MQ NMR hard and soft pulses for I = 1 spin systems. Raman multiple-quantum NMR

Abstract

The general problem of applying either soft, intermediate, or hard RF pulses to an I = 1 spin ensemble is discussed, within the framework of irreducible tensors. The nuclei in question are subject to a magnetic field, directed along the axis of an axially symmetric quadrupole interaction, plus an RF field applied along the x′ axis in the rotating frame. General results are given which hold for all values of the RF field ω 1 chemical shift Δω, and the quadrupole beating parameter ω Q. These results are subsequently used to discuss the recently reported “Raman Magnetic Resonance” experiment, in which double-quantum coherence is both created and detected in a single-shot NMR experiment. This is achieved by first preparing a T 2 2(a) multipolar state, then applying a weak cw field along the x′ axis, while detecting NMR signals along the y′ axis. It is shown that action of the weak RF field is to cause double-quantum signals to appear, along all three axes. These signals oscillate at twice the Larmor offset because they originate from the T 2 2(a) multipolar state. Explicit expressions are given which describe the time behavior of both the NMR signals and the T 2 2(a) multipolar state, as a function of the strength of the cw RF field ω 1. In particular, it is shown that for a weak cw RF field, the strength of the x′ and y′ signals is proportional to ω 1 whereas the depletion of the T 2 2(a) state is proportional to ω 1 2. Thus detectable NMR signals, at a frequency of twice the Larmor offset, can be produced, while avoiding rapid conversion of the T 2 2(a) into other multipolar states. These conclusions however only hold provided the specific resonance condition Δω = ± ω Q is avoided.