MEASURE OF STERIC HINDRANCE IN SUBSTITUTED TRIARYLMETHANES AND TRIARYLMETHYL RADICALS

The chemical shift of the methine aromatic protons in the NMR spectra of substituted triarylmethanes is determined by the distortion of the phenyl rings relative to the planes CringCαH, where Cα denotes the central C atom. Rotation around the Cα-aryl bond of the di-ortho substituted compounds 2a and 4 is sterically hindered at low temperatures. The energies of activation (11·1 and 8·8 kcal/ mole, respectively) are smaller than expected. Steric hindrance in triarylmethyl radicals is discussed in the light of the results with the corresponding triarylmethanes. © 1969.