Is light chiral?

[hellomister]

I think light is chiral because it can be polarized in a clockwise or counterclockwise direction. Is this correct?

[Fortissimo]

but isnt it light that actually lets us confirm that a substance is chiral?

[swansont]

Yes. Photons have angular momentum (spin of 1), and the spin vector is aligned (or anti-aligned) with the propagation vector, and you can polarize the beam as you stated.

 

 

more:

http://physics.unl.edu/~tgay/content/CPE.html

[hellomister]

Since it can be polarized in different planes does this mean is chiral? if light can help us determine which substances are chiral then it has to be chiral right? since only chiral objects can identify other chiral objects.

[Ze_Mole]

To be chiral you have to have symmetry. Because light is energy it doesn't have symmetry.

[hellomister]

To be chiral you have to have symmetry. Because light is energy it doesn't have symmetry.

 

i thought if a chiral molecule has any type of symmetry it is deemed achiral.

[Severian]

That is not what chiral means though, at least in the parlance of particle physics. Chiral means that they have different interactions based on their spin (in a particular direction).

 

Photons are not chiral since their interactions (electromagnetic) treat all polarizations equally. In contrast, all known fermions are chiral since the weak interaction only couples to left handed particles (and all known fermions interact weakly).

[Ze_Mole]

i thought if a chiral molecule has any type of symmetry it is deemed achiral.

 

Quite right, I stand corrected. Pays to read what you write

[swansont]

That is not what chiral means though, at least in the parlance of particle physics. Chiral means that they have different interactions based on their spin (in a particular direction).

 

Photons are not chiral since their interactions (electromagnetic) treat all polarizations equally. In contrast, all known fermions are chiral since the weak interaction only couples to left handed particles (and all known fermions interact weakly).

 

Circularly polarized light can have different atomic interactions than linearly polarized light, or circular polarization in the opposite direction.

[hellomister]

hmm i'm confused, i have never tried to classify something such as light as chiral or achiral. I have only done this to molecules. So light is not chiral because of its properties of wave-particle duality? since you said photons treat all polarizations equally.

 

By the way what's a fermion?

thanks.

Edited December 1, 2008 by hellomister

[Ze_Mole]

I see your point, I just looked up the definition of chiral. There is defiantly a relationship between polarity and chirality, but I still think that although being related they are different. I just don't think energy can have geometry, and from looking up the term, I think chirality is only related to geometry, polarity is just used to tell them apart.

[hellomister]

hmm is it even possible to classify light as chiral or achiral? haha?

[John Cuthber]

I am sure that circularly polarised light gives different rates of photochemical reactions with different enantiomers of some chemiclas. If the photons are not chiral, how can they know which molecules to decompose?

 

e.g this text "As part of these studies he probed the

assumption that interstellar/circumstellar asymmetric photochemistry is capable of inducing

significant enantiomeric enrichments in solid state amino acids, solid-state leucine films were

irradiated with VUV circularly polarised synchrotron radiation (CPSR). The most suitable

conditions for the enantioselective photodecomposition experiments were determined by recording

the VUV absorption and circular dichroism spectra of solid-state leucine films between 110 and 220

nm. Solid-state D,L-leucine films were then exposed to the intense and quasi-perfect r-CPSR at 6.8

eV (182 nm) emitted by the OPHELIE Undulator of the VUV SU5 beamline at the LURE, in

Orsay. After photodecomposition of 70 % of the starting material, enantioselective GC-MS analysis

showed a +2.6 % enantiomeric excess of D-leucine, i.e. an enantioselective decomposition of Lleucine,

in agreement with the (*, 1)-electronic transitions recorded by CD."

 

from here

http://www.cost.esf.org/typo3conf/ext/bzb_securelink/pushFile.php?cuid=253&file=fileadmin/domain_files/CHEM/Action_D27/mid-term_report/mid-term_report-D27.pdf.

 

Can I scale this up? Can someone make 2 macroscopic objects that interact differently with microwaves even though the objects only differ because they are left and right handed versions of one another?

 

It seems to me that since light (at least in the case of circulalry polarised light) interacts differently with different enantiomers it must be chiral.

[swansont]

It seems to me that since light (at least in the case of circulalry polarised light) interacts differently with different enantiomers it must be chiral.

 

That's my thinking.

[Ze_Mole]

I still think it is like as the relationship between current and magnetic field, there is a relationship but they are not the same thing. The definition of chiral is that it is not able to be superimposed on its mirror image, not that it interacts with other chiral things. If energy has no mirror image how can it be superimposed or not on it?

[Zellinger]

Let's start from the term chiral - it means that an object (or phenomenon) isn't identical to it's mirror image and it comes from greek word cheir which means hand (hands are chiral in broader sense of the word).

 

In chemical sense chiral refers to molecules with same order of binding between atoms but with different spatial relations between them. These molecules lack symmetry.

 

See: http://home.clara.net/rod.beavon/chiralit.htm

 

Light was ruled out from periodic table of elements around one century ago since the discovery of it's true nature so there's your answer. To correct someone who said molecules are chiral because of the light - wrong. Molecules are chiral in the dark too because they can lack symmetry even then - chiral nature of a molecule allows it to rotate polarized light so this is simply a manifestation of asymmetry of the molecule. To be chiral molecules have to have chiral center.

