Paths from weak to strong coupling in NMR

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 B-i(up) and B-i(low) 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 B-i(up) at high field specifies the perturbation order of the dominating Zeeman interaction and for B-i(low) 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-I-N group where S and I are rare and abundant spins 1/2 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 B-2(up) < B < B-1(up) (10(-3) T < B < 10(2) T), one at low field with B-1(low) < B < B-2(low) (10(-8) T < B < 10(-7) T), and a strong coupling regime with B-2(low) < B < B-2(up) (10(-7) T < B < 10(-3) T). The corresponding NMR spectra for the S-I-N 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 B-1(low) < B < B-2(low) 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-I-N 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.