Probing Phosphorus Nitride (P≡N) and Other Elusive Species Formed upon Pyrolysis of Dimethyl Phosphoramidate

The thermal behavior of organophosphorus compounds is intricate and poorly understood but crucial for understanding gas-phase flame inhibition, syntheses of thermally active phosphorus-based reactive precursors, catalytic combustion, incineration of toxic nerve gases, and astrochemistry. In this work, the pyrolysis of dimethyl phosphoramidate was investigated using photoion photoelectron coincidence spectroscopy in combination with vacuum ultraviolet synchrotron radiation. This technique enables isomer-selective detection of reactive intermediates, which are crucial in the understanding of the decomposition process. Combined with quantum chemical calculations, the experimental results permit the formulation of a comprehensive pyrolysis reaction pathway for dimethyl phosphoramidate, consisting of several reactive phosphorus species on four possible decomposition pathways. Compared to the decomposition of dimethyl methyl phosphonate, which leads exclusively to the formation of PO radicals, substitution of the methyl with an amino group most notably yields phosphorus nitride (P≡N). This mostly favored reaction pathway involves the subsequent loss of methanol and formaldehyde to yield three PONH₂ tautomers, which eliminate water to generate P≡N. The thermally induced production of PN species and its possible role in flame inhibition has not previously been reported. In addition, the adiabatic ionization energy of O=P(OCH₃ )₂ NH₂ was determined to be 9.79±0.02 eV.