Structure, Conformation, and Dynamic Processes of the Stereolabile Atropisomers of Hindered Terphenyl Hydrocarbons

Ortho-substituted p-terphenyl hydrocarbons exist as trans and cis atropisomers that were identified by low-temperature NMR spectroscopy. The interconversion barriers increase with the dimensions of the ortho substituents, the experimental values being matched by ab initio calculations. X-ray diffraction shows that only the trans atropisomer is present in the solids. Spectra of a tert-butyl derivative in nonequilibrium conditions indicate that the cis is more populated than the trans atropisomer in solution, favored by attractive interactions.