Separation and characterization of different signals from intermolecular three-spin orders in solution NMR

Abstract

In this paper, signals originating from a pure specific coherence of intermolecular three-spin orders were separated & characterized experimentally in highly polarized two-component spin systems. A modified CRAZED sequence with selective radio-frequency excitation was designed to separate the small signals from the strong conventional single-spin single-quantum signals. General theoretical expressions of the pulse sequence with arbitrary flip angle pulses were derived using dipolar field treatment. The expressions were used to predict the relaxation & diffusion properties & optimal experimental parameters such as flip angles. For the first time, relaxation & diffusion properties of pure intermolecular single-quantum, double-quantum, & triple-quantum coherences of three-spin orders were characterized & analyzed in one-dimensional experiments. All experimental observations are in excellent agreement with the theoretical predictions. The theoretical results show that the quantum-mechanical treatment leads to exactly the same predictions as the dipolar field treatment. The quantitative study of intermolecular multiple-quantum coherences of three-spin orders presented herein provides a better understanding of their mechanisms.