The simplest iminophosphane HPNH and its photoisomerization to aminophosphinidene H_2NP

Domain: neuroscience
Published: 2026-01-22
Editorial outcome: minor_revision

Abstract
As a heavy analogue of dinitrogen, the diatomic phosphorus mononitride (PN) was the first detected phosphorus-containing compound in the interstellar medium. Hence, small molecules derived from PN including the simplest iminophosphane HPNH and isomeric aminophosphinidene H2NP with prototype phosphorus-nitrogen multiple bonds were proposed as candidate interstellar species possibly involving in the astro-chemical network of PN. However, the existence and reactivity of HPNH and H2NP remain elusive. Here, we report the synthesis of the cis and trans conformers HPNH through high-vacuum flash pyrolysis of di-tert-butylphosphanamine in the gas phase. In addition to the photo-induced conformational interconversion in HPNH, its photoisomerization to H2NP in the singlet ground state has also been observed under the matrix-isolation conditions at 10 K. As a singlet phosphinidene, H2NP is very reactive as demonstrated by its photoreactions with CO and O2. The synthesis and characterization of the long sought-after [H2, N, P] isomers contribute to the fundamental knowledge about PN in main-group and interstellar chemistry.

Introduction
The chemistry of multiply bonded compounds involving group 15 elements keeps attracting enormous interest, not only due to the novel structural and bonding properties but also their broad applications in synthetic chemistry and material science1–3. Particularly, an extremely important role in the main group chemistry has been played by low-valent phosphorus. As a heavy analogue to molecular nitrogen (N2), the diatomic phosphorus mononitride (PN) was first spectroscopically detected in the gas phase in 19334. Chemically, PN is very reactive, and it aggregates spontaneously above 10 K to form polymeric (PN)n 5,6 and the Hückel aromatic cyclotriphosphazene P3N3 7,8. As the first phosphorus-containing species detected in the interstellar medium9,10, PN plays a key role in the chemical network of phosphorus astrochemistry11. Quite recently, PN and another small phosphorus compound •PO were observed in the dense molecular cloud WB89-621 at the edge of the Galaxy12,13. Considering the essential role of the phosphorus element in the origin and evolution of life, the chemistry of PN11,14–16 and other astronomically detected P-bearing molecules such as •PO17, PH3 18, and HCP19 have been extensively explored. To explore the synthetic applications, stable compounds containing terminal PN unit have been synthesized and used as precursors for the release of PN under catalytic conditions20–24.

As the simple derivatives of PN, the three isomers HPNH, H2NP, and H2PN serve as prototype molecules consisting of phosphorus-nitrogen multiple bonds. Particularly, HPNH is the simplest iminophosphane that has been proposed as a candidate interstellar species, which could be formed through hydrogenation of PN via intermediacy of the HPN• and HNP• radicals25,26. The structural and bonding properties of HPNH have been extensively explored27–31 by quantum chemical calculations since the first theoretical study by Trinquier in 198232. Theoretically, HPNH can exist in planar trans or cis conformations, and the former is slightly lower in energy by 1 kcal mol–1 (Fig. 1). Moreover, trans-HPNH (trans-1, Fig. 1) is the global minimum on the potential energy surface of the [2H, P, N] system, and its thermal stability is secured with the large barriers (>60 kcal mol–1) for its isomerization to the two less stable isomers H2NP (3) and H2PN (2) via 1,2-hydrogen migration. However, all the early attempts to generate HPNH were unsuccessful33, whereas, a number of the sterically protected and the carbene-stabilized iminophosphanes has been synthesized and broadly used as building blocks in organophosphorus chemistry34–40.

Among the [2H, P, N] isomers, the least stable isomer H2PN features as the parent phosphinonitrene, and it was calculated to have closed-shell singlet ground state (singlet-2) due to electron delocalization of the lone pair electrons, leading to a strongly polarized shorter PN double bond than that in HPNH. In contrast, the triplet state of H2PN (triplet-2) is less stable by 16.7 kcal mol–1. Experimentally, H2PN has so far resisted all attempts at its detection including the recently attempted generation from photodecomposition of H2PNCO25. Notably, room-temperature stable phosphinonitrenes stabilized by sterically demanding substituents have been synthesized41–43 and used for the activation of small molecules44.

In contrast to the large singlet-triplet energy gap for H2PN, the ground-state spin multiplicity of the isomeric H2NP remains unclear as the sign for the predicted singlet-triplet energy gap depends on the applied theoretical methods28,32,45. According to the latest calculations by Schaefer III et al.46, the singlet state is slightly lower by 0.2 kcal mol–1 due to electron delocalization of the lone pair electrons in the closed-shell singlet state (singlet-3), and it contains an ideal phosphorus-nitrogen double bond with a natural bond order of 2, whereas, triplet-3 features a smaller bond order of 1.5. Similar to the elusiveness of HPNH and H2PN, recent attempts to generate H2NP from photodecomposition of H2NPCO in cryogenic matrix47 and thermal decomposition of a dibenzo-7-phosphanorbornadiene precursor in the gas phase48 were unsuccessful. This is in sharp contrast to the analogous systems H2PP/HPPH49 and H2NN/HNNH50, which have already been well characterized. Herein, we report the preparation and characterization of cis-HPNH, trans-HPNH, and H2NP in cryogenic matrices at 10 K. In addition to the photo-induced interconversion among these isomers, the photoreactions of H2NP with CO and O2 have also been studied.

