Rate-determining factors in nucleophilic aromatic substitution reactions.

Quantum chemical calculations (OPBE/6-311++G(d,p)) have been performed to uncover the electronic factors that govern reactivity in the prototypical S(N)Ar reaction. It was found that intrinsic nucleophilicity--expressed as the critical energy (the energy required for forming the Meisenheimer structure Ph(X)(2)(-)) in the identity substitution reaction X(-) + PhX --> X(-) + PhX (Ph = phenyl)--shows the following approximate trend: NH(2)(-) approximately OH(-) approximately F(-) >> PH(2)(-) approximately SH(-) approximately Cl(-) > AsH(2)(-) approximately SeH(-) approximately Br(-). The periodic trends are discussed in terms of molecular properties (proton affinity of X(-) expressing Lewis basicity of the nucleophile and C(1s) orbital energy expressing Lewis acidity of the substrate) based on a dative bonding model. Furthermore, the stepwise progress of the reactions and the critical structures are analyzed applying energy decomposition analysis. Increased stability, and thereby increased intrinsic nucleophilicity, correlates with decreasing aromatic character of the Meisenheimer structure. This apparent contradiction is explained in consistency with the other observations using the same model.