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 Bᵢᵘᵖ and Bᵢˡᵒʷ 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 Bᵢᵘᵖ at high field specifies the perturbation order of the dominating Zeeman interaction and for Bᵢˡᵒʷ 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 ${B}₂ᵘᵖ<B<{B}₁ᵘᵖ$ ($10{}^{{-}3}T<B<{10}²$ T), one at low field with ${B}₁ˡᵒʷ<B<{B}₂ˡᵒʷ({10}^{{-}8}T<B<{10}^{{-}7}$ T), and a strong coupling regime with ${B}₂ˡᵒʷ<B<{B}₂ᵘᵖ$ ($10{}^{{-}7}T<B<{10}^{{-}3}$ 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 ${B}₁ˡᵒʷ<B<{B}₂ˡᵒʷ$ 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.