Resonance energy in benzene and ethene derivatives in the gas phase as a measure of resonance ability of various functional groups

Three sets of isodesmic reactions in the gas phase were constructed in which a variable functional group is transferred from a saturated to an unsaturated hydrocarbon residue: methyl→phenyl, tert-butyl→phenyl, ethyl→ethenyl. Reaction enthalpies, obtained either from the known enthalpies of formation or by calculations at an MP2 or DFT level, were considered as new possible scales of resonance effect of these functional groups; in the case of charged groups (CHOH+, NO2H+, O−, COO−, etc.) it is the only way to estimate their effects reliably. One scale was also transferred from the gas phase into aqueous solution by adding the Gibbs energies of solvation of all compounds involved. All scales are only partly parallel to the common scales of substituent constants σR°, σR+ or σR−. Some particular deviations were specified: stabilizing homoconjugation of the CH2X substituents with the benzene π-electrons, or stabilizing interaction of polar groups with the tert-butyl group. However, most of the deviations are of an unknown nature: they are not due to solvation which affects the resonance energy rather little, and are only slightly influenced by the polarizability of the phenyl group. One cause of the deviations could be that all σR values have been defined on models involving ions.