Small steric effects in isolated molecules: alkyl-substituted benzonitriles

摘要

A series of seven alkyl-substituted benzonitriles was investigated as a model for small steric effects (van der Waalsntension) excluding any steric inhibition due to resonance. The energies were calculated at a B3LYP/6-311u0001G(d,p)nlevel and the gas-phase basicities of some compounds were measured by Fourier-transform ion cyclotron resonance.nSubstituent effects were evaluated separately for neutral molecules and protonated forms by means of isodesmicnreactions, and dissected into polar and steric effects by comparing ortho and para derivatives. One or two orthonmethyl groups have a relatively small steric effect operating almost exclusively in the protonated form which leads tonweakened basicity. An ortho tert-butyl group exerts a strong steric effect both in the protonated and nonprotonatednforms, manifested also in a rather strong distortion of geometry. The effect on basicity is then a small difference ofntwo large values and is base strengthening. Acid–base properties are in such cases a poor measure of substituentneffects and cannot be interpreted in simple terms.