If you're out for a stroll and reach a hill, the most obvious paths to the other side are over or around. But under the right set of circumstances-such as when there's a convenient tunnel-sauntering through can be the simplest path forward.Chemical reactions must traverse hill-like barriers, too-in this case, energy penalties for reaching products. The path over requires enough energy to reach the hill's peak or transition state. It takes a certain amount of energy, usually from heat, for a chemical reaction to happen. Normally, that allows the molecule to wiggle enough to climb over the energy hill. But the counterintuitive rules of quantum mechanics sometimes offer a runnellike third option, so that the reaction can happen even without the added energy.
展开▼
