A variety of pulse sequences have been described for converting nuclear spin magnetisation into long-lived singlet order for nuclear spin-1/2 pairs.

Nuclear spin singlet states are often found to allow long-lived storage of nuclear magnetization, which can form the basis of novel applications in spectroscopy, imaging, and in studies of dynamic...

A range of nuclear magnetic resonance spectroscopy and imaging applications are limited by the short lifetimes of magnetization in solution. Long-lived states, which are slowly relaxing configurations of nuclear spins, have been shown to alleviate this limitation. Long-lived states have decay lifetimes T LLS significantly exceeding the longitudinal relaxation time T 1 , in some cases by an order of magnitude. Here we present an experimental case of a long-lived state for a 15 N labelled molecular system in solution. We observe a strongly biexponential decay for the long-lived state, with the lifetime of the slowly relaxing component exceeding 40 minutes, ∼21 times longer than the spin-lattice relaxation time T 1. The lifetime of the long-lived state was revealed by using a dedicated two-field NMR spectrometer capable of fast sample shuttling between high and low magnetic fields, and the application of a resonant radiofrequency field at low magnetic field. The relaxation characteristics of the long-lived state are examined.

Synthesis of Kr@C60 is achieved by quantitative high-pressure encapsulation of the noble gas into an open-fullerene, and subsequent cage closure. Krypton is the largest noble gas entrapped in C60 using...

Some nuclear spin systems support long-lived states, which display greatly extended relaxation times relative to the relaxation time of nuclear spin magnetization. In spin-1/2 pairs, the such a long-lived state is given by singlet order, representing the difference of the population of the nuclear singlet state and the mean population of the three triplets. In many cases, the experiments with long-lived singlet order are very time-consuming because of the need to wait for singlet order decay before the experiment can be repeated; otherwise spin order remaining from a previous measurement may lead to experimental artefacts. Here we propose techniques for fast and efficient singlet order destruction. These methods exploit coherent singlet-triplet conversion; in some cases, multiple conversion steps are introduced. We demonstrate that singlet order destruction enables a dramatic reduction of the waiting time between consecutive experiments and suggest to use this approach in singlet-state NMR experiments with nearly equivalent spins.

The population imbalance between nuclear singlet states and triplet states of strongly coupled spin-1/2 pairs, also known as nuclear singlet order, is well protected against several common relaxation mechanisms. We study the nuclear singlet relaxation of 13C pairs in aqueous solutions of 1,2-13C2 squarate, over a range of pH values. The 13C singlet order is accessed by introducing 18O nuclei in order to break the chemical equivalence. The squarate dianion is in chemical equilibrium with hydrogen-squarate (SqH−) and squaric acid (SqH2) characterised by the dissociation constants pKa1 = 1:5 and pKa2 = 3:4. Surprisingly, we observe a striking increase in the singlet decay time constants TS when the pH of the solution exceeds ~ 10, which is far above the acid-base equilibrium points. We derive general rate expressions for chemical-exchange-induced nuclear singlet relaxation and provide a qualitative explanation of the TS behaviour of the squarate dianion. We identify a kinetic contribution to the singlet relaxation rate constant which depends explicitly on kinetic rate constants. Qualitative agreement is achieved between the theory and the experimental data. This study shows that infrequent chemical events may have a strong effect on the relaxation of nuclear singlet order.

Coupled pairs of spin-1/2 nuclei support one singlet state and three triplet states. In many circumstances the nuclear singlet order, defined as the difference between the singlet population and the mean of the triplet populations, is a long-lived state which persists for a relatively long time in solution. Various methods have been proposed for generating singlet order, starting from nuclear magnetization. This requires the stimulation of singlet-to-triplet transitions by modulated radiofrequency fields. We show that a recently described pulse sequence, known as PulsePol (Schwartz et al. Science Advances, 4, eaat8978 (2018)), is an efficient technique for converting magnetization into long-lived singlet order. We show that the operation of this pulse sequence may be understood by adapting the theory of symmetry-based recoupling sequences in magic-angle-spinning solid-state NMR. The concept of riffling allows PulsePol to be interpreted using the theory of symmetry-based pulse sequences, and explains its robustness. This theory is used to derive a range of new pulse sequences for performing singlet-triplet excitation and conversion in solution NMR. Schemes for further enhancing the robustness of the transformations are demonstrated.

A proposal of quantum cognition advances the hypothesis that quantum entanglement between 31P nuclei could serve as a means of information storage in the brain. Testing this hypothesis requires an understanding of how long-lived these quantum effects may be. We used NMR spectroscopy and molecular dynamics simulations to study the mechanisms that limit these quantum processes in 18O-enriched molecules of pyrophosphate, the simplest biomolecule that can sustain quantum-entangled 31P nuclear spin singlet states. We confirmed that chemical shift anisotropy limits the singlet magnetization order lifetimes in high magnetic fields, and we discovered that rapid rotation of the phosphate groups limits the lifetime in low magnetic fields. These findings represent an important starting point in studying whether quantum cognition can be a true biological phenomenon.