A nuclear magnetic resonance study of hindered rotation in 8-phenylpurines

In the PMR spectrum of 8-phenylpurines, the multiplet of the o-protons appears downfield of the multiplet, characteristic for m,p-protons. The separation of the centres of these two signals (Δ-value) diminishes with increasing steric interference between the phenyl ring and substituents in the imidazole moiety. The contribution of the purine ring current to the chemical shifts of the aromatic protons was calculated according to the theory of Johnson and Bovey, and the torsion angles θ between the phenyl ring and the plane of the purine system were derived. For 8-phenylpurines with an NH-group in the imidazole ring, θ is 10-15°; for compounds with an N-methyl group in this ring, θ ≈ 35-45°; in 3,9-dimethyl derivatives, Δ becomes zero, while θ is about 50°.