Theoretical Analysis on substituent effects for Chemical Reactions by using Ab Initio Molecular Orbital Calculations.

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It was observed that there is a linear relationship between the activation free energies of the Menshutkin reaction (Eq.1) and the proton affinities (deltaPA) relative to the ammonia for corresponding bases with a nitrogen donor. The Ea's are dependent on the hybridization of the nitrogen, i.e., the sp^2 donors make a straight line which is different from that for the sp3 donors.CH_3 I + B * CH_3 B^++GAMMA B (Base with a nitrogen atom) (1)Although N,N-dimethylaniline has an sp^3 nitrogen, its deltaPA deviates from the sp^3 line. The 4-substituted derivatives of the base are not on the line, either. In the present study, ab initio molecular orbital calculations were used to explain why there are the linear relationship between the two energies. It was ascertained that the calculated activation energies (Ea) show a linear relationship with deltaPA.The Ea's also have a similar relationship with the calculated protonation energies.We also compared experimental results of Professor Tsuno's group with the calculated results concerning with the S_n I - S_n2 mechanism transition. It was ascertained that the reaction center is stabilized by the substituents of reactants and nucleophiles. This is the origin of the mechanism transition.In the present study, we investigated how substituents change reactivity and physico-chemical properties of molecules. These results include important informations for experimental chemists and useful for designing new molecules. It becomes, therefore, clear that theoretical chemists have to work together with experimental chemists in order to develop chemistry further.