Urea molecule contains two -NH 2 groups joined by a carbonyl (C=O) functional group. To analyse the general structure of J-spectra of [ 15 N]-urea and [ 13 C, 15 N 2 ]urea, first, numerical simulations are performed using J-coupling constants shown in Fig. . Since one-bond 1 H-15 N coupling in the -NH 2 group is the strongest interaction in this system, the main features in the J-spectra of both forms of urea are centered around (3/2)| 1 J NH | ≈ 133.65 Hz (marked by a dashed line in Fig. ), as expected for the XA 2 nuclear spin system corresponding to the transitions in the manifold with the total proton spin 1, see for example . Hereafter, we refer to this group of signals as high-frequency peaks. Other (weaker) homonuclear and heteronuclear interactions result in the appearance of so-called lowfrequency peaks (<10 Hz), as well as, further modifications (splitting and shifting) of the main high-frequency peaks. Specifically, the presence of an additional, relatively strong 13 C-15 N interaction in [ 13 C, 15 N 2 ]-urea increases the shifts in corresponding energy manifolds, giving rise to a wider span of low-and high-frequency peaks compared to the spectrum of [ 15 N 2 ]-urea.