One of the strange consequences of quantum mechanics is the phenomenon of indistinguishability—that two quantum particles can be impossible to tell apart, even in principle. This happens, in part, because it is impossible to determine the precise position of quantum particles. So when two particles interact at the same location, there is no way of knowing which is which.
That gives rise to some exotic behavior, particularly at low temperatures when large numbers of particles can behave in the same way. The indistinguishability of photons makes lasers possible, the indistinguishability of helium-4 nuclei at low temperature leads to superfluidity and the indistinguishability of other nuclei like rubidium leads to Bose-Einstein condensates. Indistinguishability is rich in mysterious phenomena.
But some quantum particles are not indistinguishable in this way. Electrons, for example, are forbidden from sharing the same state by a law known as the Pauli exclusion principle. And that leads to a different kind of physics. The interactions between electrons, governed by this Pauli exclusion principle, is called chemistry and it is equally rich in exotic behavior.
The worlds of chemistry and indistinguishable physics have long been thought of as entirely separate. Indistinguishability generally occurs at low temperatures while chemistry requires relatively high temperatures where objects tend to lose their quantum properties. As a result, chemists have long felt confident in ignoring the effects of quantum indistinguishability.
Today, Matthew Fisher and Leo Radzihovsky at the University of California, Santa Barbara, say that this confidence is misplaced. They show for the first time that quantum indistinguishability must play a significant role in some chemical processes even at ordinary temperatures. And they say this influence leads to an entirely new chemical phenomenon, such as isotope separation and could also explain a previously mysterious phenomenon such as the enhanced chemical activity of reactive oxygen species.
In short, Fisher and Radzihovsky are turning chemistry on its head.
The key question behind this new thinking is whether quantum properties can really be ignored in most chemical reactions. Fisher and Radzihovsky say that while it may be generally true that quantum properties are lost at high temperatures, certain quantum phenomena endure.
They point in particular to the quantum coherence of atomic nuclei. Physicists have long known that the spins of nuclei can remain coherent over timescales of minutes or hours. Indeed, they exploit this phenomenon in a wide range of quantum computing experiments that rely on nuclear spins to store quantum information.
It’s easy to think that nuclear spins have no significant effect on the way that electrons interact with each other in chemical reactions.
But that isn’t the case, say Fisher and Radzihovsky. Nuclear spins can easily become coupled to other physical states, such as the way a molecule vibrates. When this happens, the properties of indistinguishability that are normally confined to nuclei leak out and influence the molecule as a whole.
Fisher and Radzihovsky say this has a particularly strong effect on small symmetric molecules, such as water or hydrogen. The reason is that when the spins of two nuclei interact, symmetry dictates that they can take on certain configurations but not others.
When that symmetry leaks into the chemical world, it means that the molecule can interact only in situations with similar spin symmetry.
For example, a hydrogen or water molecule contains two hydrogen nuclei which can either spin in the same direction, in which case the molecule is known as ortho-water, or in the opposite directions in which case the molecule is known as para-water. These different arrangements of the same molecule are known as spin-isomers.
That has implications for the way molecules interact with each other. In many chemical reactions, the way molecules lock together is important. If the molecules can’t fit together like a key in a lock, the reaction can’t happen.
Fisher and Radzihovsky show that quantum indistinguishability influences the way molecules fit together because it prevents interactions that don’t match the symmetry of the of nuclei.
The researchers go on to show that this effect causes para molecules to be significantly more reactive than ortho molecules, because their symmetry matches that of a wider range of other molecules.
One area where this may play an important role is in enzymatic catalysis. Many enzymes rely on hydrogen to do their work. Now Fisher and Radzihovsky show that quantum indistinguishability must have a significant influence on this process.
Testing this prediction will be tricky. The obvious way is to measure the outcome of the same reaction performed with ortho- and para-versions of the molecules. But this is easier said than done. Ortho and para versions of the same molecule are hard to separate. Chemists achieved it for water for the first time only in 2014.
The chemical behavior of water and hydrogen is just the beginning. Fisher and Radzihovsky give numerous examples of other chemical processes that should also be influenced by quantum indistinguishability. These include isotope fractionation for which quantum indistinguishability provides a new mechanism, the phenomenon also explains the enhanced chemical activity of reactive oxygen species and provides a way for the spins of nuclei to influence biochemical molecules in general.
There is a rich trove of exotic behavior to study here. Testing these ideas will be hard but the rewards—a better understanding of some of chemistry’s most subtle and important biological phenomenon—will provide substantial motivation. Expect to hear more.
Ref: arxiv.org/abs/1707.05320: Quantum Indistinguishability in Chemical Reactions
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