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Abstract

J-coupled nuclear magnetic resonance (NMR) spectroscopy in the strong coupling regime at low magnetic field (10−7 T <B<10−3 T) is more complex than at high field (B>10−3 T) and at ultralow field (B<10−7 T). We show that several upper and lower boundaries Bupi and Blowi of the magnetic field B exist, where the complexity of J-coupled NMR spectra changes in terms of the number of lines. The index i=1,2,… for Bupi at high field specifies the perturbation order of the dominating Zeeman interaction and for Blowi at ultralow field the perturbation order of the dominating J-coupling interaction. Mathematical expressions for these boundaries are derived for the case of a J-coupled S-IN group where S and I are rare and abundant spins 12 and N counts the abundant spins I. The entire B-field range can further be delineated into two weak coupling regimes, one at high field with Bup2<B<Bup1 (10−3 T <B<102 T), one at low field with Blow1<B<Blow2(10−8 T <B<10−7 T), and a strong coupling regime with Blow2<B<Bup2 (10−7 T <B<10−3 T). The corresponding NMR spectra for the S−IN group are investigated by experiment and by simulation. In the strong coupling regime, the maximum number of lines is (N+1)2. In the weak coupling regime Blow1<B<Blow2 at low field, symmetric multiplet structures group around the frequencies 0, J, (3/2)J, 2J, (5/2)J, etc. These spectra determine the structure of the S−IN group unambiguously and are in dual correspondence to the weakly coupled spectra at high field. High-resolution NMR spectroscopy at ultralow field may open up new ways for chemical analysis by small and mobile instruments with many applications in science and technology.