Theoretical Studies on Reaction Constants in Yukawa-Tsuno Equation

Abstract

Linear Free Energy Relationship (LFER) is an extremely useful tool in the exploration of reaction mechanisms. Especially, the Yukawa-Tsuno (Y-T) Eq. (1) has been applied in success to a lot of systems including benzylic cation systems. It is expected that obtained reaction constants (r and rvalues) predict characters of transition states or cationic spieces. In this point of view, the Y-T relationship was applied to the stabilities of theoretically optimized benzylic cation systems using theoretically derived substituent parameters. This examination is of critical importance in order to establish the physical meanings of reaction constants. All species have been calculated at MP2/6-31G*//RHF/6-31G* level using GAUSSIAN-98. For a theoretical s0-standard system, ring substituted a,a-dimethylbenzyl cations whose cationic p-orbitals are fixed orthogonally against benzene p-system are selected. Relative stabilities to unsubstituted derivative are determined in hydride transfer equilibria and are calculated to determine theoretical s0. The set of s+ are determined in the same manner with s0 using a,a-dimethylbenzyl cations. With these s values, Eq. (1) was applied to a lot of benzylic cation systems that have electrically and sterically various substituents at a-positions. Obtained reaction constants were examined with theoretical indices to discuss the physical meaning.