One thing that all chemical reactions have in common—whether they are the reactions that take place inside a battery, the metabolic reactions inside a living organism, or the combustion reactions that cause an explosion—is that they all involve some kind of change in energy. On a large scale, the changes in energy of a reaction can usually be measured in various ways for practical purposes, but attempting to understand the fundamental origins of this energy at smaller and smaller scales becomes more complicated, especially as chemistry enters the quantum realm.
In a new study, Cornell University chemists Dr. Martin Rahm and Prof. Roald Hoffmann (who received the Nobel Prize in Chemistry in 1981 for theories on the course of chemical reactions) have explored a new way of understanding the origins of energy in chemical reactions at the quantum level. Their paper is published in the Journal of the American Chemical Society.
Energy breakdownAt the heart of the paper is the idea—which is generally widely accepted in chemistry—that all of the interactions between the molecules, atoms, and the electrons that bind atoms together can collectively be understood in terms of energy. However, the origins of this energy, and how the energy changes during a chemical reaction, remains an open question. Over the years, researchers have proposed various “energy decomposition analyses,” or interpretations of how this energy can be broken down.
In their paper, Rahm and Hoffmann propose a new energy decomposition analysis in which the total changing energy of any chemical reaction can be broken down into three components: nuclear-nuclear repulsion (the repulsive energy between the positively charged nuclei of different atoms), the average electron binding energy (the average energy required to remove one electron from an atom), and electron-electron interactions (the repulsive energy between negatively charged electrons).
To paint a picture of how this works, the scientists explain in their paper what happens when any two atoms are brought closer together. First, the repulsion between the two nuclei increases, which causes the electrons to move in between the nuclei in an attempt to shield some of this repulsion. In the presence of the two nuclei, the average binding energy of the electrons changes due to differences in electron-nuclear attraction. As the electrons move closer together, they also begin to interact more strongly with each other. Quantifying these electron-electron interactions is one of the greatest challenges in computational chemistry. Source: Scientists explore the origins of energy in chemical reactions using experimental quantum chemistry