I could look into? $\endgroup$
    – jheindel
    Dec 2, 2016 at 2:59
  • 3
    $\begingroup$ @jheindel Check out "Molecular light scattering and optical activity"(2004) by Lawrence Barron. Mechanistically, it will help if you consider chiral molecules like the helicenes (en.wikipedia.org/wiki/Helicene) rather than a 'typical' chiral molecule with a stereocenter. Simplistically, think about how it will be easier to polarize the electron density if the circular polarization matches the contour of the helicene rather than polarizing with the opposite 'handedness'. If, say, LCP light polarizes the molecule more strongly than RCP, there will be a difference in absorption intensity. $\endgroup$
    – jjgoings
    Dec 2, 2016 at 21:42

1 Answer 1

Reset to default
2
$\begingroup$

Some part of a mechanical macroscopic explanation, acoustical activity where mechanical transverse oscillation direction is being rotated, is described in

Frenzel, T., Köpfler, J., Jung, E. et al. Ultrasound experiments on acoustical activity in chiral mechanical metamaterials. Nat Commun 10, 3384 (2019).

The notion “activity” refers the rotation of the linear polarization axis of a transversely polarized mechanical wave. The rotation angle is proportional to the propagation distance and does not depend on the orientation of the incident linear polarization. It has nanometer precision under time-harmonic excitation at ultrasound frequencies in the range from 20 to 180 kHz. Polarization rotations are as large as 22° per unit cell.

Add a net neutral electrical charge distribution to the components of the metamaterial where the negative charge distribution has a different elastic or inertial nature than the positive charge distribution to get a circularly polarized electromagnetic wave.

A quantum mechanical explanation was requested. Just add stochastic bias to the mechanical input so that only inputs of at least a minimum value get statistical significance. This makes the setup equivalent to quantum mechanics.

Improve this answer
$\endgroup$
7
  • $\begingroup$ Not a singel vote. I have done what I can with this answer. What is missing with this answer? $\endgroup$
    – David Jonsson
    Dec 1, 2020 at 19:30
  • 2
    $\begingroup$ Perhaps this is more like what the OP was looking for: Physical Principles of Circular Dichroism, Steven S. Andrews* and James Tretton, J. Chem. Educ. 2020, 97, 4370−4376. Anyway, as I type this, you have 1 upvote and 1 downvote. I will give an upvote for an interesting take on the matter. $\endgroup$
    – Ed V
    Dec 23, 2020 at 18:27
  • $\begingroup$ I would prefer if downvotes and upvotes were shown separately. As it is shown now I can not tell that someone found something wrong with the answer. I just guess that they mean that there can be no twist of the polarization plane, just a probability of a larger one. Is this the issue? Effectively there is no difference since there are so many molecules involved. $\endgroup$
    – David Jonsson
    Jan 12, 2021 at 17:20
  • $\begingroup$ I have no idea what others voted or what reason(s) they may have had. Right now, you have 3 upvotes, one being mine, and 1 downvote. Best not to get too discouraged: my most upvoted answer is about fill gas in potato chip bags! Some other of my answers, with some thoughtful effort put into them, have one or a couple of upvotes. And in a couple of cases, clearly wrong answers blew my correct answer away, rep wise. Annoying, but nothing much to do about it. $\endgroup$
    – Ed V
    Jan 12, 2021 at 18:44
  • $\begingroup$ I see. Maybe editors are too few, or the scores required for voting are too low. Even if I edit an answer sometimes nothing happens. I think the votes, closing and deletions should expire after a while after an edit if the voters, closers and deleters aren't responding. $\endgroup$
    – David Jonsson
    Jan 12, 2021 at 23:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.