December 26, 2011 | 10 Comments
For 84 years chemists have suspected the conservation of angular momentum (CAM) had direct applications in chemistry. December 22, 2011 saw Michigan State University (MSU) researchers first report that CAM in chemistry is in fact at work and the demonstration offers that scientists can use it to control and predict reactions in general. This is nearly revolution in picking up the pace in doing chemistry research. Now the certainty of mathematics can play a larger predictive role, saving time, offering new ideas to check out, and quicker experimentation.
Luckily for we writers the conservation of angular momentum is easy to visualize and discuss (simpler Wikipedia page). (Save those emails, just add your voice to the comments.) By taking some liberty in terms, the conservation is about holding the energy as a mass circles tied to a point. You can see the conservation as a figure skater draws in the arms and gains rotating speed – the mass has conserved the energy with more speed in a smaller circle – and when the arms come out the energy in the mass is still about the same as the mass travels slower around a larger circle. Isaac Newton figured this out and set it to mathematics in his second law of mechanics.
What’s happening is an object in motion trying to stay in motion. A particle going 100 inches a minute in a straight line pulled into a one-inch diameter circle is going to get around 100 times in a minute. Open the circle up to a 4-inch diameter circle and it will get around 25 times. The impact energy from a particle stop will be the same. Physics in nature has managed some marvelous examples, the gyroscope stays put; the earth revolves around the sun, the moon around the earth, the whole solar system works and galaxy as well.
The MSU team set out to use the conservation of angular momentum to understand how molecules move energy around following the absorption of light. In the current issue of Science (available in full as a pdf document from MSU), MSU chemist Jim McCusker demonstrates for the first time the effect is real and also suggests how scientists could use it to control and predict chemical reaction pathways in general.
The notion CAM was at work in chemistry was first floated back in 1927 when E. Wigner introduced the notion of spin conservation in chemical reactions where a chemical reaction would be designated “spin-allowed” if the spin angular momentum space spanned by the reactants intersects the spin angular momentum space spanned by the reaction’s products.
Jim McCusker takes us forward with, “The idea has floated around for decades and has been implicitly invoked in a variety of contexts, but no one had ever come up with a chemical system that could demonstrate whether or not the underlying concept was valid. Our result not only validates the idea, but it really allows us to start thinking about chemical reactions from an entirely different perspective.”
McCusker and his team used two closely related molecules that were specifically designed to undergo a chemical reaction known as fluorescence resonance energy transfer, or FRET. Upon absorption of light, the system is predisposed to transfer that energy from one part of the molecule to another.
Then they changed the identity of one of the atoms in the molecule from chromium to cobalt. This altered the molecule’s properties and shut down the reaction. The absence of any detectable energy transfer in the cobalt-containing compound confirmed the hypothesis.
McCusker said, “What we have successfully conducted is a proof-of-principle experiment. One can easily imagine employing these ideas to other chemical processes, and we’re actually exploring some of these avenues in my group right now.”
The team says in the paper, “. . . it does not appear to us that this formalism should be limited to energy transfer. In principle, a parallel set of expressions for any chemical reaction could be drafted in which consideration of reactant and product angular momenta serves to differentiate various thermodynamically viable pathways. It seems likely that the issues raised herein will manifest more readily in inorganic rather than organic systems because of the broader array of spin states generally accessible in such compounds.”
The expectation here is the use of CAM is going to have immediate application to catalyst research. The intensity of interest in organic reactions is very high for creating fuels and in solving the catalyst issues in fuel cells. The documentation of CAM or “spin-allowed” molecules could take off at a furious pace when a few applications are published.
It’s certainly a Happy New Year for the chemists – if they’re up on their calculus and physics.