Das ausmass sterischer hinderung in substituierten triarylmethanen und triarylmethyl-radikalen

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 C_ringC_αH, where C_α denotes the central C atom. Rotation around the C_α-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.