| Popis: |
A critical review of applications of chlorine kinetic isotope effects in studies of organic reactions mechanisms is presented. Reports from the last five years are considered. During this time chlorine kinetic isotope effects were applied mainly in studies of elimination reactions and nucleophilic substitution reactions. In several cases chemical models were also used as models of the intrinsic kinetic isotope effects in several enzymatic reactions. Both experimental and theoretical approaches have been used. Chlorine kinetic isotope effects (Cl-KIEs) played an important role some 30 years ago in learning the details of mechanisms of organic reactions and understanding the rules that govern isotope effects. The experimental side of these studies was possible due to developments in high-precisio n measurements of chlorine isotopic composition of methyl chloride. The theoretical aspect was connected with the BEBOVIB program (1), a tool that allowed systematic studies of relationships between geometries, bond orders and the resulting isotope effects. A large number of these studies (7) was carried out on solvolytic reactions (2). Recent years brought Cl-KIEs back to the spotlights. There are two reasons for this. Firstly, new experimental techniques have been developed for measurements of the chlorine isotopic composition that allow these measurements to be carried out directly on silver (3,4) or caesium chloride (5) without the need for tedious conversion to volatile methyl chloride. While they do not offer improvement in the precision, the simplicity of these procedures permits their application to biological systems, such as for example enzymatic reactions. Since chloroorganic compounds constitute the largest group of environmental pollutants their (bio)degradation becomes one of the major ecological problems (6). Understanding the mechanisms of chemical and biochemical dehalogenation can be greatly enhanced by studies of chlorine isotope effects. Secondly, modern computational tools allow us to scrutinize origins and details of isotope effects in much more precise and unbiased way than the studies performed within the BEBOVIB framework. Enormous increase in the speed of computers together with the simplicity of use and availability of quantum-chemi cal software resulted in reinvestigati ons of the foundations of Cl-KIEs. |
