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 whic h a variable functional group is transferred from a saturated to an unsatur ated 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 g as 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 sigma(R)degrees, sigma(R)(+) or sigma(R)(-) . Some particular deviations were specified: stabilizing homoconjugation of the CH2X substituents with the benzene pi-electrons, or stabilizing intera ction of polar groups with the tert-butyl group. However, most of the devia tions are of an unknown nature: they are not due to solvation which affects the resonance energy rather little, and are only slightly influenced by th e polarizability of the phenyl group. One cause of the deviations could be that all sigma(R) values have been defined on models involving ions.