Abstract
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.
Highlights
Nuclear magnetic resonance (NMR) techniques are often limited by low polarisation levels and the restricted lifetime of nuclear spin order
Our main interest is the transformation amplitude of -spin singlet order into spin Zeeman polarisation which is bounded by −1 ≤ ≤ 1 [66]. This indicates that any amount of singlet order may in principle be fully converted into -spin Zeeman polarisation by coherent evolution under a pulse sequence
The T00 block acts as a filter suppressing all NMR signals not passing through singlet order [71]
Summary
Nuclear magnetic resonance (NMR) techniques are often limited by low polarisation levels and the restricted lifetime of nuclear spin order. The difference in the heteronuclear couplings induces singlet-triplet mixing of the -spin pair. For small mixing angles ST (the nearequivalence regime), the S2hM sequence is able to efficiently transform singlet polarisation into heteronuclear Zeeman polarisation with an amplitude close to the theoretical maximum. For small mixing angles ST, the S2hM and gS2hM sequences both transforms singlet order into heteronuclear Zeeman polarisation with efficiencies close to the theoretical maximum. The operational regime of the gS2hM sequence is considerably wider than that of S2hM, covering a range of mixing angles spanning 0◦ < ST ≲ 50◦, albeit with prominent dips in performance at certain ST values. A more thorough comparison of gS2hM with other pulse sequences, especially those used in the context of para-hydrogen induced polarization, is given in the discussion section
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