REVEALING NMR-INVISIBLE CONFORMATIONAL PROCESSES IN AMINES THROUGH NMR ISOTOPE SHIFTS

Symmetry and the ordering of the inversion and isolated rotation barri ers can render inversion ''NMR-invisible'' in many simple amines, such as all dimethylamino compounds. NMR-invisible processes in RCH(2)NR'R ' amines can be revealed through NMR isotope shifts if the methylene i s monodeuteriated to create a chiral center that makes the R' groups d iastereotopic. A Delta G(double dagger) of 8.2 kcal mol(-1) for invers ion in N,N-dimethylpentanamine-1-d(1) was found through complete DNMR line-shape analysis of the N-methyl signals in 75 MHz C-13{H-1} spectr a. A barrier of 9.5 kcal mol(-1) for the exchange process in N,N-dieth yl-2,2-dimethylpropanamine, 3, was found from both H-1 spectra of 3 an d C-13 spectra of 3-d(1). The barrier for 3 is higher than expected fo r inversion and can be explained by inversion of a conformation that h as a nearly eclipsing neopentyl group to a conformation that has a hig h barrier for a rotation to complete the exchange process.