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Methyl cation affinity vs proton affinity in substituted benzenes: An ab initio study

The proton (PA) and methyl cation (MCA) affinities of benzene and its di- and polysubstituted derivatives are examined at the MP2(fc)/6-31G**//HF/6-31G*+ZPE(HF/6-31G*) theoretical level. The calculated MCA of benzene is in good accordance with the experimental estimate. It is shown that MCAs of polysubstituted benzenes follow the additivity rule, which is analogous to the additivity property of PAs in multiply substituted aromatics. The additivity rule of thumb is based on the independent substituent approach (ISA). It exhibits a high performance being at the same time simple and intuitively appealing. Finally, conclusive evidence is provided which shows that the MCA of polysubstituted benzenes is linearly related to the corresponding proton affinities.

polysubstituted benzenes; additivityof the methyl cation affinity; ab initio calculations

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