Tunnelling effects in chemistry

Most of the molecules react to form products by climbing a barrier. The energy involved in this climbing of the barrier is known as the activation energy of the reaction. Questions like how fast a reaction occurs can be answered by considering the height of the barrier. However, in some reactions, the reactants transform to products by directly tunnelling across the barrier instead of climbing over it. Such a purely quantum mechanical effect, which becomes more prominent for reactions at low temperatures can lead to interesting and even completely unexpected products. This effect and its consequences in representative examples are discussed.