Study Combining Experiment and Theory Affirms Key Premise of Quantum Chemistry

“We can confidently predict from first principles not only the structure of molecules but the outcomes of chemical reactions," Alexander says, "and, thereby, understand the chemical nature of our universe.”“We can confidently predict from first principles not only the structure of molecules but the outcomes of chemical reactions,” Alexander says, “and, thereby, understand the chemical nature of our universe.”

Millard Alexander, Distinguished University Professor of Chemistry and Biochemistry, in UM’s College of Chemical and Life Sciences, and a team of colleagues have validated a key premise of theoretical chemistry and biomolecular modeling, known as the Born Oppenheimer approximation, through an experimental-theoretical study exploring the reaction of chlorine (Cl) and hydrogen (H2). Their findings were published in the January 2008 issue of Science.

“We can confidently predict from first principles not only the structure of molecules but the outcomes of chemical reactions,” Alexander says, “and, thereby, understand the chemical nature of our universe.”

A previous study of this reaction, led by Kopin Liu, of Taiwan’s Academia Sinica, challenged the applicability of this approximation, which underlies  all simulations of chemical reactions. Recently, a team of scientists led by UM’s Alexander; Daniel Neumark, Professor of Chemistry at UC, Berkeley; and David Manolopoulos, Professor of Chemistry at the University of Oxford; used a new high-resolution experimental technique to probe the validity of the Born-Oppenheimer approximation. “Born Oppenheimer is an approximation, not a law, but we have no reason to think that it should break down,” Alexander says.

First developed in 1927 by physicists Max Born (Nobel Laureate) and J. Robert Oppenheimer (best known for his role as the director of the Manhattan Project), theBorn-Oppenheimer approximation says that because electrons move much faster than nuclei in atoms and molecules their motions can be treated separately in quantum mechanical calculations. Any “coupling” between the motions will generally be insignificant.

“If the Born Oppenheimer approximation is valid, only the ground spin-orbit state of Cl atoms should react and not the excited orbit state,” Alexander explains. Using quantum mechanical calculations, Alexander and Manolopoulos predicted what Neumark would see in his investigation of the interaction between chlorine and hydrogen. “We were able to simulate Neumark’s experiments by taking the Born Oppenheimer approximation to be totally true or by permitting it to break down to the extent allowed by the physics of the system ,” he explained.

alexander_illustrationView of the potential energy surfaces which control the reaction of Cl atoms with molecular hydrogen. 

In the reaction, the atoms enter at the bottom and proceed out as products on the far right. The spin-orbit excited state of the Cl atom (Cl*) approaches on the upper potential energy surface and can not react unless a Born-Oppenheimer forbidden transition (curly vertical arrow) occurs.

Using a technique called slow electron velocity-map imaging (SEVI), Alexander’s colleagues (Neumark and his group at Berkeley) used a laser to eject an electron from the ClH2 anion and measured its kinetic energy.  From this, they could derive previously unobtainable information about the coupling between the electronic state of the Cl atom and the position of the Cl and H2 nuclei. “You have to catch the electron and find out how fast it’s going.” Alexander says  “This measurement is at the very edge of detectability.  It’s like trying to listen to a radio station that is nearly out of range. There’s a lot of noise and you are trying to pick out signals amidst the noise. It is very hard to do.”

Previous studies led by Alexander showed that the Born-Oppenheimer approximation held true for reactions between another halogen –fluorine (F) – and deuterium (D2).  However, Liu’s experiments indicated that chlorine in its excited orbit state reacted much more efficiently than the ground state, opposite to what the Born Oppenheimer approximation would predict.

Contrary to those results, Alexander and colleagues predict that the approximation should hold true for the chlorine-hydrogen system.  “We confirmed that the electron-nucleus coupling has only a minor effect on reactions, and that the approximation is not breaking down,” Alexander says. If the Born-Oppenheimer approximation is valid, then, as Alexander says, “We can confidently predict from first principles not only the structure of molecules but the outcomes of chemical reactions – and, thereby, understand the chemical nature of our universe.”

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