Results
The preparation of HPNH was performed by high-vacuum flash pyrolysis (HVFP) of di-tert-butylphosphanamine ((t-Bu)2PNH2) in the gas phase at ca. 950 K (Fig. 2). A typical IR spectrum for the thermolysis products isolated in a solid N2-matrix at 10 K (Fig. 3A) shows IR bands of the fragments C4H8 and C4H10 51, along with some weak bands of the counterpart fragment HPNH. When using the deuterated precursor (t-Bu)2PND2, the mono-deuterated dissociation products HPND and [D]-C4H10 were identified (Fig. S2), implying that the dissociation of (t-Bu)2PNH2 likely proceeds in a stepwise pathway (Fig. 2). When using the less condensable Ar or Ne as the carrier gas in the HVFP experiments, the yields of the matrix-isolated HNPH become lower (Figs. S4 and S5).

Considering the TD-DSD-PBEP86/aug-cc-pVTZ calculated absorption at 432 nm for the π*←n transition in trans-HPNH (Table S1), the matrix containing the pyrolysis products of (t-Bu)2PNH2 was subjected to the irradiation at 410 nm. The resulting IR difference spectrum (Fig. 3B) reflecting the changes of the matrix shows selective depletion of trans-HPNH with a group of IR bands at 3309.1, 2225.0, 1107.0, 1012.8, 953.5, and 911.0 cm–1 along with the concomitant appearance of another group of IR bands at 3334.2, 2135.2, 1074.8, 1023.5, 897.4, and 816.9 cm–1 for cis-HPNH. Notably, the IR bands for cis-HPNH are also detectable in the spectrum of the decomposition products of the precursor, indicating that both conformers of HPNH were produced in the gas phase. To aid the identification, quantum chemical calculations on the IR spectra for the two conformers of HPNH (Table 1) were carried out at the CCSD(T)-F12B/VTZ-F12 level of theory using configuration-selective vibrational configuration interaction theory (VCI)52. Additionally, isotope labeling experiment was performed by using (t-Bu)2P15NH2 as the precursor to HP15NH (Figs. S1 and S5).

In the IR spectrum of trans-HPNH, the band at 3309.1 cm–1 shows D- and 15N-isotopic shifts of −844.3 and −7.1 cm–1, respectively, supporting its assignment to the NH stretching mode with a calculated frequency of 3299.5 cm–1. The frequency is lower than that in cis-HPNH (obs. 3334.2 cm–1; cal. 3345.5 cm–1), for which the D- and 15N-isotopic shifts are −863.7 and −7.4 cm–1, respectively. The NH stretching frequencies in the two conformers of HPNH are close to the same mode in HNSO (3308.5 cm–1)53,54. The frequency for the PH stretching mode in trans-HPNH (obs. 2225.0 cm–1; cal. 2212.9 cm–1) is significantly higher than that in cis-HPNH (obs. 2135.0 cm–1; cal. 2120.3 cm–1), whereas, both are lower than the same mode in HPCO (2311.8 cm–1)55,56 and HPNN (2322.1 cm–1)57,58. The band at 1012.8 cm–1 with a large 15N-isotopic shift of −18.9 cm–1 for trans-HPNH mainly involves the PN stretching vibration, and it couples with the NH bending at 1107.0 cm–1 as evidenced by the 15N-isotopic shift of −8.5 cm–1. This vibrational coupling becomes more evident in the cis conformer due to the nearly equivalent 15N-isotopic shifts of −11.8 and −12.1 cm–1 for the two bands at 1074.8 and 1023.5 cm–1, respectively. The PN stretching frequencies in the two conformers of HPNH are lower than those in FPNF (1116.8 cm–1, Δν(14/15N)=−25.3 cm–1)59 and HNP• (1136.3 cm–1, Δν(14/15N)=−22.7 cm–1)25. The last two bands for HPNH belong to the out-of-plane and in-plane deformation modes of the PH and NH moieties, respectively. Additionally, two very weak bands at 2207.6 and 2086.5 cm–1 show similar changes with the other bands of trans-HPNH and cis-HPNH, respectively, and they belong to the ν1 overtone of trans-HPNH and the ν3 + ν4 combination of cis-HPNH according to the corresponding D- and 15N-isotopic shifts, as well as the theoretically predicted anharmonic frequencies of 2198.0 and 2083.0 cm–1, respectively. In line with the calculated absorption at 376 nm for cis-HPNH (Table S1), the reverse cis to trans conformational transformation in HPNH was observed upon subsequent photoexcitation at 365 nm (Fig. 3C).

Consistent with the trans-HPNH → cis-HPNH photoisomerization observed by matrix-isolation IR spectroscopy, a weak absorption at around 420 nm in the UV–vis spectrum of the matrix-isolated HVFP products of (t-Bu)2PNH2 was depleted by the irradiation at 410 nm, leading to the concomitant increase of another very weak absorption at around 340 nm (Fig. 4A). The assignment of the visible absorption (ca. 420 nm) to trans-HPNH is supported by the good agreement with the calculated transition at 432 nm (Table S1) for the excitation of the phosphorus lone-pair electron to the anti-bonding π* orbital. The UV-absorption (ca. 340 nm) corresponds to the similar transition of the cis conformer with a calculated band at 376 nm.

In addition to the photo-induced conformational rotation in HPNH, further photoexcitation of HPNH at 395 nm promotes 1,2-H migration to yield H2NP with IR bands at 3386.3, 3313.4, 947.6, 943.2, and 739.8 cm–1 (Fig. 4B). Accordingly, the absorptions for HPNH (cis and trans) decrease in the UV–vis spectrum (Fig. 4A). Consequently, new absorptions at 290 and 240 nm for the isomer H2NP appear (Fig. 4A), and they are in good agreement with the calculated vertical transitions at 302 and 232 nm for H2NP in the singlet state (Table S1). Consistent with the observed absorption at 290 nm for H2NP, subsequent irradiation of the matrix at 310 nm causes reformation HPNH from H2NP (Fig. 4C).

