Phosphorus mononitride

Phosphorus mononitride is an inorganic compound with the chemical formula PN. Containing only phosphorus and nitrogen, this material is classified as a binary nitride. From the Lewis structure perspective, it can be represented with a P-N triple bond with a lone pair on each atom. It is isoelectronic with N2, CO, P2, CS, NO+, CN− and SiO. The compound is highly unstable in standard conditions, tending to rapidly self polymerize. It can be isolated within argon and krypton matrices at 10 K (−263.1 °C). Due to its instability, documentation of reactions with other molecules is limited. Most of its reactivity has thus far been probed and studied at transition metal centers. Phosphorus mononitride was the first identified phosphorus compound in the interstellar medium and is even thought to be an important molecule in the atmospheres of Jupiter and Saturn.

Discovery and interstellar occurrence The existence of free, gas-phase phosphorus mononitride was confirmed spectroscopically in 1934 by Nobel laureate, Gerhard Herzberg, and coworkers. J. Curry, L. Herzberg, and G. Herzberg made the accidental discovery after observing new bands in the UV region from 2375 to 2992 Å following an electric discharge within an air-filled tube that had been earlier exposed to phosphorus. In 1987, phosphorus mononitride was detected in the Orion KL Nebula, the W51M nebula in Aquila, and Saggitarius B2 simultaneously by Turner, Bally, and Ziurys. Data from radio telescopes allowed for observation of rotational lines associated with the J = 2-1, 3-2, 5-4, and 6-5 transitions. In the following decades, a rapid expansion of interstellar PN observations ensued, detected frequently alongside PO. Examples include within shocked regions of L1157, within the Galactic Center, in carbon-rich envelopes in CRL 2688 (alongside HCP) and oxygen-rich envelopes toward VY Canis Majoris, TX Camelopardalis, R. Cassiopeiae, and NML Cygni. ALMA data alongside spectroscopic measurements from the Rosetta probe have shown PN being carried from the comet 67P/Churyumov–Gerasimenko alongside the far more abundant PO. These observations may offer insight to how pre-biotic matter could be transported to planets. In cases where PN and PO are observed in the same region, the latter is more abundant. The consistency of the molecular ratio between these two interstellar molecules across many different interstellar clouds is thought to be a sign of a shared formation pathway between the two molecules. PN is mostly detected in hot, turbulent regions, where the shock induced sputtering of dust grain is thought to contribute to its formation. However, it has also been confirmed in massive dense cores which are by comparison "cold and quiescent". In 2022, researchers used data from the ALMA Comprehensive High-resolution Extragalactic Molecular Inventory (ALCHEMI) project and reported evidence of phosphorus mononitride in giant molecular clouds within the galaxy, NGC 253. This finding marks phosphorus mononitride as the first extragalactic phosphorus containing molecule detected as well. In 2023, Ziurys and coworkers showed the existence of PN and PO in WB89-621 (22.6 kpc from the Galactic Center) using rotational spectroscopy. Prior, phosphorus was only observed in the inner Milky Way (12kpc). Since supernovae do not occur in outer regions of the galaxy, the detection of these phosphorus-bearing molecules in WB89-621 provides evidence of additional alternative sources of phosphorus formation, such as non-explosive, lower mass asymptotic giant branch stars. The levels were detected at comparable values to that in the Solar System.

Electronic structure, spectral and bonding properties

PN formation from gaseous phosphorus and nitrogen is endothermic. ½ P2 + ½ N2 = PN (ER = 117 ± 10kJ/mol) Early mass spectrometry studies by Gingerich yielded a PN dissociation energy D0 of 146.6 ± 5.0 kcal/mol (613 ± 21 kJ/mol; 6.36 ± 0.22 eV). It is predicted to have a high proton affinity (PA = 191 kcal/mol (800 kJ/mol)). Early rotational analysis of 24 of the bands from Herzberg's original study suggested a PN internuclear distance of 1.49 Å, intermediate between N2 (1.094 Å) and P2 (1.856 Å). The associated electronic transition, 1Π → 1Σ, was noted to be similar to that of the isoelectronic CS and SiO molecules. Later rotational spectra studies aligned well with these findings, for example analysis of millimeter wave rotational PN spectra from a microwave spectrometer yielded a bond distance of 1.49085 (2) Å. Infrared studies of gaseous PN at high temperatures assign its vibrational frequency (ωe) to 1337.24 cm−1 and interatomic separation of 1.4869 Å. Simple comparisons to tabulated experimental and calculated bond lengths match well with a PN triple bond according to Pyykkö's Triple-Bond Covalent Radii. NBO analyses support a single neutral resonance structure with a PN triple bond and one lone pair on each atom. However, natural population analysis shows nitrogen as significantly negatively charged (-0.82603) and phosphorus as significantly positively charged (0.82603). This is in line with the large dipole moment and partial ionic character reflecting the electron density contour plots. Monomeric PN in a krypton matrix at 10 K (−263.1 °C) gives rise to a single IR band at 1323 cm−1. Auer and Neese have produced calculated gas phase 31P and 15N NMR chemical shifts of 51.61 and -344.71 respectively at the CCSD(T)/p4 level of theory. However, different functionals and basis sets yield dramatically different predictions for chemical shielding and so far experimental NMR shifts for phosphorus mononitride remain elusive. Molecular beam electric resonance spectroscopy has been used to determine the radio frequency spectrum of phosphorus mononitride generated from P3N5 thermolysis; the experimental results showed an experimental PN dipole moment (μ) of 2.7465 +- 0.0001 D, 2.7380 +-0.001 D, and 2.7293 +-0.0001 D for the first three vibrational levels respectively.

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