On the other hand, rapid proton exchange leads to a modification of the effective coupled spin system that can greatly simplify the spectra. This is demonstrated in the spectra of [ 13 C, 15 N 2 ]-urea in highly acidic and basic solutions (red boxes in Fig. ), where a narrow peak appears at ≈30 Hz. This signal arises at (3/2)J CN and originates from the J-coupling between 13 C and 15 N nuclei in a CN 2 spin system, where, due to the rapid exchange, protons are effectively decoupled from the rest of the nuclei. The emergence of the low-frequency peak is also supported by the numerical simulations for the spin system under rapid chemical exchange (shown in the bottom of Fig. ). It should be stressed that, by taking advantage of the accelerated chemical-exchange effect on the zero-field NMR, a The peaks arising from onebond strong J-coupling interaction between 15 N and 1 H are green shaded (120-150 Hz), while the narrow peaks (around Hz) originating from one-bond J-coupling between 13 C and 15 N, are highlighted in red.