Behavior of longitudinal spin orders in NMR measurements of self-diffusion coefficients using radiofrequency field gradients

Abstract

Weakly coupled spin 1/2 systems of the type AX and AX2 can be easily brought into states of longitudinal order represented (in terms of operator products) by 2IzAIzX and 4IzAIzXIzX′, respectively. It proves possible to follow the evolution of these states under translational diffusion processes through a simple experiment involving two identical radiofrequency field gradient pulses separated by an interval allowing diffusion to take place. Relevant theory and experimental verifications are provided. The main conclusion is that, for a part of the decay, the effective gradient is multiplied by a factor of two in the case of a two spin order (2IzAIzX) and a factor of three in the case of a three spin order (4IzAIzXIzX′). These properties can be employed for accessing to self-diffusion coefficients, smaller by factors of four and nine respectively, as compared with those measured by conventional methods.