Robust transformation of singlet order into heteronuclear magnetisation over an extended coupling range

Several important NMR procedures involve the conversion of nuclear singlet order into heteronuclear magnetisation, including some experiments involving long-lived spin states and parahydrogen-induced hyperpolarisation. However most existing sequences suffer from a limited range of validity or a lack of robustness against experimental imperfections. We present a new radio-frequency scheme for the transformation of the singlet order of a chemically-equivalent homonuclear spin pair into the magnetisation of a heteronuclear coupling partner. The proposed radio-frequency (RF) scheme is called gS2hM (generalized singlet-to-heteronuclear magnetisation) and has good compensation for common experimental errors such as RF and static field inhomogeneities. The sequence retains its robustness for homonuclear spin pairs in the intermediate coupling regime, characterised by the in-pair coupling being of the same order of magnitude as the difference between the out-of-pair couplings. This is a substantial improvement to the validity range of existing sequences. Analytical solutions for the pulse sequence parameters are provided. Experimental results are shown for two test cases.

Automated pneumatic shuttle for magnetic field cycling and parahydrogen hyperpolarized multidimensional NMR

We present a simple-to-implement pneumatic sample shuttle for automation of magnetic field cycling and multidimensional NMR. The shuttle system is robust allowing automation of hyperpolarized and non-hyperpolarized measurements, including variable field lifetime measurements, SABRE polarization optimization, and SABRE multidimensional experiments. Relaxation-protected singlet states are evaluated by variable-field T₁ and T_S measurements. Automated shuttling facilitates characterization of hyperpolarization dynamics, field dependence and polarization buildup rates. Furthermore, reproducible hyperpolarization levels at every shuttling event enables automated 2D hyperpolarized NMR, including the first inverse ¹⁵N/¹H HSQC. We uncover binding mechanisms of the catalytic species through cross peaks that are not accessible in standard one-dimensional hyperpolarized experiments. The simple design of the shuttling setup interfaced with standard TTL signals allows easy adaptation to any standard NMR magnet.

Singlet NMR methodology in two-spin-1/2 systems

This paper discusses methodology developed over the past 12years in order to access and manipulate singlet order in systems comprising two coupled spin-1/2 nuclei in liquid-state nuclear magnetic resonance. Pulse sequences that are valid for different regimes are discussed, and fully analytical proofs are given using different spin dynamics techniques that include product operator methods, the single transition operator formalism, and average Hamiltonian theory. Methods used to filter singlet order from byproducts of pulse sequences are also listed and discussed analytically. The theoretical maximum amplitudes of the transformations achieved by these techniques are reported, together with the results of numerical simulations performed using custom-built simulation code.

Alternating Delays Achieve Polarization Transfer (ADAPT) to heteronuclei in PHIP experiments

A new methodology for producing hyperpolarized ¹³C nuclei in small organic systems via parahydrogen induced polarization (PHIP) is proposed: ADAPT (Alternating Delays Achieve Polarization Transfer). The theoretical foundation of the process is investigated in some detail and experimental examples demonstrating the viability of the approach are provided as well. The number of adjustable parameters is fewer than most of other conversion schemes. The achieved theoretical heteronuclear polarization is close to unity for any examined magnetic equivalence regime. The duration of the pulse sequence, which was successfully implemented, can be shorter than other established methods reducing possible relaxation losses. The conversion scheme is robust to B₁ inhomogeneities, but more sensitive to off-resonance RF irradiation.

Singlet order conversion and parahydrogen-induced hyperpolarization of 13C nuclei in near-equivalent spin systems

We have demonstrated two radiofrequency pulse methods which convert the nuclear singlet order of proton spin pairs into the magnetisation of nearby ¹³C nuclei. These irradiation schemes work well in the near-equivalence regime of the three-spin system, which applies when the difference in the two ¹H-¹³C couplings is much smaller than the ¹H-¹H coupling. We use pulse sequences to generate thermally polarized singlet states in a reproducible manner, and study the singlet-to-magnetisation transfer step. Preliminary results demonstrate a parahydrogen-enhanced ¹³C polarization level of at least 9%, providing a signal enhancement factor of more than 9000, using 50% enriched parahydrogen.

Long-lived localization in magnetic resonance imaging

The longitudinal nuclear relaxation time, T1, sets a stringent limit on the range of information that can be obtained from magnetic resonance imaging (MRI) experiments. Long-lived nuclear spin states provide a possibility to extend the timescale over which information can be encoded in magnetic resonance. We introduce a strategy to localize an ensemble of molecules for a significantly extended duration (∼30 times longer than T1 in this example), using a spatially selective conversion between magnetization and long-lived singlet order. An application to tagging and transport is proposed.

Para-hydrogen perspectives in hyperpolarized NMR

The first instance of para-hydrogen induced polarization (PHIP) in an NMR experiment was serendipitously observed in the 1980s while investigating a hydrogenation reaction (Seldler et al., 1983; Bowers and Weitekamp, 1986, 1987; Eisenschmid et al., 1987) [1-4]. Remarkably a theoretical investigation of the applicability of para-hydrogen as a hyperpolarization agent was being performed in the 1980's thereby quickly providing a theoretical basis for the PHIP-effect (Bowers and Weitekamp, 1986) [2]. The discovery of signal amplification by a non-hydrogenating interaction with para-hydrogen has recently extended the interest to exploit the PHIP effect, as it enables investigation of compounds without structural alteration while retaining the advantages of spectroscopy with hyperpolarized compounds [5]. In this article we will place more emphasis of the future applications of the method while only briefly discussing the efforts that have been made in the understanding of the phenomenon and the development of the method so far.