The photolytic interconversion between HPNH (cis and trans) and H2NP under the matrix-isolation conditions enables unambiguous identification of the weak IR bands for H2NP. The two bands at 3386.3 and 3313.4 cm–1 with D-isotopic shifts of −33.1 and −829.9 cm–1 in H(D)NP are reasonably assigned to the NH stretching modes, and they show better agreement with the VCI calculated values for H2NP in the singlet state (3379.9 and 3322.2 cm–1) than those in the triplet state (3493.1 and 3423.9 cm–1, Table 2). In accordance with the VCI calculated frequencies of 951.6 and 949.4 cm–1 for the NP stretching and NH deformation modes in singlet H2NP, two bands at 947.6 and 943.2 cm–1 are observable in the IR difference spectrum (Fig. 4C). However, the VCI calculated frequencies for the same modes in triplet H2NP (845.6 and 813.9 cm–1) differ from the singlet state by more than 100 cm–1. Additionally, the NH wagging mode locates at 739.8 cm–1 (cal. 723.0 cm–1) as a broad band, and it shows a large D-isotopic shift of −74.5 cm–1 (cal. −74.5 cm–1) in H(D)NP.

In an attempt to generate the least stable isomer H2PN on the potential energy surface of the [2H, N, P] system, photoexcitation of the matrix-isolated HPNH was carried out by using short-wavelength irradiations at 254 and 193 nm. However, no isomerization but dehydrogenation to yield PN was observed. Additionally, chemical trapping reaction was performed by photolysis of HPNH at 395 nm in solid CO ice at 10 K. The corresponding IR difference spectrum (Fig. 5A) shows exclusive formation of H2NPCO47, implying first formation of H2NP from HPNH with subsequent reaction by CO. Similar photolytic CO-trapping reactions have been observed for singlet phosphinophosphinidenes60 and triplet phenylphosphinidene61,62. Absence of the CO-trapping product of the putative intermediate H2PN (+CO → H2PNCO) suggests that it is not involved in the photochemistry of HPNH, since similar CO-trapping reaction was observed before for the fluorine derivative F2PN (+CO → F2PNCO)59.

To further explore the reactivity of H2NP, its oxidation by molecular oxygen was studied in an O2-doped N2-matrix (5%) at 10 K. Upon 395 nm irradiation, a new species forms with a group of IR bands at 3529.6, 3423.4, 1540.0, 1434.7, 1191.7, and 909.8 cm–1 (Fig. 5B). The two bands at 1434.7 and 1191.7 cm–1 exhibit large 18O-isotopic shifts of –40.2 and –37.6 cm–1, supporting their assignment to the asymmetric and symmetric OPO stretching vibration modes in the oxidation product H2NPO (←H2NP + O2). The two stretching modes are close to the same modes in other dioxophosphoranes CH3PO2 (1404.3 and 1159.6 cm–1, N2-matrix)63 and PhPO2 (1416 and 1170 cm–1, Ar-matrix)61. The three bands at 3529.6, 3423.4, and 1540.0 cm–1 correspond to the stretching and deformation of the NH2 moiety, and they resemble the same modes in the nitrogen analogue NH2NO2 (3478.0, 3359.3, and 1558.1 cm–1, Ar-matrix)64,65. The identification of H2NPO is further supported by the good agreement of the observed IR spectrum with the theoretical calculations (3550.4, 3439.7, 1531.3, 1417.7, and 1160.7 cm–1, Table S2). Considering the previously observed spontaneous O2-oxidation of triplet phosphinidenes61 and nitrenes66–68 at 30 K, the O2-doped matrix containing H2NP was annealed to 30 K to allow O2 diffusion for possible thermal reaction with H2NP. However, no reaction occurs to H2NP.

The frontier molecular orbitals of trans-HPNH, cis-HPNH, H2NP, and H2PN are depicted in Fig. 6. In line with the double-bond characters of the PN moieties in these species, the lowest unoccupied molecular orbitals (LUMOs) correspond to the antibonding character of the PN bond. The highest occupied molecular orbitals (HOMOs) in trans-HPNH, cis-HPNH, and H2NP show pronounced lone-pair character on the phosphorus atom, whereas, the HOMO in H2PN is mainly composed by the lone-pair character on the nitrogen atom. The HOMO-1 and HOMO-2 for all the four species correspond to the π- and σ-bonding interactions of the PN moiety, respectively. In agreement with the high reactivity of these phosphorus-containing compounds, the natural bond orbital (NBO) calculations indicate significant polarization in the PN double bonds. The nitrogen atoms bear large negative partial charges of −1.02 e, −1.04 e, −1.09 e, and −0.88 e, while the phosphorus atoms carry positive charges of +0.77 e, +0.82 e, +0.33 e, and +0.98 e for trans-HPNH, cis-HPNH, H2NP, and H2PN, respectively.

Methods
Sample preparation
Di-tert-butylaminophosphane ((t-Bu)2PNH2) was synthesized by a slightly revised method according to the published protocol71. Briefly, a mixture of (t-Bu)2PCl (2 mmol, 0.2 mL) and dried Et2O (1.5 mL) was added to a reaction vessel and degassed twice to remove oxygen. Subsequently, a large excess of ammonia (10 mmol) was condensed into the vessel, and the mixture was kept at −55 °C and stirred for 1 h. The volatile products were separated by passing through three successive cold U-traps (‒8, −45, and −196 °C). Pure (t-Bu)2PNH2 was obtained in the first trap. The purity of (t-Bu)2PNH2 was checked with 31P NMR spectroscopy (Fig. S9). For the 15N- and D-labeled (t-Bu)2PNH2, 15NH3 and ND3 gases were used. 15NH3 was synthesized by heating the mixture of Ca(OH)2 and 15NH4Cl (99% atom 15N, Cambridge) with an alcohol burner, and ND3 is commercially available.

