Electronic and steric effects of substituents on the conformational diversity and hydrogen bonding of N-(4-X-phenyl)-N′,N″-bis(piperidinyl) phosphoric triamides (X = F, Cl, Br, H, CH3): A combined experimental and DFT study

Phosphoric triamides of the general formula (4-X-C₆H₄NH)P(O)(NC₅H₁₀)₂, X = F (1), Cl (2), Br (3), H (4) and CH₃ (5), have been synthesized and characterized. X-ray crystallography at 120 K reveals that the compounds 1, 3, 4·H₂O and 5 are composed of one, four, two and four conformers, respectively. DFT calculations were performed to investigate the electronic structures of the compounds. The X-ray data and DFT calculations revealed that the conformational diversity in these compounds is mainly governed by the steric effects of the substituent X rather than by electronic effects. Although substituent X does not participate directly in hydrogen bonding, the crystal packing of the compounds is influenced by the size of X. Atoms in molecules (AIM) and natural bond orbital (NBO) analyses confirm that the para substituent X has no significant effect on the electronic features of the amidic proton and the phosphoryl oxygen atom (O_P). Using X-ray crystallography, AIM and NBO analyses, the structural and electronic aspects of inter- and intramolecular hydrogen bonds of the compounds have been studied. The charge density (ρ) at the bond critical point (bcp) of the N-H bond decreases from the fully optimized monomers to their corresponding hydrogen bonded clusters. The N-H stretching frequency decreases from the calculated values to the experimental results.