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

Abstract

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 OP...H—N (OP 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(OP) → σ*(N—H), LP(OP) being the lone pair of OP, 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.