Ab Initio Calculation for Benzylic Cations

α位（ベンジル位）に各種置換基を導入した20種類のベンジル位カチオン及びそれらの共役塩基の構造をRHF/6-31G^*レベルの非経験的分子軌道計算により最適化した。エネルギーはMP2/6-31G^*//RHF/6-31G^*+ZPE（scaled 0.9）レベルで計算した。それらの値を用いて算出したプロトン移動反応のエネルギー計算値は対応する実験値を良い精度で再現した。そこで得られた物理量と対応するソルボリシス系の湯川―都野式によるr値を比較した。結合距離、結合次数、電荷の分布はr値に対して直線相関を与えた。相関結果からr値が共鳴の度合を表すパラメータであるという理論的根拠が得られた。Structures of 20 kinds of α-substituted benzylic cations and their conjugate base, which cover a wide variation of the stability, were optimized by means of ab initio molecular orbital method at the RHF/6-31G^* level. Total energies were calculated at the MP2/6-31G^*//RHF/6-31G^* + ZPE (scaled 0.9) level. Calculated relative proton affinities of conjugate bases agreed well with the corresponding basicities in the gas phase. The obtained theoretical indices associated with the resonance interaction, such as population, bond order, and bond length were compared with the resonance demand parameter (r value) of corresponding solvolysis systems, that were given in the Yukawa-Tsuno substituent effect analysis. The r value was linearly correlated with the theoretical resonance indices. The relation of each theoretical index and the r value agreed completely with the prediction by the electronic theory. This provides theoretical support that the r value is a good parameter indicating the degree of resonance interaction between benzylic pπ-orbital and benzene π-system