A density functional theory investigation was carried out to characterize (14)N electric field gradient tensors, EFG, in crystalline sulfamerazine and sulfathiazole. To include hydrogen-bonding effects in the calculations, the most probable interacting molecules with the target were considered as tetrameric and pentameric clusters, respectively. The calculated EFG tensors were used to evaluate nuclear quadrupole coupling tensors (chi(ii)) and asymmetry parameters (eta(Q)) for the target molecule in the clusters. Results are in satisfactory agreement with the experimental data. The EFG calculations reveal different contributions of nitrogen atoms in hydrogen-bonding network of the sulfamerazine and sulfathiazole. Moreover, based on the results obtained via atoms in molecules (AIM) analyses, an acceptable linear relation between (14)N nuclear quadrupole coupling constants and charge density values at N-H...N and N-H...O bond critical points, rho(b)(r(cp)), is observed.