for the aryl–aryl rotation of 17 biphenyl derivatives, bearing a heavy heteroatom (S, Se, Te, P, Si, Sn) as ortho substituent, have been measured by variable temperature NMR. These numbers, so called B values, represent a meaningful measure of the steric hindrance exerted by the selected substituents. DFT computations match quite satisfactorily the experimental barriers and the ground state geometries
cyano groups were found to be slightly smaller than a phenyl ring. In contrast, vinyl and, in particular, formyl groups proved to be larger than phenyl. The latter difference is due to the loss of conjugation forces at the planar transition state. α-Hydroxyhexafluoroisopropyl is slightly more bulky than tert-butyl. Pentafluorophenyl and trifluoromethoxy exhibit nearly the same effectivesize as phenyl
Someone who says “A” should be prepared to also say “B”: In contrast to cyclohexane model‐based A values, biphenyl model‐derived B values are powerful tools to quantify steric repulsion in and conformational behavior of ortho‐substituted aromatic compounds.