Fourier transform NMR pulse methods for the measurement of slow-exchange rates

Abstract

The seven-membered ring of Valium exists in two equivalent conformations. Exchange between the two forms exchanges two coupled nonequivalent hydrogen. atoms in the ring and this system has been used to examine NMR Fourier transform pulse methods for measuring rates of slow exchange. Experiments are reported both for the exchanging system (at 320 K) and for the effectively nonexchanging system at a lower temperature (270 K). The selective inversion of one exchanging resonance allows the determination of all the rate and relaxation constants by an iterative fitting of the data to the general solution of the modified Bloch equations. However, the use of a combination of non-selective inversion, selective inversion, and presaturation sequences is shown to provide definitive and consistent values for the rate and relaxation constants without the need for difficult curve fitting to several variables. By this means the rate of exchange in a 100 m M Valium solution in deuterochloroform at 320 K is found to be 5.4 ± 0.2 sec −. An appendix discusses the application of transverse relaxation measurements and concludes that for the slow exchange in Valium this method of determining the rate of exchange is inferior to the measurement of longitudinal relaxation.