Matrix-isolation IR and UV–vis spectroscopy
The setups for matrix-isolation experiments were described before49. Briefly, matrix-isolation IR spectra were recorded at resolution of 0.5 cm−1 on an FT-IR spectrometer (Bruker 70V). To prepare the matrix, gaseous samples were mixed by passing a flow of matrix gas (N2, Ar, and Ne) through a cold U-trap (–20 °C) containing ca. 20 mg of freshly synthesized (t-Bu)2PNH2. Then the mixture of (t-Bu)2PNH2 vapor/matrix gas (~1:1000 estimated) was passed through an Al2O3 tube furnace (o.d. 2.0 mm, i.d. 1.0 mm), which can be heated over a length of ca. 30 mm by a tantalum wire (resistance 2.9 Ω, voltage 10.0 V, and current 3.50 A). The resulting pyrolysis products were deposited (2 mmol h−1) onto the gold-plated copper block matrix support (10 K) in a high vacuum (~10−6 Pa) using a closed-cycle helium cryostat (Sumitomo Heavy Industries, SRDK-408D2-F50H) inside the chamber. Matrix-isolation UV–vis spectra were recorded on a PerkinElmer Lambda 850+ spectrometer (190–600 nm, scanning speed of 1 nm s−1). The high-vacuum flash pyrolysis products were deposited onto a CaF2 matrix support (10 K) for the measurements. Photolysis experiments were performed by using UV flashlight LEDs including 410 nm (10 W), 395 nm (15 W), 365 nm (15 W), and 310 nm (15 W).

