Leslie B. Sims

Model calculations of hydrogen–deuterium isotope effects for E2 and E1cB dehydrochlorinations of 1,1-diaryl-2,2-dichlorethanes

A 14-atom transition-state model is employed as the basis for computations of primary β-deuterium and secondary α-deuterium isotope effects for the dehydrochlorination of the substrates Ar2CH·CHCl2 by alkoxide bases. The structure of the transition state is systematically altered by varying the bond orders of O ⋯ H, H ⋯ Cβ,CβCβ and Cα⋯ Cl, with a progressive change in the angular geometry about Cα and Cβ from tetrahedral to trigonal and proportional to the degree of CαCβ double bond character. These changes are translated into force constant changes by means of empirical relations involving bond orders, bond angles, and the associated force constants. The results of changing the way in which the imaginary reaction co-ordinate vibration is formulated are also investigated. Best agreement between experimental values of primary kH/kD isotope effects and computed isotope effects is obtained when proton transfer is allowed to dominate the reaction co-ordinate motion, with the reacting heavy atoms remaining relatively motionless.

Model calculations of heavy atom isotope effects for E2 and E1cB dehydrochlorination of 1,1-diaryl-2,2-dichloroethanes

Calculations of heavy atom kinetic isotope effects for the alkoxide-induced dehydrochlorination of Ar2CH·CHCl2 are presented. Previously employed cut-off methods and transition state models are used, and the atoms concerned are α-chlorine (k35/k37 for primary intermolecular and intramolecular effects, and for the secondary effect) and Cα and Cβ primary carbon isotope effects, k12/k14). Changes in kinetic isotope effect with simulated variations in transition state structure are discussed. The effect of simulating partial solvation of the incipient anionic leaving group in the transition state is investigated. Theoretical isotope effects are compared with preliminary experimental results, and tentative transition state structures are proposed.