Selective nuclear Overhauser effect experiments on coupled spin systems

Abstract

A formalism describing selective nuclear Overhauser effect (NOE) experiments on coupled spin systems is developed for the study of relaxation processes. The formalism is completely general, and is developed in terms of the fractional intensity changes Gab,cd of various transitions ab γ relative to the irradiated line cd. The general properties of the Gab,cd are discussed. The symmetry properties of coupled spin systems can be investigated conveniently using group theoretical considerations by setting up a geometrical figure to correspond to the relaxation problem. This procedure was illustrated for weakly coupled three-spin systems, where the systematics of selective NOE experiments can be described in terms of the transformation properties of a cube. Analytical expressions for the Gab,cd were obtained for the intramolecular dipole-dipole and external random field relaxation mechanisms. These expressions provide a good approximate description even for strongly coupled systems. Relaxation analyses were performed on the ABX proton system vinyl bromide and the A2X proton system 1,1,2-trichloroethane. It was shown that the particular experiment where an outer line of the X multiplet is saturated is quite sensitive to the detailed relaxation mechanism. The remaining lines of the X multiplet directly reflect the fractional contribution of the intramolecular dipole-dipole mechanism. The results for vinyl bromide were interpreted in terms of anisotropic reorientation using an inertial model. It is suggested that the selective NOE technique can be useful when information regarding molecular conformations is required, and the conventional NOE experiments (where an entire multiplet is saturated) are difficult to perform.