Kinetic Isotope Effects
Kinetic isotope effects (KIE) refer to the impact on reaction rate when one of the atoms in a molecule is replaced by its isotope. For example, replacing hydrogen (H) in a molecule with its isotope deuterium (D), which is heavier by one neutron, can slow down a reaction by up to 20 times. From rate information scientists learn about what happens to molecules during a reaction. They gain information about the changes as the reactants form new products during the course of the reaction,.
Atomic isotopes are closely related. Consider, for example, the isotopes of radium. They are atoms of the same element, so they have the same number of protons and electrons. The only way they differ is in the number of neutrons. One isotope has 202 neutrons, and the other isotope has 203 neutrons. The isotope with the extra neutron is a little heavier than its sister. Therefore, the isotopes behave differently. For example, they may have different reaction rates. Isotopes of heavier elements, like radium, however, are so close in weight (257 neutrons are not that much heavier than 256 neutrons) that this difference is insignificant.
The difference is much larger however, for the isotopes of the lightest elements because one neutron changes the mass of the atom by a larger percentage. So deuterium, which has one neutron, is heavier than hydrogen, which has none. And in a chemical reaction, deuterium reacts much more slowly than hydrogen. This chemical difference is put to use by scientists in the technique of kinetic isotope effects. Besides hydrogen and its isotope deuterium, researchers use the isotopes of boron, oxygen, nitrogen and carbon.
The reason isotopes H and D behave differently relates to their "dissociation energy," or the amount of energy required to break their bonds with other atoms. Generally, the lighter the atom, the less energy is needed to excite its bond to its breaking point, and the lower the "dissociation energy". Consider, for example, two bonds, a carbon-hydrogen (C-H) and a carbon-deuterium (C-D) bond. The atoms at each end of the bond can be viewed as two balls vibrating back and forth on two ends of a spring. The lighter hydrogen atoms are like Ping-Pong balls, and the heavier deuterium atoms like golf-balls. The golf-balls are harder to get moving than the ping pong balls just because they are heavier, and more energy is required vibrate them to the point where they will separate"dissociate." In this example then, the C-D bond will take more energy to break than the C-H bond.
A reaction involving deuterium will proceed more slowly than the same reaction with hydrogen reactants provided the isotope is involved in the "rate limiting step." In a chemical reaction, a rate limiting step is the slowest part which sets the pace for the whole reaction. The remainder of the reaction occurs extremely quickly in comparison. To find out which part of the reaction is the rate limiting step, chemists look for a kinetic isotope effect. For example, consider a two-step reaction process where the first step involves combining two molecules, and the second step involves breaking a C-H bond. One of these steps occurs more slowly than the other, and so the rate of this step "limits" the rate for the whole reaction. Scientists can run this reaction twice, once with deuterium and once with hydrogen. If the reaction is much slower with deuterium, then the "rate-limiting step" is probably the one in which the C-H bond is broken. This difference in reaction rates is called a primary KIE because the isotopes were part of the bond broken during the rate limiting step.
Scientists also evaluate secondary KIEs. A KIE is secondary when the heavier isotope is not part of a bond which breaks during the rate limiting step, but is close by in the same molecule. Such a secondary effect is much smaller than a primary KIE, but it can provide useful information about the "transition state" of a reaction. A "transition state" is the term for a temporary chemical structure which may be formed during the course of the reaction, but which is not present as either a reactant or product. Therefore, the "transition state" compound does not exist as a separate substance. Nevertheless, we can observe its effects on secondary KIEs. A larger secondary KIE indicates the transition state compound looks more like the products, a smaller one means it is more like the reactants.
So far, we have explained how KIEs are used to investigate the middle of a reaction. Sometimes however, these effects can be used differently, as a way to 'sort' isotopes.
For example, investigators at Oak Ridge National Laboratory in the United States use KIEs to remove an isotope which is a threat to our environment.Tritium, another isotope of hydrogen, contains two neutrons, and is in waste-water from nuclear reactors. It must be disposed of carefully and is easier to deal with if it is in a concentrated form. One way to concentrate this isotope is to use KIEs.
The primary KIE for this reaction is very strong; the reaction occurs 124 times faster for water with hydrogen than for water with tritium. So, when the water contains both isotopes, the lighter hydrogen-water reacts much more quickly than the heavier tritium- water. After a short while, most of the hydrogen resides in the product, while the reagents contain a concentrated form of tritium. With the isotopes in different chemical substances, the chemists at Oak Ridge find them much easier to separate from each other. Applications like this one are part of the reason why KIEs are a growing area of fruitful research.
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