Generalized methodology for control of long-lived nuclear spin order in magnetic resonance

Abstract

The battle between signal and noise is the fundamental predicament of all experimental science. Amidst the plethora of interactions in the playing field of physical space, it is the supreme constraint that distinguishes what is readily measurable from what is undetectable outright to current methods. In the world of nuclear magnetic resonance, signal is typically the vector polarization of nuclear spins. Consequently, increasing interest has been dedicated to research in hyperpolarization. However, hyperpolarized spin order is fragile, decaying rapidly in the process of relaxation. Heated efforts have been devoted to studying long-lived states (LLS) as vessels for hyperpolarization. Since their discovery by our group in 2004, robust and generalized methods for accessing LLS have remained elusive until recently. The goal of this thesis is to make modest contributions to the art of generating, pre­serving, and controlling long-lived spin order. We clearly delineate between the two available modes of long-lived spin order in 2-spin-1/2 systems - singlet order and long-lived eigenorder - presenting for the first time a complete relaxation theory for the latter, as well as proof-of-concept of novel pulse sequences for efficiently accessing both of these configurations in a wide range of inequivalence regimes. In the course of these investigations, we uncover solution-state lifetimes exceeding an hour, the first such observations in molecular spin systems beyond a single previously known example. We investigate singlet scalar relaxation of the second kind (S-SR2K) in the unexplored millitesla regime, with a comprehensive study of the behaviour of this mechanism under selective low-frequency rf irradiation of both a spin-1/2 pair hosting long-lived order, and external quadrupolar nuclei - demonstrating controlled preservation or alternatively destruction of singlet order. We also show striking evidence of an S-SR2K effect induced via distant quadrupolar nuclei, allowing for the first time the quantification of mHz­scale heteronuclear coupling to short-lived spins, and providing ample evidence for the possibility of exploiting LLS as probes for previously inaccessible small-scale interactions.

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

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