Unreal reactions elucidate energy flow.

Unreal reactions elucidate energy flow

As computers get more powerful and less expensive, more chemists spend time in the world of make-believe. By simulating chemistry in a computer, researchers have now developed a detailed account of how fleeting energy fluctuations in an imaginary solution manage to initiate equally imaginary chemical reactions. They say the work provides a new window on the flow of energy in real-world reactions.

The chemists modeled an atom exchange reaction in which a molecule made of two identical atoms transfers one of them to a single atom of the same kind. Composed of chlorine-like atoms, the simulated reactants were dissolved in a solvent composed of 100 nonsticky, argon-like atoms. The chemists told the computer to arrange the pseudo-argon atoms in a liquid-like phase, insert the two reactants into the solvent at randomly selected locations, and then set all of the particles into motion.

"The reaction is simple, but many [real] reactions follow a single step like this," says Ilan Benjamin of the University of California, Santa Cruz, who helped perform the simulations in Kent R. Wilson's laboratory at the University of California, San Diego. Even complicated reactions often involve a series of single-atom exchanges, Benjamin notes.

The simulations offer "a particularly simple but revealing picture of energy flow from the solvent bath to the reaction system," the researchers conclude in the Jan. 17 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. The energetic journey that culminates in the modeled reaction begins as fluctuations in the solvent. Through atomic collisions within the solvent, the fluctuations focus energy into miniature hotspots involving several solvent atoms. Through solvent-reactant collisions, these high-speed atoms in turn transfer some of their energy to the reactants, which express the energy in a variety of vibrational, rotational and linear motions. By converting this kinetic energy into potential energy, the reactants get closer to each other until they surmount the reaction's energy barrier and finally exchange an atom.

Until the 1970s, experimental and computational difficulties prevented chemists from rigorously probing the physical events that occur at superbrief time scales. Scientists could say little, for instance, about exactly how energy in a solution would flow into dissolved reactants and push them over a specific reaction's energy barrier. Such processes take place in less than a quadrillionth of a second.

"Now we can ask more detailed questions of what form the energy takes during a simple reaction," Benjamin says. The computer simulations enable researchers to avoid many of the experimental difficulties and expenses of looking at the superfast energy flow of chemical reactions.

Benjamin notes that a few experimentalists now have lasers with pulses fast enough to probe the energy flow in real chemical reactions. However, simulations are more easily and variably controlled because they can incorporate evolving theories of how solvent and reactant species store energy, how atoms and molecules move in an environment of atomically derived pushing and pulling forces, and how solvents get reactants close enough to each other to actually react.