Different orientations of P(O) versus C(O) in P(O)NHC(O) skeleton have been discussed in two new phosphorus(V)-nitrogen compounds with formula XP(O)Y and XP(O)Z2 where X = NHC(O)C6H4(4-F) and Y = NHCH2C(CH3)2CH2NH (1), Z = NHC6H4(4-CH3) (2). Compound 1 is the first example of an aliphatic diazaphosphorinane with a gauche orientation which has been studied by X-ray crystallography; the P=O bond is in the equatorial position of the ring. Both compounds show nJ(F,C) and mJ(F,H) coupling constants (n = 1, 2, 3 and 4; m = 3 and 4) and 3J(P,C)[2J(P,C). Quantum chemical calculations were performed with HF and Density Functional Theory (DFT) methods using 6-31?G(d,p) basis set. A tentative assignment of the observed vibrational bands for these molecules is discussed. Compound 1 shows a deshielded C atom of the carbonyl moiety (in 13C NMR spectrum) relative to that of 2, which is supported by IR spectroscopy in which the considerably lower C=O frequency is observed for 1. Comparing the X-ray crystallography and IR spectra of 1 and 2 shows that the acyclic compound 2, containing P=O and C=O bonds in an anti position, are involving in a stronger N–H...O=P hydrogen bond in crystal network. This leads to a weaker P=O and NC(O)NHP(O)–H bonds and stronger N...O interaction. The Namide–H is involved in an intramolecular N–H...O hydrogen bond.

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