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DBL%20Hendrix%20small.png College chemistry, 1983

Derek Lowe The 2002 Model

Dbl%20new%20portrait%20B%26W.png After 10 years of blogging. . .

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. To contact Derek email him directly: derekb.lowe@gmail.com Twitter: Dereklowe

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« Walking Away From the ACS | Main | Another One of Those Startling Molecular Images »

September 14, 2012

Chemistry in the Quantum Vacuum. No, Really.

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Posted by Derek

When I was clearing a space on my desk the other day, I came across this paper, which I'd printed several months ago to read later. Later's finally here! A brief look at the manuscript will make clear why I didn't immediately dig into it - it's titled "Modifying Chemical Landscapes by Coupling to Vacuum Fields", and it's about as physics-heavy as anything that Angewandte Chemie would be willing to publish. The scary part is, this is one of a pair of papers from the same group (Thomas Ebbesen's at Strasbourg), and it's the other one that really gets into the physics. (If you can't get the first paper, here's a summary of it, the only mention I've been able to find of this work).

But it's worth a bit of digging, because this is very strange and interesting work. So bear with me for a paragraph - I always thought that someone should write a textbook titled "Quantum Mechanics: A Hand-Waving Approach", and that's what you're about to get here. The theory tells us, among many other weird things, that the vacuum between molecules is not what we might think it is. That's more properly the quantum electrodynamic vacuum, the ground state of electromagnetic fields. Because the Planck constant is not zero - tiny, but crucially not zero - the QED vacuum is not the empty nothingness that we think of classically. It's a dielectric, it's diamagnetic, and its properties can be altered. The theory that tells us such odd things is to be taken very seriously indeed, though, since it has made some of the most detailed and accurate predictions in the history of science.

And the vacuum-field fluctuation part of the theory has to be taken very seriously, too, because these effects have actually been measured. This was first accomplished via the Lamb shift and the Casimir effect is the latest poster child. That relates to the properties of the vacuum between two very closely spaced physical plates, and we're now to the point, technologically, where we actually make structures of this kind, measure their sizes and compositions, and determine what's going on inside them.

So what, those few of you who are still reading would like to know, does this have to do with chemistry? Well, when a real molecule is placed between such plates, its energy levels behave in strange ways. And this latest paper demonstrates that with a photochemical rearrangement - the reaction rates change completely depending on whether or not the starting material is confined in the right sort of space, and they change exactly as the cavity is tuned more closely to the absorption taking place. In effect, the molecule is now part of a completely new system (molecule-plus-cavity), and this new system has different energy levels - and can do different chemistry.

The photochemistry shown is not exciting per se, but the fact that it can be altered just by putting the molecule in a very tiny box is exciting indeed:

The rearrangement of the molecular energy levels by coupling to the vacuum field has numerous important consequences for molecular and material sciences. As we have shown here, it can be used to modify chemical energy landscapes and in turn the reaction rates and yields. Strong coupling can either speed up or slow down a reaction depending on the reorganization of specific energy levels relative to the overall energy landscape. Both rates and the thermodynamics of the reaction will be modified. . .The coupling was done here to an electronic transition but it could also be done to a specific vibrational transition for instance to modify the reactivity of a bond. In this way it can be seen as analogous to a catalyst which changes the reaction rate by modifying the energy landscape.

I look forward to seeing how this field develops. If we end up being able to make reactions go the way we want them to by coupling our starting materials to actual fabric of space, I will officially decide that I am, in fact, living in someone's science fiction novel, and I will be very happy about that. I can picture a vacuum-field flow chemistry machine, pumping reactants through various ridiculously small and convoluted lattices, as someone turns a chrome-plated crank on the side to adjust the geometry of the cavities to change the product distributions. OK, there are perhaps a couple of engineering challenges there, but you get the idea.

And speaking as an organic chemist, I have a few other questions: can these vacuum field effects occur in some of the other confined spaces that we're more used to thinking about? The insides of zeolites, for example? The interior of a cyclodextrin? Between sheets of graphene? Inside the active site of an enzyme? I'm sure that there are reasons why not all of these would be able to show such an effect (irregular geometry, just to pick one), but it does make you wonder.

Comments (24) + TrackBacks (0) | Category: Chemical News


COMMENTS

1. Anonymous on September 14, 2012 9:26 AM writes...

Wow! Be sure to look at that one! Question is: is it really a quantum effect? I heard that a lot of physicists believe the Casimir Effect can be explained in simpler terms.

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2. Chemist on September 14, 2012 9:29 AM writes...

"...which I'd printed several months ago to read later."
Had to laugh when i read this. My "interesting but not related" pile only seems to grow, but there is so much interesting stuff out there.

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3. cookingwithsolvents on September 14, 2012 9:34 AM writes...

Thanks for the very interesting article. I can't wait to read it and it got bumped to the top of my "to read" pile.

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4. Anonymous on September 14, 2012 9:53 AM writes...

