Whereas there is no solid experimental support for either model, recent accurate measurements of isotopic shift of the transition temperature Tc upon boron and carbon isotopic substitutions favor the BCS theory.
12.
In most cases, isotopic substitution at A, especially if it is a heavy atom, will not alter k 1 or k 2, but it will most probably alter k 3.
13.
Isotopic substitution of B will clearly alter k 3, but it may also alter k 1 to a lesser extent if the B-C bond vibrations are affected in the transition state of the first step.
14.
The isotopic substitution changes the vibrational frequencies of various bonds in the molecule, which can have observable effects on the chemical reactivity via the kinetic isotope effect, and even by extension the biological activity in some cases.
15.
In practice, together with isotopic substitution, this means that a continuous comb of frequencies separated by around 1 cm " 1 ( 30 GHz ) can be used that extend from 880 to 1090 cm " 1.
16.
Since kinetic isotope effects arise from differences in isotopic masses, the largest observable kinetic isotope effects are associated with isotopic substitutions of hydrogen with deuterium ( 100 % increase in mass ) or tritium ( 200 % increase in mass ).
17.
For this reaction, isotopic substitution happens to produce an inverse kinetic isotope effect, and the authors report a kinetic isotope effect as low as 1.74 x 10 " 4, which is the smallest kinetic isotope effect ever reported.
18.
Silva, R . A . G . D ., Kubelka, J ., Bour, P ., Decatur, S . M ., and Keiderling, T . A . ( 2000 ) " Site-Specific Conformational Determination in Thermal Unfolding Studies of Helical Peptides Using Vibrational Circular Dichroism with Isotopic Substitution, " " The Proceedings of the National Academy of Sciences"
