The static and Dynamic NMR spectra of 4,4′‐bis( N ‐isopropyl‐ N ‐nitrosoamino)biphenyl, a four‐site exchange problem

The 1 H and 13 C NMR spectra of 4,4′‐bis( N ‐isopropyl‐ N ‐nitrosoamino)biphenyl at 22°C were consistent with the presence of three isomers due to slow rotation about the N—N bonds of the two N ‐nitroso groups. The structures of the three isomers were assigned by means of their symmetries and from the known effects of the two orientations of the N ‐nitroso group on 1 H and 13 C NMR chemical shifts. Each conversion of an N ‐nitroso group from anti to syn to an aryl group was associated with the same free energy difference (0.26 kcal mol −1 ) after symmetry corrections were made. The changes in the 13 C NMR spectra recorded from 95 to 150°C were simulated using a program for classical four‐site exchange. The potential barrier to rotation was calculated to be 20.76±0.06 kcal mol −1 at 400 K.