This is another example of "empty space" not simply being "nothing". It reminded me I'm reading Milo Wolff's "Schrodinger's Universe", a small and interesting book.

By the way, my "interesting but not related" pile has outgrown my "relevant and must do" pile

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5. Curious Wavefunction on September 14, 2012 10:47 AM writes...

Haven't read the paper but it seems to ask a potentially valid question; at what scales do quantum effects become significant enough to influence chemical observations?

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6. fred on September 14, 2012 11:17 AM writes...

QED may influence biology....you mean it is all connected and mushed up quantum world that may not mesh with the ball and sticks the modelers show us...my goodness. dont open that door of the mind...nativis may be lurking behind.

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7. barry on September 14, 2012 12:30 PM writes...

the active site of an enzyme is an interesting box of just this sort (although the dielectric of an enzyme may not be different enough from that of bulk water?).
The photoisomerization of conjugated iminium ions in retinal cones may be the most sensitive test system for these effects.

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8. luysii on September 14, 2012 1:28 PM writes...

Fascinating stuff all right. As a neurologist the very tight pores in ion channels in neurons come to mind (along with enzymes as you mention). Also there is fairly little space in the center of globular proteins. Could what we're calling thermal motion actually be vacuum motion?

A lot of fun to think about over the weekend. Mark Twain said it the best --- There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact -- Life on the Mississippi

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9. grad on September 14, 2012 1:30 PM writes...

The chrome plated crank link is spectacular. I laughed out loud, bravo!

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10. Semichemist on September 14, 2012 3:22 PM writes...

"the active site of an enzyme is an interesting box of just this sort"

Very, very interesting point

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11. Rhenium on September 14, 2012 4:20 PM writes...

Damn... where's Uncle Al when you need him...

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12. Anonymous on September 14, 2012 4:37 PM writes...

As a physicist-turned-chemical engineer who once studied the Casimir force, this was very interesting to me. Here are a couple of comments from my perspective.

(1) "...is it really a quantum effect?"

Yes, definitely. There are some debates about whether it is a quantum VACUUM effect, but I don't know anyone who seriously claims there is no quantum in the explanation. See for example [1].

(5) "...at what scales do quantum effects become significant enough to influence chemical observations?"

Trite answer: at every scale, because if you turned off quantum mechanics then atoms would not be stable, electrons wouldn't live in distinct energy levels, and everything would be totally different. Less trite: resonant bonds are a familiar quantum effect, and the Van Der Waals force is a close cousin of the Casimir force. Surprising answer: although I can't turn up a reference at the moment, I think I remember hearing that some reactions of chiral molecules depend on the weak nuclear force being non-zero. Ugh, this is going to bother me now...

OP: "can these vacuum field effects occur in some of the other confined spaces that we're more used to thinking about? The insides of zeolites, for example?..."

The Casimir effect is the same general type of interaction as the Van Der Waals force[1], so I would say the answer is probably yes.

[1] From Wikipedia: 'Alternatively, a 2005 paper by Robert Jaffe of MIT states that "Casimir effects can be formulated and Casimir forces can be computed without reference to zero point energies. They are relativistic, quantum forces between charges and currents. The Casimir force (per unit area) between parallel plates vanishes as alpha, the fine structure constant, goes to zero, and the standard result, which appears to be independent of alpha, corresponds to the alpha → infinity limit," and that "The Casimir force is simply the (relativistic, retarded) van der Waals force between the metal plates."'

Permalink to Comment

13. Paul Podhorn on September 14, 2012 10:04 PM writes...

Excellent, both your article and the research. The potential gain to controlling chemistry is amazing. Thank you for your article.

Permalink to Comment

14. Sili on September 15, 2012 6:01 AM writes...

The insides of zeolites, for example? The interior of a cyclodextrin? Between sheets of graphene?
Graphene, perhaps. The Casimir effect requires conducting plates. Permalink to Comment

15. metaphysician on September 15, 2012 8:47 AM writes...

We already are living in a science fiction novel. Its just that, most days, it feels awfully cyberpunk. ;)

Permalink to Comment

16. rhodium on September 15, 2012 10:32 AM writes...

Where this may show up is in molecular dynamics simulations. Shaw's home-built supercomputer Anton is producing some interesting simulations on biologically relevant timescales. However, as he notes, the potential energy functions are classical. He has said that if quantum effects are actually important then getting these programs to provide accurate results will be impossible, at least with current technology and PE functions.

Permalink to Comment

17. luysii on September 15, 2012 12:57 PM writes...

I'm not sure the readership knows the exquisite chemistry behind the channel selectivity for various cations. As the ionic diameter of Na+ is 1.16 Angstroms and that of K+ is 1.52, I leave it to your imagination how an ion channel could exist permitting potassium flow while preventing Na+ from crossing.

Back in medical school in the 60's we were told that this was because the activity of K+ and Na+ inside the cell was different. This, of course, drove me nuts knowing that activity coefficients are essentially a fudge factor to make experiment agree with theory. Typical of 'medical science' back then, using a different wording of a phenomenon as an explanation for it.

