Substituent effects on the stabilities of anion derivatives

Abstract

Relative gas-phase stabilities of anions carrying electronically varied substituents on the benzene ring were determined by means of computational chemistry at a sufficiently high level of theory to reproduce the experimental data. The substituent effects obtained were compared with one another to reveal that the stabilities of anions are governed not only by the basic electronic effect of all ring-substituents but also by the through-resonance effect induced by para π -electron withdrawing groups ( para + R groups) and the saturation effect induced by electron donating groups (EDG). Statistical analyses on these additional stabilizing effects showed that the substituent effects on gas-phase stabilities of anion derivatives are accurately analyzed by a three-term extended Yukawa-Tsuno equation: − Δ E X = ρ σ 0 + r - Δ σ ¯ R - + s Δ σ ¯ S . This outcome is in contrast to the fact that the stabilities of cations are governed by the basic electronic effect of all ring-substituents and the through-resonance effect induced by para π -electron donating groups ( para −R groups) and their substituent effects are accurately analyzed by the Yukawa-Tsuno equation: − Δ E X = ρ σ 0 + r + Δ σ ¯ R + . The σ 0 and σ − substituent constants were determined by computed gas-phase stabilities of benzoate and phenoxide anions, respectively, to prepare the set of substituent constants that are utilized in the extended Yukawa-Tsuno equation. The extended Yukawa-Tsuno equation was then used to analyze substituent effects for various anion derivatives, and the physical meaning of the through-resonance and saturation effects were discussed in terms of computational chemistry. Whether the extended Yukawa-Tsuno equation is a useful tool to explore the properties of anions is discussed together with the original Hammett equation and the Yukawa-Tsuno equation which is a useful research tool for cationic species.