Steric effects of the alkyl groups: evaluation on isolated molecules by means of isodesmic reactions†
Abstract
Several possible scales of steric effects of the alkyl groups were suggested on the basis of isodesmic model reactions, in which a sterically crowded compound is formally synthesized from simpler derivatives. The reaction energies were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d.p)//B3LYP/6-311+G(d.p) for 6 model systems and 7 various alkyl groups. The most important systems were cis-1,2-dialkylcyclopropanes 1 synthesized from two mono derivatives and sterically crowded derivatives of bicyclo[2.2.2]octane 2 with C3 symmetry. The scales of steric effects evaluated from the two models were rather different: the first scale depended in effect only on the C atoms in the α and β positions and the effects were almost equal for all primary alkyls. The second scale depended also on the γ position and the effect of the CH2–t-Bu group was much greater than that of the