The answer is that the tightest spot of the K selective channel is a formed by transmembrane alpha helices helices with 4 carbonyl groups forming the wall of the pore exactly the diameter of the K+ ion apart. So when K+ is stripped of its water when approaching the pore (from wider parts of the channel), the carbonyl oxygens are there to 'solvate' it.

Sodium, with its 33% smaller diameter is also stripped of its waters, but the energetics of shielding essentially a naked cation in the pore are much less favorable. Elegant isn't it? This sort of chemical elegance appearing over and over in cellular biochemistry makes it very hard for me to accept that it arose by chance.

So perhaps the Casimir effect could be relevant in situations like this.

Permalink to Comment

18. ezra abrams on September 16, 2012 5:22 PM writes...

dear sir - politely, I didn't understand this at all (PhD, molecular biology)
I don't mean to be rude, but can you try again (I suspect all that stuff about qed and so forth at the beginning is irrelevant; the key thing is what coupling means

Permalink to Comment

19. Curious Wavefunction on September 16, 2012 9:49 PM writes...

luysii: "This sort of chemical elegance appearing over and over in cellular biochemistry makes it very hard for me to accept that it arose by chance."

I don't find it surprising at all. What you are describing was worked out by Roderick McKinnon in his Nobel Prize winning work about fifteen years ago. Part of his work demonstrated that there's lots of ion channels with varying degrees of selectivity across different species. In fact for key processes like sodium and potassium ion transport, I can see very easily how minute differences in solvation properties can be immediately seized and amplified by natural selection.

Anon: What I was asking is how many observables on a macroscale you can tie directly to quantum phenomena like entanglement. I don't claim to understand the gory details, but there is a school of thought pioneered by Roger Penrose for instance which proposed that consciousness can be directly linked to the on/off properties of microtubules which are a direct consequence of quantum coherence, until Max Tegmark from MIT pointed out that thermal noise would cause decoherence so fast that there would be no possibility of coherence contributing to microtubule dynamics. On the other hand, coherence has been shown to contribute to electron transport in photosynthesis. I think the next few years are going to bring exciting news regarding this kind of dichotomy.

Permalink to Comment

20. Derek Lowe on September 17, 2012 8:00 AM writes...

#18 - unfortunately, the QED stuff was relevant, although hardly enough to explain "strong coupling". I'm no physicist myself, but as I understand it, this occurs when the energies of the virtual particles of the vacuum correspond to energy levels of the molecule/particle that's confined. You then have a new hybrid system, with new energy levels of the ground and excited states as compared to the uncoupled non-hybrid state. These energy levels split (Rabi coupling) into two new ones, higher and lower than the original. This paper is pointing out that such Rabi splitting can be quite large for organic molecules, and definitely large enough to affect chemical reactions (since the new transition states may be either harder or easier to reach, energetically). And the paper mentions that these changes may well reach all the way down into the ground state as well, which would certainly change the thermodynamic landscape, too.

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21. jtd7 on September 17, 2012 9:57 AM writes...

Reminds me of a head-shaking remark made by one of my undergrad physics profs, Art Benade: "Empty space now has more properties than the aether ever did."

Permalink to Comment

22. Alex on October 21, 2012 11:32 AM writes...

There's a lot of biological speculation about the earliest self-replicating molecules, the RNA world hypothesis, and what the substrates might have been. Clays and ice are candidates. Miller of Urey-Miller experiment fame was a supporter of the ice-RNA route.

Permalink to Comment

23. Bill Mansker on December 15, 2012 4:11 PM writes...

Casimir Effect is also demonstrated for organics and clays. Clays? . . . h-m-m-m- 'dust thou are to dust returneth . . ." BTW: I got kicked-off a forum for EVEN suggesting that the Casimir Effect may have initiated complex organic combinations (e.g., DNA? . . Life?) in clay platelet interstitia.

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24. Yonatan Zunger on January 5, 2013 11:56 PM writes...

You should be able to make these effects manifest in any of the above containers, although the details will vary. Basically, the Casimir Effect follows from there being nontrivial boundary conditions on the electromagnetic field. In the idealized version found in textbooks, you create a gap between two infinite conducting plates, so that the E field is constrained to be zero at each plate; this limits the spectrum of photons which can even exist in that gap, which in turn tunes the possible electromagnetic interactions which can happen in that gap, which proceeds to change the effective form of the Coulomb potential within that gap.

The big advances of Casimir physics in the past few years have been around creating boundaries which are enough like infinite conducting planes that are close enough together that these effects become significant. I suspect that building such things out of traditional molecules may be quite fruitful; what you really want is a conducting cage of a tunable shape. (Perhaps some kind of organometallic polymer that's coiled around the working area, whose shape can be tuned by adjusting Hydrogen bonds, or even by adjusting some externally applied electric field?)

I don't really know much about the state of such polymer chemistry. How feasible is it to wrap a reaction in some other big molecule?

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