Quantum chemistry calculation
Structures and IR frequencies for stationary points and transition states of the [2H, P, N] system were initially calculated at the DSD-PBEP86/aug-cc-pVTZ level of theory72. For the calculations on the decomposition of (t-Bu)2PNH2, the B3LYP/6-311++G(3df,3pd)73,74 method was used. Local minima were confirmed by vibrational frequency analysis, and the transition states were ascertained with additional intrinsic reaction coordinate (IRC) calculations75. These calculations were performed using the Gaussian 16 software package76. Time-dependent (TD)77 calculations at the DSD-PBEP86/aug-cc-pVTZ level was performed for the prediction of UV–vis transitions energies. Single-point energies were corrected at the CCSD(T)/CBS level78,79. These calculations were performed using the ORCA 5.0 software package80. Further vibrational frequency calculations were carried out at the CCSD(T)-F12B/VTZ-F1281,82 level using configuration-selective vibrational configuration interaction (VCI)52 theory with Molpro 2019 package83. This method has been shown to achieve high accuracy in predicting IR spectra for phosphorus-containing species such as the P2H2 isomers49. Rotational constants and dipole moments were also calculated at the CCSD(T)-F12B/VTZ-F12 level (Table S4). The NRT calculations were carried out with the NBO 6.084. The molecular orbital graphs are rendered by the VMD 1.9.3 program85. Ring-polymer instanton optimizations were carried out at the B3LYP/6-311++G(3df,3pd) level of theory, using a Python program developed based on the instanton package of Prof. J. O. Richardson86.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Reviewer’s report to NCOMMS-25-65087\nThis manuscript reports the first synthesis and spectroscopic characterization of the simplest iminophosphane (HPNH, cis\nand trans) and its photoisomerization to aminophosphinidene (H₂NP). The study combines high-vacuum flash\npyrolysis, cryogenic matrix isolation, isotopic labeling, and high-level quantum chemical calculations. The results are significant\nbecause these are fundamental, long-sought [2H, P, N] species, relevant both to main-group chemistry and astrochemistry.\nHowever, I find it difficult to envision a sufficiently broad scientific role or impact of these species that would meet the\nstandards of Nature Communications. The novelty lies primarily in the generation and detection of elusive prototype\nmolecules, but the manuscript does not convincingly demonstrate why these particular species are of major significance\nbeyond the specialized field of phosphorus–nitrogen chemistry.\nThe work is carefully executed and clearly written, but I am not convinced it reaches the level of broad conceptual impact\nexpected for Nature Communications. The results may be better suited for a specialized high-quality chemistry journal\nfocused on molecular spectroscopy, main-group chemistry, or astrochemistry.\nSome aspects require clarification and further discussion to strengthen the paper.\n(1) The singlet–triplet gap is reported to be ~0.2 kcal/mol, a very small value. How robust is the experimental assignment of\nthe singlet state? Could matrix effects influence this ordering ?\n(2) Most experiments are performed in solid N₂, with some in CO or Ar matrices. Could matrix stabilization bias the\nisomerization pathways ?\n(3) Upon 395 nm irradiation, H₂NP is observed, but not H₂PN. The authors conclude H₂PN is not an intermediate. Can this\nbe definitively excluded, or might H₂PN form transiently but decay too fast to detect?\n(4) The authors assign the singlet ground state of H₂NP based on the observation that reaction with O₂ occurs only under\nphotoexcitation, but not upon thermal annealing at 30 K. They argue that this implies a spin-forbidden process for the closed-\nshell singlet, and therefore supports singlet character. However, this reasoning is not fully convincing: The reaction observed\nunder 395 nm irradiation may proceed via excited-state chemistry of H₂NP or O₂ (e.g., singlet O₂ generation or transient spin-\ncrossing), rather than reflecting the nature of the ground state. Thus, the photoreaction is not necessarily diagnostic of the\nelectronic ground state.\n(5) The introduction extensively discusses PN and related molecules, but the conclusion only briefly touches on\nastrochemistry. Could the authors expand on plausible interstellar formation pathways for HPNH/H₂NP, e.g., hydrogenation\nof PN on icy grains or photochemistry in dense clouds? How likely are these molecules to survive in astrophysical\nenvironments given their demonstrated high reactivity ?\n(6) The main contribution is spectroscopic identification supported by computation. There is little in terms of unexpected\nreactivity, mechanistic novelty, or conceptual paradigm shift. Beyond spectroscopic curiosity, what important role could\nHPNH or H₂NP play in chemical, materials, or astrochemical systems?", "reviewer_2": "This article reports an excellent work on very unique molecules. The article is very well written. First report of unique\nmolecule HPNH has been included in this work. Photoisomerization of the species is also impressive.\nThe experimental evidence provided for the formation is based on matrix isolation IR and UV-vis. Photoisomerization details\nare on the cryogenic matrices. The chemical reactivity was reported with reference to reactions with CO and O2. Whereas all\nthese evidences are quite worthy, they are related to transient existence of the species. What is the half-life of the cis and\ntrans isomers and what are their stability parameters. What are the experimental structural parameters. What is the half-life of\nthe trapping products H2NPCO and H2HPO2? How the products are characterized -- any crystal structure or NMR? This\nwork will be of interest to astrophysics / astrochemistry scientists. But not to broad spectrum chemistry community.", "reviewer_3": "The manuscript reports on the generation and spectroscopic identification of the simplest iminophosphane (HPNH) and\naminophosphinidene (H₂NP), as well as their photochemical interconversion and their reactions with O₂ and CO molecules.\nThe identification of these species, based on their IR spectra and supported by high-level calculations, is convincing. The\nwork is interesting and, with minor revisions, could be suitable for publication.\nThe authors do not explain why the experiments were carried out in an N₂ matrix. The more common choices are Ar, Ne, or\npara-H₂ matrices. N₂ matrices interact more strongly with isolated molecules, leading to larger matrix shifts, and due to site\neffects, many of the spectral features may split. I may have some ideas about why the authors chose an N₂ matrix, but it is\nimportant that they provide their own explanation in the manuscript.