Lein Group - Research

Reaction Mechanisms

Chemical reactions proceed from starting materials, the educts, to the products. Energetically, the system has to go from one minimum to a different, deeper, minumum. Along the path the system will pass a point of maximum energy, before it starts to descend into the product valley. The corresponding structure at this point is called the transition structure. In chemical notation such transition structures are often labeled with a "‡" symbol. The figure on the left illustrates the energetic changes along the reaction coordinate. As the reaction progresses along the white path. It moves from the relatively low-lying educt valley over the transition structure into the deep product valley. The energy that is necessary to overcome the barrier between the two valleys is called the activation energy. In principle there are many paths connecting the educts with the products. Of special interest are usually only very few of those. The path shown in the figure on the left is special, because it follows a path of minimum energy.

Any other path that could be imagined in the illustration would be higher in energy than the path shown in white. Such paths that go through a maximum in energy on their way from the educts to the products, but otherwise minimise the energy are also called intrinsic reaction channels (IRC). By calculating every step along the path of a chemical reaction a detailed map of the corresponding potential energy hypersurface can be generated. The animation on the left shows such a trajectory along the IRC of the migration of a hydrogen atom assisted by a water molecule. The animation pauses briefly at the transition structure before proceeding to the product.