Theory and practice of singlet nuclear magnetic resonance

Abstract

Sensitivity is a signature problem of NMR. In its most basic description, an NMR experiment involves encoding information into an ensemble of nuclear spins followed by readout at a later time. The sensitivity is the extent to which the information content is distinguishable from system noise.

Principal factors that determine sensitivity are ensemble initialisation or polarisation, detection efficiency and relaxation effects that occur in between. This thesis addresses the last of these by examining opportunities of nuclear singlet states. Singlet states are exchange-antisymmetric quantum states of spin-1/2 pairs that, under favourable conditions, are the slowest-relaxing spin states of the NMR ensemble. In certain cases, singlet states may also exceed the relaxation times of isolated spins-1/2.

The goal of the work is to explore 'singlet NMR' as concept. The fundamentals of coherent control in a spin-1/2 pair are studied in depth, from which pulse sequences to generate and take advantage of singlet states are discussed. Several new methods for singlet excitation and detection are introduced. Existing methods are discussed within an overview context. Basic principles of singlet relaxation are also presented. Singlet lifetimes depend strongly on the correlation between magnetic fields at the nuclear spin pair sites and are sensitive most to the local magnetic environment. This can be used to retrieve information on local molecular structure and motion. Study of the relaxation relax rates may also help one determine the dominant singlet relaxation mechanisms for a given spin pair environment. This information may help one design molecules for maximum longevity of nuclear spin order.

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Identifiers

ORCID for Malcolm H. Levitt: orcid.org/0000-0001-9878-1180

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