\nThe computational method employed is also not the most typical one for this kind of study. It would be helpful if the authors\ncould provide references to earlier works demonstrating the expected accuracy at the chosen level of theory.\nThe introduction is somewhat unfocused in its early part, but it rightly emphasizes the astrochemical relevance of the studied\nmolecules toward the end. Given that the identification of such species is more likely to come from radioastronomy rather\nthan IR astronomy, it would be valuable to provide highly accurate calculated rotational constants and dipole moments, with\na brief interpretation.\nIn my opinion, the calculated UV–vis spectrum shown in Figure 2 should not appear only as an inset, but rather below the\nexperimental spectrum with matching axis scales. Similarly, displaying the calculated and experimental IR spectra in the\nfigures may be worthwhile, even if the agreement is already clear from the tables."}, "author_response": "General comment: This manuscript reports the first synthesis and spectroscopic\ncharacterization of the simplest iminophosphane (HPNH, cis and trans) and its\nphotoisomerization to aminophosphinidene (H₂NP). The study combines high-vacuum flash\npyrolysis, cryogenic matrix isolation, isotopic labeling, and high-level quantum chemical\ncalculations. The results are significant because these are fundamental, long-sought [2H, P, N]\nspecies, relevant both to main-group chemistry and astrochemistry. However, I find it difficult\nto envision a sufficiently broad scientific role or impact of these species that would meet the\nstandards of Nature Communications. The novelty lies primarily in the generation and\ndetection of elusive prototype molecules, but the manuscript does not convincingly\ndemonstrate why these particular species are of major significance beyond the specialized\nfield of phosphorus-nitrogen chemistry. The work is carefully executed and clearly written,\nbut I am not convinced it reaches the level of broad conceptual impact expected for Nature\nCommunications. The results may be better suited for a specialized high-quality chemistry\njournal focused on molecular spectroscopy, main-group chemistry, or astrochemistry.\nAuthor reply: We greatly appreciate the Reviewer’s positive comments. As mentioned by the\nreviewer and also demonstrated by the references 5-33 in the manuscript, the prototype\nPN-containing molecules including the diatomic species PN, HPN/HNP, and HPNH/H NP are\n2\nof broad interest in the fields of molecular spectroscopy, main-group chemistry, and\nastrochemistry. For instance, PN is a key interstellar P-bearing specie that has been also\ndetected at the edge of the Galaxy [reference 12: Koelemay, L. A., Gold, K. R. & Ziurys, L. M.\nPhosphorus-bearing molecules PO and PN at the edge of the Galaxy. Nature 623, 292−295\n(2023)]. On the other hand, new methods for generation of PN from different precursors have\nbeen recently reported as main breakthrough in main-group chemistry [reference 22: Eckhardt,\nA. K. et al. Taming phosphorus mononitride. Nat. Chem. 14, 928−934 (2022); reference 23:\nEdin, S. et al. Unleashing phosphorus mononitride. Nat. Commun. 16, 5596 (2025)].\nFurthermore, the reactivity of PN has also been very recently disclosed [reference 7: Zhu,\nC. et al. The elusive cyclotriphosphazene molecule and its Dewar benzene-type valence isomer\n(P N ). Sci. Adv. 6, eaba6934 (2020); reference 24: Qian, W., Wende, R. C., Schreiner, P. R. &\n3 3\nMardyukov, A. Selective preparation of phosphorus mononitride (P≡N) from phosphinoazide\nand reversible oxidation to phosphinonitrene. Angew. Chem. Int. Ed. 62, e202300761 (2023)].\nBy analogy, it can be expected that the simple derivatives HPNH/H NP should be of interest\n2\nas well in different fields. However, both species remain elusive due to the lack of effective\nsynthetic routes, although their properties keep attracting numerous theoretical calculations\n(references 27-33) since the first theoretical study 40 years ago [Nguyen, M. N., McGinn, M.\nA. & Hegarty, A. F. A theoretical study of the phosphinonitrene (H P=N)-iminophosphane\n2\n(HP=NH) rearrangement. J. Am. Chem. Soc. 107, 8029−8033 (1985)]. The first-time synthesis\nand characterization of HPNH/H NP in the present study will enlighten further experimental\n2\nstudies on the fundamental properties and potential applications of HPNH/H NP in the future.\nrobust is the experimental assignment of the singlet state? Could matrix effects influence this\nordering?\nAuthor reply: According to the recent theoretical study by Schaefer III et al. (reference 46),\nthe calculated singlet–triplet energy gap for H NP is very small. However, the theoretically\n2\ncalculated IR spectra for the species in the two states vary dramatically with frequency\ndifferences exceeding 100 cm⁻¹ (Table 2) due to very different molecular structures. Our\nexperimental spectrum shows excellent agreement with the predicted spectrum for the singlet\nstate, with average deviations by less than 15 cm–1, providing solid evidence for the\nidentification of the singlet state. Furthermore, the assignment is supported by the isotope\nlabeling experiments. As mentioned by the Reviewer, the matrix environment may cause\nminor perturbations on the band positions. Indeed, our experiments in Ar-matrix repeat the\nphotochemistry of HPNH/H NP (Fig. S4), and the observed spectrum for H NP also support\n2 2\nthe identification of the singlet state with two main IR bands at 758.3 and 3380.7 cm–1,\nalthough the isomerization from HPNH to H NP becomes less efficient.\nTable 2 | Calculated and observed IR data for H NP.\n2\nν Δν(H/D)[c] Δν(14/15N)[d] mode[e]\ntriplet[a] singlet[a] Obs.[b] Cal. Obs. Cal. Obs.\n3493.1 (37) 3379.9 (33) 3386.3 (27) −30.9 −33.1 −9.8 −9.4 ν1, νas(NH2)\n3423.9 (25) 3322.2 (<1) 3313.4 (1) −838.7 −829.9 −4.8 −6.0 ν2, νs(NH2)\n1558.3 (20) 1608.2 (2) n.o.[f] −167.4 n.o. −5.7 n.o. ν3, δ(NH2)\n845.6 (76) 951.6 (33) 947.6 (6) −3.1 n.o. −18.2 n.o. ν4, ν(NP)\n813.9 (2) 949.4 (17) 943.2 (4) −170.1 −157.9 −4.0 −2.5 ν5, ρ(NH2)\n170.3 (110) 723.0 (177) 739.8 (100) −74.5 −74.5 −4.7 −3.8 ν6, ω(NH2)\n[a] Calculated IR frequencies (> 400 cm–1) and intensities (km mol–1, in parentheses) at the CCSD(T)-F12B/VTZ-F12 level\nof theory using configuration-selective vibrational configuration interaction theory (VCI) for H2NP. [b] Observed band\nposition (cm–1) for the strongest matrix site in N2-matrix at 10 K. [c] Calculated and observed D-isotopic shifts for singlet\nH(D)NP. [d] Calculated and observed 15N-isotopic shifts for singlet H2 15NP. [e] Assignment of the vibrational modes. [f] Not\nobserved due to overlap with the IR band of H2O.