 

Some molecules can have chiral center without manifesting the rotation of light's plane of polarization - so they aren't labeled as chiral. These are called meso-compounds and have a plane of symmetry - in other words can be superimposed. See meso-tartaric acid. I think that it is wrong to say they aren't chiral because in reality they do rotate light just do not manifest it because as soon as one group rotates the light polarization plane other one reverses it so no effect is observed as light interacts with them, in other word the sum effect is zero. Or maybe these molecules don't reverse light at all... hmmmm. I'm not sure exactly on the nature of interaction between light and molecules. It's electromagnetic but do they first rotates and then reverse light or simply don't rotate light at all. because electromagnetic influences of these kind of groups close together negate themselves prior to interaction with light, I'm not sure. Can some clarify this to me? In theory will a solution that of meso-compound with long aliphatic chain between two opposite chiral groups rotate light if we can separate only identical groups to different side of the solution? See this picture. I think it will and if this is right then meso-compounds are in deed chiral.

 

Apparently light can be chiral in physical sense but this completely different phenomenon. It's like comparing apples and pears. I am not much familiar with this phenomenon so I can't really explain it but I'm sure that except same name and it's meaning in broader sense they don't have much in common. Please correct me if I'm wrong.

 

See here for more info:

 

http://en.wikipedia.org/wiki/Chirality_(chemistry)

 

http://en.wikipedia.org/wiki/Chirality_(physics)

 

p.s. By the way I don't get this chiral interacts with chiral story and what it's point. Can someone explain?

Edited December 2, 2008 by Zellinger

[hermanntrude]

seems to me trying to call light chiral is like trying to call a milk-carton a mammal because it produces milk.

[Tartaglia]

Circularly polarised light has a rotating E vector with the same symmetry as an infinite screw thread. Since screw threads can be right and left handed, so can circularly polarised light

[Zellinger]

Circularly polarised light has a rotating E vector with the same symmetry as an infinite screw thread. Since screw threads can be right and left handed, so can circularly polarised light

 

Quote Wiki: In electrodynamics, circular polarization of electromagnetic radiation is a polarization such that the tip of the electric field vector, at a fixed point in space, describes a circle as time progresses. The electric vector, at one point in time, describes a helix along the direction of wave propagation.

 

This actually means that the trajectory that is described along the direction of wave propagation by electric vector is chiral to be exact. Is light thus chiral?

 

Light is said to be chiral (left or right polarized) to describe this phenomenon but it isn't the kind of chirality we talk about in molecules. It is a description of light's behavior.

 

Then I could say that the molecule that moves trough the vacuum with a helical trajectory is chiral, right? Light is an electromagnetic wave (combination of electric and magnetic fields) and thus cannot be chiral in the way that molecules are. You have to have a solid matter for something to be chiral.

Edited December 2, 2008 by Zellinger

[Tartaglia]

Zellinger, OK point taken about the difference beween a solid object existing in enantiomeric forms, being different from a "behaviour" without a plane of symmetry.

 

The non-chiral molecule moving in a helical path is an interesting one. Consider this "thought" experiment. Assuming we could bombard an enzyme active site (chiral in nature) with non chiral charged substrate moving in right and left handed helices. I would expect the activation energy for enzyme/substrate complex formation to differ as the pathways into the activated site to be hindered to different extents.

Thus the non chiral substrate takes on the sort of property that chiral substrates would exhibit in forming diastereomeric enzyme/substrate complexes ie differing activation energy. This obviously isn't a practical proposition but is this phenomenom really as clear cut as you think?

[John Cuthber]

If the only criterion for being a mammal were giving milk then a milk carton would be a mammal.

On the other hand, I wouldn't be. Like roughly half the human race I'm male.

 

Chiral means "handed" in the sense of a screw thread or a glove- nothing more and nothing less.

 

If I were to grow a single crystal of silicon it would (if it were perfect) be a single molecule If it were a cylinder then it wouldn't be chiral. If I then cut a spiral thread in the thing it would be chiral and it would also be a chiral molecule.

My gloves are chiral, but my socks are not (on a macroscopic scale- of course the thread has a twist to it).

 

Circulalry polarised ligh behaves differntly towards different enantiomers. To do this it must be, in some way, handed or it would not (if you forgive the anthropomorphism) "know" which enantiomer to photodisociate.

 

Whether that's a function of the spiral path of the electric vector of the em field; photon spin; political spin, or what, I don't know. But I know that there's something "left handed" about left circularly polarised light that distinguishes it from "right handed" right polarised light.

[swansont]

Quote Wiki: In electrodynamics, circular polarization of electromagnetic radiation is a polarization such that the tip of the electric field vector, at a fixed point in space, describes a circle as time progresses. The electric vector, at one point in time, describes a helix along the direction of wave propagation.

 

This actually means that the trajectory that is described along the direction of wave propagation by electric vector is chiral to be exact. Is light thus chiral?

 

Light is said to be chiral (left or right polarized) to describe this phenomenon but it isn't the kind of chirality we talk about in molecules. It is a description of light's behavior.

 

Then I could say that the molecule that moves trough the vacuum with a helical trajectory is chiral, right? Light is an electromagnetic wave (combination of electric and magnetic fields) and thus cannot be chiral in the way that molecules are. You have to have a solid matter for something to be chiral.

 

 

The polarization is not a description of the light's trajectory, it's a description of what's happening to the E vector with time. Put another way, the angular momentum vector is either aligned or anti-aligned with the direction of propagation. This makes it right-handed or left-handed. The light has a helicity.