Selection of intra- or inter-molecular multiple-quantum coherences in NMR of highly polarized solution

Abstract

In highly polarized scalar-coupled liquid systems, nuclear magnetic resonance signals from multiple-quantum coherences (MQCs) may be formed by inter-molecular dipolar and/or intra-molecular scalar couplings. Selection of specific signals can simplify the spectra, which may help us understand the underlying physical mechanisms in complex coupled spin systems. In this paper, a pulse sequence with three selective radio-frequency (RF) pulses and phase cycling was designed for this purpose. For an I pS q ( p, q = 1 , 2 , 3 , … ) spin system, there are three kinds of MQC signals, which originate from intra-molecular I– S, inter-molecular I– S, and inter-molecular S– S (or I– I) coherences. These three kinds of signals can be detected separately by proper phase cycling of RF pulses, which is independent of coupling constants. The intra- and inter-molecular MQC signals can also be detected separately with specific preparation periods, but this method is sensitive to coupling constants. Our theoretical predictions are in good agreement with experimental observations. The method proposed herein can be extended to heteronuclear cases.