\nTo further estimate the matrix effects on the spin state of H NP, the prototype complexes\n2\nbetween H NP and matrix material (Ne, Ar, and N ) were calculated at the CCSD(T)/CBS\n2 2\n//DSDPBEP86/aug-cc-pVTZ level of theory (Table S5). The results show that the binding\nenergies are quite small, and the gap between the singlet and triplet increases from −0.6 to −1.3\nkcal mol−1 for H\n2\ncomplexes, respectively. Therefore, the intermolecular\ninteractions of H NP with the matrix material do not switch the spin state. For clarity, Table\nTable S5. Calculated binding energies (D ) and ΔE (kcal mol−1) at the CCSD(T)/CBS\ne st\n//DSDPBEP86/aug-cc-pVTZ level of theory.\nSpecies D (singlet) D (triplet) ΔE\ne e st\nH NP / / −0.6\n2 2\nComment 2: Most experiments are performed in solid N₂, with some in CO or Ar matrices.\nCould matrix stabilization bias the isomerization pathways?\nAuthor reply: In addition to the experiments in solid N and Ar, we further performed new\n2\nexperiments in Ne-matrix (newly added as Fig. S5). The photoisomerization pathways for\nHPNH/H NP remain fully consistent across all matrices (N , Ar, and Ne, see the below Fig.\n2 2\nR1). As aforementioned, while the matrix environment causes minor spectral perturbations,\nthe stabilization energy provided by the matrix is relatively small (Table S5). The calculated\nbinding energies (~1 kcal mol−1) between the matrix and our species are much smaller than\nthe energy barriers (> 15 kcal mol−1) required for the isomerization reactions (Fig. S8).\nTherefore, matrix stabilization is insufficient to alter the isomerization pathways of the\nstudied species.\nFig. S5. (A) IR spectrum (3400−800 cm–1) of the matrix-isolated pyrolysis (ca. 950 K)\nproducts of (t-Bu) PNH in an Ne-matrix at 3 K. (B) IR difference spectrum showing the\n2 2\nisomerization from trans-HPNH (a) to cis-HPNH (b) in the matrix upon photoexcitation at\n410 nm (10 min). The IR bands for C H (c), C H (d), and HOPO (h) are also marked.\n4 8 4 10\nFig. R1. IR difference spectrum (unscaled) showing the isomerization from trans-HPNH (a)\nto cis-HPNH (b) in the N (panel A), Ar (panel B) and Ne (panel C) matrix upon\n2\nphotoexcitation at 410 nm (10 min).\nFig. S8. CCSD(T)/CBS//DSD-PBEP86/aug-cc-pVTZ calculated relative Gibbs free energies\n(ΔG) for the isomerization of HPNH.\nComment 3: Upon 395 nm irradiation, H₂NP is observed, but not H₂PN. The authors\nconclude H₂PN is not an intermediate. Can this be definitively excluded, or might H₂PN form\ntransiently but decay too fast to detect?\nAuthor reply: Similar to H NP, no significant absorption near 395 nm was predicted for\n2\n(Fig. S8), which is sufficiently high to ensure the kinetic stability of H PN in the cold matrix.\n2\nto generate H PN from photochemistry of H NP by using different light sources were\n2 2\nunsuccessful. Moreover, further evidence against its formation comes from the CO-trapping\nexperiment. The exclusive formation of H NPCO without H PNCO as the CO-trapping\n2 2\nproducts strongly suggests that H PN is unlikely involved in the photochemistry of H NP.\n2 2\nPlease note that a previous study shows that similar species F PN can be efficiently trapped\n2\nby CO as F PNCO (Angew. Chem. Int. Ed. 2009, 48, 4828–4831). In the meantime, the IR\n2\nspectrum for the assumed CO-trapping product H PNCO has been reported (Angew. Chem.\nabsence of H PN is probably due to fast quantum tunneling by hydrogen atom, however, our\n2\nexperiments with the deuterated sample H(D)NP show the photoisomerization with HNPD\nwithout intermediacy of H(D)PN. We further performed calculations on the tunneling reaction\nrates for H PN→ HPNH with the ring–polymer instanton theory at the B3LYP/aug-cc-pVTZ\n2\nlevel. The tunneling rates were predicted to be extremely slow (60 K, 10−18 s−1, Table S6),\nindicating that the absence of H PN during the photochemistry of HPNH in the cryogenic\n2\nmatrix at 10 K is unlikely to be caused by quantum tunneling. Notably, the tunneling\ncontrolled isomerization from HPN to HNP has been very recently observed in the matrix at 3\nK (reference 25).\nOn the basis of these experimental and theoretical observations, it can be safely concluded\nthat H PN is not involved in this reaction. To make it clear, details about the CO-trapping\n2\nexperiments have been described in the manuscript: “Additionally, chemical trapping reaction\nwas performed by photolysis of HPNH at 395 nm in solid CO ice at 10 K. The corresponding\nIR difference spectrum (Fig. 3A) shows exclusive formation of H NPCO,47 implying first\n2\nformation of H NP from HPNH with subsequent reaction by CO. Similar photolytic\nphosphinidene.61,62 Absence of the CO-trapping product of the putative intermediate\nH PN (+ CO → H PNCO) suggests that it is not involved in the photochemistry of HPNH,\n2 2\nsince similar CO-trapping reaction was observed before for the fluorine derivative F PN (+\nthat reaction with O₂ occurs only under photoexcitation, but not upon thermal annealing at 30\nK. They argue that this implies a spin-forbidden process for the closed-shell singlet, and\ntherefore supports singlet character. However, this reasoning is not fully convincing: The\nreaction observed under 395 nm irradiation may proceed via excited-state chemistry of H₂NP\nor O₂ (e.g., singlet O₂ generation or transient spin-crossing), rather than reflecting the nature\nof the ground state. Thus, the photoreaction is not necessarily diagnostic of the electronic\nground state.\nAuthor reply: As detailed in our response to Comment 1, the singlet ground state of H NP\n2\nwas ascertained according to the excellent agreement between the experimental and\ntheoretical IR spectra (Table 2). The photoreaction with O was performed to further explore\n2\nthe reactivity of H NP, from which the novel compound H NPO was obtained. Given the\n2 2 2\npreviously reported spontaneous O -reactions with triplet phosphinidenes61 and nitrenes66-68\n2\nunder matrix-isolation conditions, thermal annealing was also performed for the H NP/O\n2 2\nsystem, however, no reaction was observed. As suggested by the Reviewer, the manuscript\nhas been changed by removal of the discussion linked the O reactivity to the nature of the\n2\nelectronic ground state.