In the analysis of the urea 1 H/D isotopologues Jspectra, two nitrogen atoms are treated as equivalent hence the isotopologues consist of three different spin subsystems: -NH 2 -an XA 2 spin system, -NHD -an (XA)B spin system, and -ND 2 -an XB 2 spin system. As shown in Fig. , the peaks arising from -NH 2 and -NHD groups are predicted using the first-order perturbation theory. It should be also noted that signals from the -ND 2 group are not observed in the spectra. This results from the fact that amplitude of ZULF NMR signals is proportional to the square of the difference between gyromagnetic ratios of J-coupled nuclei , which in the considered case equals (γ D -γ15 N ) 2 /(γ1 H -γ15 N ) 2 ≈ 0.0025 (see SI for detailed energy-level analysis). , respectively. The transition in XA2 spin system is represented as v4 which corresponds to 3/2JXA. The manifolds are grouped by the quantum numbers IA and IB that denote the spin number of A nuclei and B nuclei, respectively. Each manifold is labelled by its total spin quantum number F or FT (see SI for detailed energy level analysis) . Only a single sublevel in each manifold and a single transition at each frequency are shown for clarity. b Experimental spectra of [ 15 N2]-urea in the mixture of D2O/H2O (1:1). For all peaks in the spectrum, the corresponding transitions (v1-4, v * 1-2 ) are determined by a first-order perturbation theory.