Usually we talk about reactions in solution, but recently techniques have been developed to follow reactions that occur in a vacuum when a stream of reactant A and a stream of reactant B cross each other in a defined direction, as with molecular beams. From the direction in which the products are ejected and their energies, much fundamental information can be deduced about the details of the molecular processes. Lasers, which emit light-energy in a highly focused beam, are sometimes used to put energy into one of the reactants in a defined way. Such a technique reveals less about the nature of the transition state than about what is called the dynamics of the process—how molecules collide so as to react, and how the products carry away the energy of the overall reaction. The development and application of such techniques were recognized by a Nobel Prize in 1986 to Dudley Herschbach, Yuan Lee, and John Polanyi.

The interaction between experiment and theory is very important in the field of chemical transformations. In 1981 Kenichi Fukui and Roald Hoffmann received a Nobel Prize for their theoretical work on the electronic basis of reaction mechanisms for a number of important reaction types. Theory has also been influential in guiding experimental work toward demonstrating the mechanisms of one of the simplest classes of reactions, electron transfer (movement of an electron from one place to another). Henry Taube received a Nobel prize in 1983 for his studies of electron transfer in inorganic chemistry, and Rudolf Marcus received a Nobel Prize in 1992 for his theoretical work in this area. The state of development of chemical reaction theory is now sufficiently advanced that it can begin to guide the invention of new transformations by synthetic chemists.

Particular interest in recent times has centered on trying to understand the chemical mechanisms by which various biological processes occur. Such complex events as the cleavage of RNA by the enzyme ribonuclease, the multi-step synthesis of ATP in vivo (Paul Boyer and John E. Walker received Nobel prizes in 1997 for working this out), and the activity of molecular motors that power bacterial flagellae are now understood in molecular detail. George Wald received a Nobel Prize in 1967 for discovering the chemical mechanism in the eye by which light is transformed into a signal to the brain that produces vision. The recent sequencing of the human genome has provided a molecular foundation from which other complex biological processes might be tackled at a molecular level.

The basic science of chemical reactions has been put to good use for more than half a century in the fuels, plastics, environmental, and biotechnology fields. One story illustrates well the societal benefit of learning to guide the pathways of chemical reactions, in this case via catalysis and process design. The 1997 Draper Prize of the National Academy of Engineering, the engineering profession’s highest honor, went to Vladimir Haensel for his 1947 invention of platinum-catalyzed reforming of petroleum. The prize citation explains how, while working for Universal Oil Products Co. (now called UOP), Haensel sought to improve the process by which gasoline was produced from crude oil; thermal cracking of petro-