How atoms assemble into new molecules
Chemical reactions, i.e., the omnipresent rearrangement of atoms into new molecules, have been a conundrum for scientists and engineers for many centuries. An understanding of the course of chemical reactions is required to improve for example the production of plastics and drugs or to prevent the destruction of the earth’s ozone layer. Scientists from the Institute of Physics at the University of Freiburg have now been able to decipher one of the most important chemical reaction classes in detail. This interdisciplinary project in which physicists and chemists worked together appears in the recent issue of Science. The scientists have been able to show that the substitution of atoms during the reaction process works in a different way than previously assumed.
Five years ago, Dr. Roland Wester joined the group of Prof. Dr. Matthias Weidemüller and started a research project that employs a novel technique in the study of the chemical reactions of ions and neutral molecules. The scientists have developed an experimental apparatus, which is capable of visualizing reactive collisions between ions and molecules on a single particle level. This is rather like watching the collision of billiard balls on a pool table: The starting substances move towards each other with controlled velocity and react with each other; the reaction products’ scattering angle and velocity is imaged directly with a camera system. “In the last few years, we faced not only many unsolved issues, but also we had to deal with colleagues’ scepticism that the experiment would ever work. With our recent breakthroughs, we demonstrated that our unique combination of novel innovative techniques enable us to obtain much deeper insights into many chemical reactions,“ says Roland Wester.
Many unexpected effects
In their most recent work, the scientists from Freiburg have investigated the so-called nucleophilic substitution reaction. When a negatively charged chlorine atom collides with a methyl iodine molecule, CH3I, the iodine atom, I, is substituted by the chlorine to form a CH3Cl molecule and a negatively charged iodine atom. The analysis of camera images revealed many unexpected details in the reaction dynamics. Contrary to the simplistic explanation found in chemistry textbooks, this exchange reaction does not solely involve an attack of the CH3I molecule by the chlorine ion at the reactive centre, which drives the iodine atom away on the opposite side. Instead, the reactants form a transient complex, which undergoes several rotations before the reaction products are formed. The iodine leaves the complex in any direction.
“Through the cooperation with the group of Prof. Dr. Bill Hase, Texas Tech University, USA, we succeeded in discovering another unexpected reaction mechanism, i.e. one that becomes important at higher velocities,” said Dr. Jochen Mikosch who recently received his PhD for the experimental work done in Freiburg. The researchers refer to the new reaction mechanism as a “roundabout” mechanism.
Better predictions for technologically important reactions
Water promotes nucleophilic substitution reactions. It is therefore highly probable that such substitution reactions play a key role in living organisms. Therefore, the scientists plan to study the effect of water molecules in the reaction. They are particularly interested in the ability to predict the course of technologically important reactions and in gaining a deeper insight into chemical processes in living cells. Matthias Weidemüller and Roland Wester are convinced that the interface between physics and chemistry, which is characterized by the combination of complexity and quantum theory, holds many more surprises.
“Our understanding of modern quantum physics is currently undergoing a paradigm shift. It was previously assumed that quantum effects do not have a significant effect on complex processes, in particular in living nature. However, the progress in understanding complex quantum systems has broadened our perspective and I'm sure that in the near future phenomena will be discovered in living nature where quantum physics plays an important role,“ highlights Matthias Weidemüller. For the Freiburg scientists, the discovery of such phenomena is a big challenge.
Publication: (Science online, 11.01.08 DOI 1126/science.1150238)
Source: University of Freiburg - 11.01.2008