Symmetry-based singlet–triplet excitation in solution nuclear magnetic resonance

Abstract

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.

Long-lived states of methylene protons in achiral molecules

“…There are several approaches to create LLSs in strongly coupled spin pairs, including magnetization-to-singlet (M2S) sequence (24,36) and symmetry-based pulse sequences (37). More specific pulse sequences were developed to transfer singlet order from strongly coupled protons enhanced by para-hydrogen to a heteronucleus such as 13 C (38)(39)(40)(41).…”

Delivering Robust Proton-Only Sensing of Hyperpolarized [1,2-13C2]-Pyruvate Using Broad-Spectral-Range Nuclear Magnetic Resonance Pulse Sequences

“…The S 0 plot declines quickly, and at the same time, the T 0 and 1 H plot rise with the same slope. Detailed theory for these pulse sequences is found in ref for riffled R4 3 1 and in refs , for S2M.…”

“…Importantly, these pulse sequences are applied only to the CH 3 protons. As illustrated in Figures b–d, the employed pulse sequences are S2M, − composite S2M, and riffled R4 3 1 pulses, − all applied only on the 1 H channel. The S2M pulse sequence involves two spin–echo trains separated by a 90° pulse (Figure b).…”

“…A good example is riffled R4 3 1 pulses, which are relatively new pulse sequences originally developed to implement electron-to-nuclear polarization transfer in diamond nitrogen-vacancy magnetometry, 49 which have recently been used for nuclear spin manipulations as well. 49 The riffled R4 3 1 name reflects the symmetry of the employed pulses, and as shown in Figure 2d, this pulse sequence consists of four blocks, R A -R B -R A -R B . The R A block consists of three hard pulses: 90°pulse with 135°phase, 180°pulse with 45°phase, and 90°pulse with 135°phase.…”

Multiple Stroboscopic Detection of Long-Lived Nuclear Magnetization for Glutathione Oxidation Kinetics

“…We preserved magnetization in symmetry-adapted long-lived spin states in GSH/GSSG Gly-(H α1,α2 ) and Cys-(H α H β1,β2 ) aliphatic protons (Figure A) in order to ensure the lifetimes necessary for on-the-fly stroboscopic detection. Methods to excite long-lived nuclear spin states on protons have been developed − for several endogenous molecules − to act as magnetization carriers while circumventing costly molecular enrichment in carbon-13 or nitrogen-15. Herein, long-lived states were excited and detected using a stroboscopic version of the “SELLS” procedure we have proposed recently. , The detection starts by a small flip-angle pulse used to convert a fraction of the singlet state into detectable magnetization (Figure B).…”