where n is the number of possible sites where D nuclei can reside and k is the number of D nuclei that each isotopologue contains. Simulated spectra of all isotopologues are next summed after weighing each spectrum with an appropriate binomial coefficient. For the simulation, the D-15 N coupling constants are estimated using the ratio of appropriate gyromagnetic ratios of the nuclei and the J NH coupling constant, J ND ≈ (γ D /γ H )J NH , which neglects secondary isotope effects . As a result, we obtained a good agreement of the experimental spectra of urea solutions with the various ratios of D 2 O/H 2 O with their simulated counterparts (Fig. ).