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-CH3-C6H4NH)P(O)(NC4H8O)(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) -> Sigma*(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 () at the bond critical point (b.c.p.) of N-H decreases by similar to 0.012-0.014 e A-3 when the molecule participates in hydrogen bonding. This may explain the red shift of the nu(N-H) stretching frequency from a single molecule in the gas phase to a hydrogen-bonded one in the solid state.