Isotope secondary effects

Support for such an interaction of the H—C bonds with the carbon atom carrying the positive charge is provided by substituting H by D in the original halide, the rate of formation of the ion pair is then found to be slowed down by 10% per deuterium atom incorporated a result compatible only with the H—C bonds being involved in the ionisation. This is known as a secondary kinetic isotope effect, secondary... 

Deuterium kinetic isotope effects, secondary, and transition state structure, 31,143 Diazo compounds, aliphatic, reactions with acids, 5, 331... 

Further Discussion, Heavy Atom Isotope Effects, Secondary Isotope Effects 319... 

Ab initio calculations at the MP2/6-31-t- G level have been performed for gas-phase E2 elimination reactions of CH3CH2X (X = NH3+, Br, Cl, F, SH) promoted by NH2, OH , F , PH2, SH , and Cl in order to determine how changes in transition-state geometry, from reactant-hke to product-like, influence kinetic isotope effects. Secondary isotope effects (a-FI) on leaving group departure are correlated with the hybridization at Ca in the transition state, whereas there is no such correlation between secondary 0 -H) isotope effects and the transition state hybridization at Cp. The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

Unlike primary kinetic isotope effects, secondary kinetic isotope effects arise due to presence of stable isotope at sites close to point of reaction and consequently influence the geometry of the reaction intermediates. Although the absolute values are much lower than those observed for primary effects, secondary kinetic isotope effects yield important information pertaining to the progress of the reaction and transition states involved therein [100,124,125],... 

Actually, secondary isotope effects are more likely to give useful information on transition state structures than primary ones, except where coupled hydrogen motions distort the secondary isotope effects. Secondary isotope effects were quite useful in determining the carbanion mechanism for ftimarase (709). The interested reader is referred to a recent review which gives a number of other examples (96). 

All of the isotope effects discussed so far have been primary kinetic isotope effects. A secondary kinetic isotope effect can arise when a bond to the isotope is not broken during the rate-limiting step of a reaction. Generally, secondary kinetic isotope effects are much smaller in magnitude than are primary kinetic isotope effects. Secondary kinetic isotope effects can nevertheless serve as useful probes of transition state structure because the magnitude of the effect generally increases as the transition structure changes from reactant-like to product-like. ° ... 

Alteration of either the equilibrium constant or the rate constant of a reaction if an atom in a reactant molecule is replaced by one of its isotopes, distinguished are kinetic isotope effect, equilibrium isotope effect, primary isotope effect, secondary isotope effect. 

The bond between a-carbon and deuterium is not broken in the solvolysis transition state, but the presence of deuterium, bonded to the reaction center, causes the reaction rate decrease. Isotope effects such as in (1.14.20) and (1.14.21) are called secondary kinetic isotope effects, to be clearly distinguished from primary isotope effects. Secondary isotope effects were defined as rate effects caused by isotope substitution on the bond not broken in the rate-determining step. 

---