Collect. Czech. Chem. Commun. 2001, 66, 1623-1637

Resonance Energy in Benzene Derivatives with a Variable Functional Group

Otto Exnera and Stanislav Böhmb,*

a Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic
b Department of Organic Chemistry, Institute of Chemical Technology, Prague, 166 28 Prague 6, Czech Republic


Three isodesmic reactions were investigated in which a variable substituent is transferred from a saturated to an unsaturated hydrocarbon residue: ethane → ethene, methane → benzene, ethane → benzene. Their reaction energies could serve as a measure of substituent resonance ability. They were calculated for 31 substituents by the density functional theory (DFT) at the B3LYP/6-311+G(d,p) level, and served as a model for systematic comparison of the DFT energies and experimental enthalpies of formation ∆fH°(298). It turned out that calculations at this level are more reliable than experiments; they disagree sometimes with ∆fH°(298) measured at different times and in different laboratories but agree better with the values calculated by simple additive scheme. Some particularly suspicious values were pointed out. The DFT reaction energies of the isodesmic reactions, ∆1E-∆3E, were correlated with the standard scales of resonance and with other scales of substituent effects by multiple linear regression and principal component analysis. The three scales are fairly similar to each other (correlation coefficients 0.68-0.83), depend rather strongly on resonance (75% of the variance) but involve still at least one component which could not be explained in terms of common substituent effects. Certainly they do not depend - not even slightly - on the inductive effect, steric effect or polarizability.

Keywords: Arenes; Benzene; Enthalpy of formation; Isodesmic reactions; Resonance energy; Substituent effects; Ab initio calculations; DFT.

References: 45 live references.