ベンジルオキシドアニオンの気相安定性に及ぼす置換基効果

抄録

Substituent effects on gas-phase stabilities of benzyloxide anions having fixed dihedral angle φ between the C-O bond in the side chain and the benzene ring plane were computationally determined at the B3LYP/6-311+G (2d,p) level of theory, in addition to that of fully optimized anions that have the φ of 0.0°. The fixed φ was varied from 10° to 90° at the interval of 10°. All the obtained 10 series of substituent effects were analyzed by the extended Yukawa-Tsuno equation (-ΔE_X=ρ(σ⁰+r^-Δσ_R+sΔσ_S)). The saturation degree of the anion quantified by the resultant s value did not show a significant change against the dihedral angle φ. The through-resonance degree quantified by the r^- value decreased from 0.45 to 0.40 as the φ increased from 0° to 40°, and increased from 0.40 to 0.59 as the φ further increased from 40° to 90°. NBO analyses revealed that the sum of the donor-acceptor (DA) interactions from orbitals in the side chain to the π* orbitals in the benzene ring play an crucial role to induce the through-resonance effect. The detailed investigation showed that the DA interaction from the lone pair on the O atom and the σ orbital of C-O bond to the π* orbitals in the benzene ring are important when the φ is nearby 90°, and the DA interaction from the σ orbital of C-H bond in the side chain to the π* orbitals in the benzene ring are important when the φ is nearby 0°. Since the through-resonance effect appears at any φ through above-mentioned DA interactions, the present anion cannot become an ideal σ⁰-reference system.