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The T₁of a hyperpolarized site in solution is a key parameter that determines the time‐window in which its NMR signals are observable. For¹³C sites adjacent to protons,¹H‐decoupling has been shown to increase the hyperpolarized signal resolution and SNR. Additionally, polarization transfer to protons has shown utility in increasing the sensitivity of detection. However,¹H‐decoupling could lead to a change in the decay rate of a hyperpolarized¹³C site. Here we tested this possible effect in a case where the protons are directly bound to an sp³hyperpolarized¹³C site (using [1,2‐¹³C₂]choline) and¹H‐decoupling was applied continuously throughout the hyperpolarized decay measurement. We found that¹H‐decoupling did not lead to any significant changes in the¹³C polarization decay time but did result in the expected collapse of J‐coupling and produced sharper signals. This result suggested that¹H‐decoupling did not affect the decay rate of hyperpolarized sp^{3 13}C sites. The deuterium‐substitution approach (using [1,1,2,2‐D₄,1‐¹³C]choline) showed a dramatic prolongation of T₁. Upper bounds on the T₁of all investigated sites were calculated.

 

A variety of pulse sequences have been described for converting nuclear spin magnetisation into long-lived singlet order for nuclear spin-1/2 pairs. Existing sequences operate well in two extreme parameter regimes. The magnetisation-to-singlet (M2S) pulse sequence performs a robust conversion of nuclear spin magnetisation into singlet order in the near-equivalent limit, meaning that the difference in chemical shift frequencies of the two spins is much smaller than the spin–spin coupling. Other pulse sequences operate in the strong-inequivalence regime, where the shift difference is much larger than the spin–spin coupling. However both sets of pulse sequences fail in the intermediate regime, where the chemical shift difference and the spin–spin coupling are roughly equal in magnitude. We describe a generalised version of M2S, called gM2S, which achieves robust singlet order excitation for spin systems ranging from the near-equivalence limit well into the intermediate regime. This closes an important gap left by existing pulse sequences. The efficiency of the gM2S sequence is demonstrated numerically and experimentally for near-equivalent and intermediate-regime cases. 

The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T 1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet–triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet–triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors.