Next, we investigated 1 H/D isotopologues of urea. For these studies, we modify the spin coupling network by replacing 1 H (spin-1/2) with D (spin-1), by solving urea in D 2 O/H 2 O mixture. To simulate J-spectra of urea solutions with various D 2 O/H 2 O ratios, the proportion of each isotopologue in a solution is calculated using a bino- mial distribution. Since a probability of an amide-proton site being occupied by deuterium depends on the fraction p of deuterium in the solution, the molar fraction x of each isotopologue is given by