Computational study of substituent effects on gas-phase stabilities of Meisenheimer complexes

Abstract

The total stabilization energies (TSEs) and anion stabilization energies (ASEs) of ring-substituted (X-) Meisenheimer complexes featuring two NO2 groups in the ring were determined using appropriate isodesmic reactions. The structures and energies of respective species were calculated at the B3LYP/6–311+G(2d,p) level of theory. Ten series of substituent effects were examined by varying substituent Y, which is connected to the sp3 carbon of the ring. The substituent effects were successfully analyzed using an extended Yukawa–Tsuno equation, [Formula: see text]. The r− values for the TSEs were identical to those for the ASEs, whereas the s values for the TSEs were significantly different from those for the ASEs. This shows that the effect of neutral species contributes to the s values of the TSEs. The r− and s values for the ASEs of all Meisenheimer complexes were distributed in a narrow range because substituent Y was insulated from the π-conjugation system. The r− values were large and the s values were small. This shows that the r− and s values were independent of each other and that the extended three-term Yukawa–Tsuno equation was intrinsic for substituent-effect analyses of anions. Although the variation was not substantial, the change in the r− values was clearly explained by the orbital interaction between substituent Y and the π-conjugation system. The r− values exhibited a good correlation with the bond lengths between the ring and the 4-NO2 group among all Meisenheimer complexes and benzylic anions. These facts provide a physical meaning: the r− value is a parameter that reveals the degree of the additional π interactions between the electron-withdrawing substituents and the π-conjugation systems of the ring.