Combination of X-ray crystallography and theoretical study to evaluate the effect of NH⋯OP versus NH⋯OC hydrogen bonds on the NH stretching frequencies

Two new CCl3C(O)NHP(O)X compounds [X = NH(CH2)3NH (1) and NHCH2C(CH3)2CH2NH (2)] were synthesized and characterized by spectroscopic methods and X-ray crystallography. In each of 1 and 2, the phosphoryl and carbonyl groups are gauche relative to one another and the P atom is in a tetrahedral environment. The 1,3-diazaphosphorinane moiety adopts conformation between chair and envelope with the PO in an equatorial position. In the structure 1, the asymmetric unit contains one molecule; whereas, the asymmetric unit of 2 is composed of two independent molecules. In the crystal packing of 1 and 2, the NH⋯O hydrogen bonds lead to two-dimensional arrangements along (−1 0 1) plane for 1 and (0 1 0) plane for 2. In order to precise assignments of different NH units stretching frequencies in compounds 1 and 2, quantum chemical calculations were performed, with B3LYP method using 6-311++G(d,p) basis set. Theoretical procedure was performed for the individual molecule (monomer) and also for the hydrogen-bonded clusters of the molecules. In the H-bonded molecules, the effect of hydrogen bonds formation and their strengths in different NH⋯OP and NH⋯OC units were considered to close the theoretical frequencies of absorption bands to the real values. It is found that the stronger NH⋯OP hydrogen bond creates more changes in the NH stretching frequency than the weaker NH⋯OC hydrogen bond makes. The natural bond orbital analysis showed a positive character for H atom and a negative character for N and O atoms. The hydrogen bond formation was studied by the AIM method through the Laplacian analysis.