Solid-state and gas-phase structures of two conformers of N-(4-methylphenyl)-N',N''-bis(morpholinyl) phosphoric triamide; insights from X-ray crystallography and DFT calculations.

A phosphoric triamide with the formula (4-CH(3)-C(6)H(4)NH)P(O)(NC(4)H(8)O)(2) has been synthesized and characterized. X-ray crystallography at 120 K reveals that the title compound is composed of two symmetrically independent molecules in the solid state. Density functional theory (DFT) calculations reveal that two conformers A and B are very close to each other from an energy point of view. Thus there is equal chance that the presence of two conformers in the lattice may lead to hydrogen-bonded chains with an ABABAB arrangement. Hydrogen bonds of the type O(P)...H-N (O(P) being the phosphoryl O atom) are established between the two conformers with binding energies of -18.8 and -20.3 kJ mol(-1) (at B3LYP/6-31+G*). The electronic delocalization LP(O(P)) → σ*(N-H), LP(O(P)) being the lone pair of O(P), leads to a decrease in the strength of the N-H bond during hydrogen bonding between the conformers. The charge density (ρ) at the bond critical point (b.c.p.) of N-H decreases by ∼0.012-0.014 e Å(-3) when the molecule participates in hydrogen bonding. This may explain the red shift of the ν(N-H) stretching frequency from a single molecule in the gas phase to a hydrogen-bonded one in the solid state.