B Values as a Sensitive Measure of Steric Effects
Torsional barriers of 15 ortho-substituted biphenyls have been determined computationally using the B3LYP density functional and experimentally by variable-temperature ("dynamic") nuclear magnetic resonance. Taking advantage of the 3'-isopropyldimethylsilyl group as a novel and superior diastercotopicity probe and tracking coalescence temperatures down to -173 degrees C (100 K), activation energies of aryl-aryl rotation as small as 5 kcal mol(-1) can be assessed. The 2-X/2'-H repulsion increments thus derived are powerful parameters for rationalizing and predicting the conformational behavior of aromatic compounds carrying ortho substituents.
Keywords: biphenyls ; density functional calculations ; NMR spectroscopy ; steric effects ; Absolute-Configuration ; Carbon-Carbon ; Dynamic Nmr ; Stereodynamics ; Biphenyls ; Rotation ; Substituent ; Conformation ; Atropisomers ; Barriers
Record created on 2010-11-30, modified on 2016-08-09