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

Abstract

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 ⋯ C_β,C_βC_β and C_α⋯ Cl, with a progressive change in the angular geometry about C_α and C_β from tetrahedral to trigonal and proportional to the degree of C_αC_β 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 k_H/k_D 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.