Selective-inversion experiments applied to chemical exchange in coupled spin systems

Abstract

This paper addresses the problems of studying slow chemical exchange in spin systems that show scalar coupling. Line broadening due to exchange in this regime is minimal, so rates are often obtained by selective-inversion experiments. These are modified inversion-recovery experiments, in which only one part of the exchanging system is inverted. These spins can then return to equilibrium by exchange with the non-inverted sites, as well as by normal spin-lattice relaxation processes. This is a standard method for spin systems which show little or no scalar coupling. The problem is more complex for coupled spin systems, because the longitudinal magnetizations are more difficult to measure. There is no longer a one-to-one correspondence between the longitudinal magnetizations and the observable lines in the spectrum. The spectra depend on the flip angle of the observe pulse, there may be longitudinal magnetizations that can not be measured reliably, and zero-quantum transitions may interfere. These problems are addressed here. The flip angle dependence can be exploited to enhance the information collected, and a singular value decomposition (SVD) is used to obviate the numerical problems of poorly determined longitudinal magnetizations. The role of the zero-quantum interference is also discussed. With these methods, coupled spin systems can be treated like an uncoupled system. The technique is tested on two molecules: ReBr(CO)₃-2,6-pyridinedicarboxylate, a simple three-spin exchange system; and 4-nitroso-N,N-dimethylaniline, a four-spin system.