Investigating the impact of substituent variations on molecular packing has been a topic of interest in the field of crystal engineering [1,2]. Based on the Kitaigorodskii close packing model [3], the exchanging of non-polar substituents with almost similar sizes may lead to the production of isostructures. For polar substituents, the molecular arrangement deeply depends on the electrostatic potential of the substituents [4]. In this study, a Cambridge Structural Database survey is considered to find analogous phosphoramide and thiophosphoramide structures with the three equal amine groups (P(X)(R)3, X = O, S). Two pairs of similar phosphoramide/thiophosphoramide structures are found (R = N(NH2)(CH3) (I/II) and NHCH2C6H5 (III/IV)), and the structural analysis is carried out using the energy framework calculations to explore similarities and differences arisen from O/S exchange. The results show that the structures with smaller and more polar substituents (I/II) have completely different molecular packing maps (space groups = Pbca/P63), while in the structures with bulkier and less polar substituents (III/IV), despite the overall differences in the crystal packing maps, there are some similarities. Particularly, the strongest assemblies are similar and are constructed through NH…X=P hydrogen bonds with total interaction energies of –116.7/–100.3 kJ/mol and nearly equal electrostatic energies (–70.2/–68.5 kJ/mol). The differences in III/IV are related to the greater tendency of the P=O group to attract NH units, which makes one other assembly with a total interaction energy of –45.4 kJ/mol.

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