A theoretical study of substituent effects in cyclopropyl and isopropyl cations: Strain energies in cyclopropyl cations and proton affinities of propenes and ethylenes

Ab initio molecular orbital calculations are reported for 1-X-isopropyl cations, 1-X-cyclopropyl cations, 2-X-propanes and X-cyclopropanes, where X is H, CH3, NH2, OH, F, CN and NC. Ions CH2X+ and CH3CHX+ were also re-examined. Geometries were optimized at the 3-21G level followed by single-point calculations at the 6-31G* level. The relative stabilizing effect of substituents is NH2 &> OH &> CH3 &> F ≈ NC &> CN, with the cyano group being the only destabilizing group. Substituent effects are largest in primary carbenium ions, CH2X+, and smallest in tertiary ions, (CH3)2CX+. The strain energies of the cyclopropyl cations, estimated from group separation reactions, are between 48 kcal mol−1 (X = CH3) and 65 kcal mol−1 (X = F), and are approximately double the strain energies of the similarly substituted cyclopropanes. For all substituents, except when X = OH and NH2, the 2-X-allyl cations are much lower in energy than the isomeric 1-X-cyclopropyl cations.