Deuterium kinetic isotope effects, secondary, and transition state structure

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

Tetrahedral intermediates, derived from carboxylic acids, spectroscopic detection and the investigation of their properties, 21, 37 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-state chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition state structure, secondary deuterium isotope effects and, 31, 143 Transition states, structure in solution, cross-interaction constants and, 27, 57 Transition states, the stabilization of by cyclodextrins and other catalysts, 29, 1... 

Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and, 27, 1 Transition stale structure, secondary deuterium isotope effects and,... 

Secondary Deuterium Kinetic Isotope Effects and Transition State Structure... 

Secondary a-deuterium kinetic isotope effects (KIEs) have been widely used to determine the mechanism of SN reactions and to elucidate the structure of their transition states (Shiner, 1970a Westaway, 1987a). Some of the significant studies illustrating these principles are presented in this section. 

This solvation rule for 5n2 reactions can be useful in predicting the influence of a change in solvent on the structure of activated complexes. It is in agreement with studies involving leaving group heavy atom and secondary a-deuterium kinetic isotope effects, as well as theoretical calculations of solvent effects on transition-state structures. Possible limitations of this solvation rule have been discussed see [498] and relevant references cited therein. 

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. 

Another important observation was provided even before the Dewar papers, namely that the secondary deuterium kinetic isotope effect (SDKIE), at C4 of a 3,3-dicyano substituted-1,5-hexadiene was a larger normal effect in an absolute sense than the inverse SDKIE at C6. This must represent more bond breaking at C4, i.e. rehybridization from sp to sp, than bond formation at C6 involving rehybrization from sp to sp, a result inconsistent with a diyl-like transition state whose SDKIEs would be opposite in magnitude. Subsequent SDKIE determination by Conrad on 3-aIkyl substituted -1,5-hexadiene and 2-phenyl-l,5-hexadiene and on carbon-13 labeled 2,5-diphenyl-1,5-hexadiene revealed a change in transition state structure from a species with more bond making than breaking to one with a nearly a fully formed 1,4-cyclohexane diyl in that order. 

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