STERICALLY HINDERED RESONANCE IN METHYL-SUBSTITUTED ANILINES IN THE GAS-PHASE

The steric hindrance to resonance and other substituent effects in o-methylanilines and N,N-dimethylanilines were reinvestigated on the basis of gas-phase basicities and acidities (Fourier transform ion cyclotron resonance equilibrium data) and enthalpies of formation calculated by the semiempirical methods PM3 and AM1. In the gas phase, the phenomenon of hindered resonance is actually operating in the isolated molecules of N,N,2-trimethyl- and N,N,2,6-tetramethyl-aniline: it can be also observed via their basicities in water, but previous values were overestimated. The second ortho methyl group, in these cases, causes a lesser effect than the first. In the case of ortho-substituted anilines, a controversy was settled in favor of the steric hindrance to solvation of the cation as the most important effect in solution. However, steric effects also operate in both the isolated molecules and the anions: in these cases the second ortho methyl group causes a greater effect than the first. By the combination of different steric effects in the neutral base molecule and in the cation, an unexpected order in the basicity values can arise. The principle of inhibited resonance is fruitful and generally valid but should be revised quantitatively in every case with respect to the actual conformation and to a real estimate of the resonance energy.