Sensitivity enhancement and field-dependent relaxation in singlet nuclear magnetic resonance

Abstract

Nitrous oxide (N₂O), also known as "laughing" gas, is a well known compound used in medicine as a mild anaesthetic, or in engineering as a powerful oxidizer providing high output of engines. Recently, its ¹⁵N doubly-labelled isotopologue attracted attention in singlet NMR due to its long singlet relaxation time ranging between 7 minutes, when dissolved in blood, up to 26 minutes in degassed dimethyl sulfoxide (DMSO). Singlet NMR deals with nuclear singlet states, which are exchange antisymmetric quantum states of coupled pairs of spin-1/2 nuclei with zero total nuclear spin quantum number. These states are nonmagnetic and immune to exchange-symmetric relaxation processes such as intramolecular direct dipolar relaxation. Their lifetimes may be up to an order of magnitude longer than conventional relaxation times T₁ and T₂. Besides various fields of NMR, singlet states find potential application also in MRI. The direct medical application of ¹⁵N₂O as a MRI tracer is, however, complicated by a poor detection sensitivity resulting from the low ¹⁵N magnetogyric ratio, low solubility in liquids at room temperature and atmospheric pressure, and limitations of ¹⁵N signal enhancement by means of physical methods for dissolved ¹⁵N₂O. This thesis addresses two topics related to singlet NMR of ¹⁵N₂O - sensitivity enhancement and magnetic-field dependent relaxation. The NMR signal decay in liquid phase is often dominated by static magnetic field inhomogeneity, described by the time constant T'₂ , which is much faster than the transverse relaxation, characterized by T₂. Repeated refocusing by a multiple spin-echo (MSE) train maintains the ¹⁵N signal for extended times of several T₂. Acquisition of the signal during the whole MSE sequence followed by a proper processing either by matched weighting or singular value decomposition, may lead to the signal-to noise ratio (SNR) enhancement by up to an order of magnitude under favourable circumstances. The SNR enhancement is a function of T₂, T'₂, and the spectral resolution. The procedure of the SNR enhancement in combination with methods of singlet NMR was used to investigate in detail low-field ¹⁵N₂O singlet relaxation. The ¹⁵N₂O relaxation measurements were extended to field strengths up to the spectrometer high field. The observed relaxation dependencies were described by a general theory, relaxation as a time-dependent exchange of populations of the field-dependent energy eigenstates. In particular, spin-rotation relaxation in low field was discussed.

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

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