Question

6.2 In your own words, explain the isotope effect in a paragraph. In a first approximation, how much slower do deuterium and tritium diffusion relative to hydrogen (hydrogen mass-1, deuterium mass-2, and tritium mass-3 vary?)

Answer 1

Isotope effect means the variation of certain characteristics (as density , rate of reaction and spectrum) of an element in accordance with the mass of the isotopes involved. The kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for the reactions involving the light (k_L) and the heavy (k_H) isotopically substituted reactants:

$KIE={\frac {k_{L}}{k_{H}}}$

This change in rate results from heavier isotopologues having a lower velocity/mobility and an increased stability from the higher dissociation energies when compared to the compounds containing lighter isotopes.The kinetic isotope effect is considered to be one of the most essential and sensitive tools for the study of reaction mechanisms. Isotopic rate changes are most pronounced when the relative mass change is greatest, since the effect is related to vibrational frequencies of the affected bonds. For instance, changing a hydrogen atom (H) to its isotope deuterium (D) represents a 100% increase in mass, whereas in replacing carbon-12 with carbon-13, the mass increases by only 8 percent. The rate of a reaction involving a C–H bond is typically 6–10 times faster than the corresponding C–D bond, whereas a ¹²C reaction is only 4 percent faster than the corresponding ¹³C reaction.  A very common isotope substitution is when hydrogen is replaced by deuterium. This is known as a deuterium effect and is expressed by the ratio kH/kD (as explained above). Normal KIEs for the deuterium effect are around 1 to 7 or 8. Large effects are seen because the percentage mass change between hydrogen and deuterium is great. Heavy atom isotope effects involve the substitution of carbon, oxygen, nitrogen, sulfur, and bromine, with effects that are much smaller and are usually between 1.02 and 1.10. The difference in KIE magnitude is directly related to the percentage change in mass. Large effects are seen when hydrogen is replaced with deuterium because the percentage mass change is very large (mass is being doubled) while smaller percent mass changes are present when an atom like sulfur is replaced with its isotope (increased by two mass units).