Group 14 elements kinetic isotope effects

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . 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. 

Such a reaction is termed El(anion) or Elcb(anion)- For a deuterium-labeled reactant (D—C -C -L), this t5 e of reaction is characterized by rapid exchange of deuterium with protons from protic solvents and by a j8-hydro-gen 1° kinetic isotope effect ( h/ d) of 1.0. There is also an appreciable element effect, meaning that the rate constant for the reaction depends on the leaving group ability of L. An example of an El(anion) mechanism is the elimination of methanol from 2-phenyl-f ra s-2-methoxy-l-nitrocyclopentane (4) by the mechanism shown in Figure 10.13. ... 

Using MAS C NMR, the resonances at 127 and 136 ppm were attributed to the cis and trans double bonds and that at 178 ppm was for the carbonyl groups. The composition of the copolymers was determined by elemental analysis, radioassay of C content in the labeled copolymers and NMR. The copolymers were found to have a composition of 95, 90, and 87 mol% acetylene. The molecular weight of the insoluble copolymers was determined by radioassay of the T content after adjustment for the kinetic isotope effect (KEF). At a reaction temperature of —78 C, copolymers in the form of thin film were obtained with A/n = approximately 4000, whereas at a reaction temperature of 25 C, only powder-like products with A/n = approximately 1000 were obtained. The homopolyacetylene was found to have A/ = 11 000 using... 

Mechanism Kinetic" Order P-Hydrogen Exchange Faster Than Elimination General or Specific Base Catalysis hAd Electron Withdrawal atCp Electron Release at C Leaving- Group Isotope Effect or Element Effect... 

Mechanism Kinetic order Hydrogen exchange faster than elimination General or specific base catalysis knlko Electron withdrawal atC- Electron release or C-o" Leaving group isotope effect or element effect... 

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