Theoretical study of the intramolecular CH/π interaction effect on rotation energy barriers in 1-pentene, 2,2′-diisopropyl biphenyl and some amino and nitro derivatives

The 2,2′-diisopropyl biphenyl conformers, and their amino and nitro para-disubstituted derivatives present two typical characteristics: In the ground state, CH/π interactions may induce local structures by positioning H atoms above some C atoms of the unsaturated cycles, and next the main skeleton is of biphenyl type. From ab initio theoretical calculations, we analyse first these characteristics separately by considering smaller molecules, i.e. 1-pentene for CH/π interactions and the biphenyl molecule itself. Sophisticated methods can be used for 1-pentene. We point out that the CH/π interaction present in the syn-conformer is not sufficiently stabilizing to compensate steric repulsions and the anti-conformer is found as the ground state. In the case of the biphenyl molecule, like many authors did before, and experiment compared, we were not able to improve significantly the calculated rotation energy barrier between the ground state and the conformation with a coplanar arrangement of the π-cycles. MP2 and B3LYP calculations, with basis sets of double-ζ plus polarization quality, on 2,2′-diisopropyl biphenyl conformers and their amino and nitro para-disubstituted derivatives, emphasize the difficulties found with 1-pentene and biphenyl, but damped down. The electron-donating and electron-withdrawing group effects of the amino and nitro substituents are analysed in term of σ and π contributions. This is mainly a π effect, which imposes its behaviour to the total electronic population only when considering also the atoms bonded to the amino and nitro groups. An increase of the electronic population on the atoms of the CH/π bond, mainly located on the C atom of the π system, is observed and is rather a σ effect. Moreover we show that CH/π interactions in the ground state only arise between H atoms of CH in iPr groups with C_ipso, not with C_ortho, and fit the experimental substituent effect on the basicity of the aryl group.