\nComment 5: The introduction extensively discusses PN and related molecules, but the\nconclusion only briefly touches on astrochemistry. Could the authors expand on plausible\ninterstellar formation pathways for HPNH/H₂NP, e.g., hydrogenation of PN on icy grains or\nphotochemistry in dense clouds? How likely are these molecules to survive in astrophysical\nenvironments given their demonstrated high reactivity?\nAuthor reply: We thank the reviewer for this insightful comment. The chemical evolution of\nphosphorus-containing species such as PN and PO is a hot topic in astrochemistry (Please see\nthe two references below).\nIn recent chemical models of phosphorus astrochemistry, H PN, H NP and HNPH have been\n2 2\nproposed as candidates that could be derived from successive hydrogenation of phosphorus\nnitride (PN), which is one of the most abundant phosphorus-containing species in the\ninterstellar medium (for examples, see references: 11-16). Moreover, hydrogenation reactions\nhave been known to play a key role in the chemical growth/evolution of interstellar species in\ndense molecular cloud ices (for an example, see: McClure, M. K. et al. An Ice Age JWST\ninventory of dense molecular cloud ices, Nat. Astron. 2023, 7, 431–443). Additionally,\ngas-phase collisional reactions may offer a complementary pathway, particularly in warmer or\nshocked regions, since HNP and HPN (key intermediates for the formation of HPNH and\nH NP from PN) were proposed in the astrochemical model of phosphorus through collisional\n2\nreactions P + NH and N + PH (for an example, see reference 14: Silva, M. X. et al. New\nroutes for PN destruction and formation in the interstellar medium via neutral-neutral\ngas-phase reactions and an extended database for reactions involving phosphorus. Astron.\nAstrophys. 2025, 696, A170). However, none of the hydrogenated species of PN (including\nHNP/HPN and HPNH/H NP) has been yet detected in the interstellar medium partly due to\n2\nthe lack of experimental data about these species. We think that the facile generation of\nHPNH by high-vacuum flash pyrolysis of di-tert-butylphosphanamine in this work will\nfacilitate further studies about its astrochemistry, so as to understand its role in the interstellar\nPN-chemistry.\nConsidering the reactivity of these small molecules, their survival is strongly\nenvironment-dependent. In the shielded interiors of dense molecular clouds, where\ntemperatures are low and irradiations are negligible, HPNH/H NP isomers are expected to be\n2\nrelatively stable. The absence of tunneling reaction and the low thermal energy under these\nconditions prevent rearrangement or destruction via further reactions with other interstellar\nspecies such as CO, which has been proposed as a major component of interstellar ices. In\ncontrast, the survival timescale of these isomers might be shorter in irradiated regions.\nRecently, photochemistry has been proposed as an important driving force for formation of\ncomplex organic molecules (COMs) in the interstellar medium (for an example, see: Caro, G.\nM. M. et al. Photochemistry of interstellar ice forming complex organic molecules. Nat. Rev.\nChem. 2025, 9, 2025). To estimate the chemical behavior and distribution of HPNH/H NP in\n2\nthe interstellar medium, further modeling studies are necessary, which is beyond the topic of\nthe present study.\nAs suggested by the Reviewer, the last part of the Conclusion has been changed to:\n“Similar to the broad applications of the closely related singlet phosphinophosphinidenes60\nand phosphinonitrenes,41 H NP features as a fundamentally new and highly reactive\n2\naminophosphinidene that might be used as a building block in synthetic phosphorus chemistry.\nThe photoreactions of H NP with small molecules CO and O to the formation of H NPCO\n2 2 2\nand H NPO underscore its role as a potentially useful intermediate in constructing novel\n2 2\nN−P−C and N−P−O motifs. Considering the possible involvement of the [2H, N, P] isomers\nin the astrochemical network of the important phosphorus-bearing species PN in the\ninterstellar medium (ISM), the facile generation of HPNH in the gas phase will stimulate\nfurther studies on its role in interstellar phosphorus chemistry. Particularly, the disclosed\nphotochemistry of the [2H, N, P] isomers including the reactions of H NP with CO and O\n2 2\ncould be of interest in understanding the phosphorus chemistry in cold dense molecular\nclouds, where photochemistry has been proposed as an important driving force for the\nformation of complex organic molecules.69,70”.\nComment 6: The main contribution is spectroscopic identification supported by computation.\nThere is little in terms of unexpected reactivity, mechanistic novelty, or conceptual paradigm\nshift. Beyond spectroscopic curiosity, what important role could HPNH or H₂NP play in\nchemical, materials, or astrochemical systems?\nAuthor reply: As described in the introduction, the fundamental importance of HPNH and\nH NP in main-group chemistry has been demonstrated by the novel bonding/electronic\n2\nproperties and reactivity associated with the prototype phosphorus-nitrogen multiple bonds, as\nevidenced by numerous theoretical studies (for examples, see references 27-33), including the\nfirst theoretical study by Trinquier in 1982 [reference 32: Phosphinonitrene, phosphazene, and\naminophosphinidene. Structures and stabilities. J. Am. Chem. Soc. 104, 6969−6977 (1982)]\nand also the very recently predicted singlet ground state nature for H NP using the high-level\npnictinidenes. Chem. Eur. J. 27, 14461−14471 (2021)]. Although phosphazenes RP=NR and\naminophosphinidenes R N≡P are versatile building blocks in synthetic phosphorus chemistry,\n2\nall the early attempts to prepare the parent [2H, N, P] isomers were unsuccessful due to the\nlack of synthetic methods. Therefore, the first time synthesis of the long sought-after HPNH\nand H NP contributes to the fundamental knowledge of main-group chemistry.\nmolecule PN thanks to the newly discovered methods for its preparation [for examples, see\nreference 22: Eckhardt, A. K. et al. Taming phosphorus mononitride. Nat. Chem. 14, 928−934\n(2022) and reference 23: Edin, S. et al. Unleashing phosphorus mononitride. Nat. Commun.\n16, 5596 (2025)]. By analogy, the successful preparation and characterization of HPNH and\nits isomer H NP will enable further studies on their structures and reactivity. Given the\n2\ninvolvement of [2H, N, P] isomers in interstellar phosphorus chemistry, the disclosed\nphotochemistry in this work is crucial for building the chemical network for the important\ninterstellar species PN.\nTo make it clear, the conclusion has been changed to “Similar to the broad applications of the\nclosely related singlet phosphinophosphinidenes60 and phosphinonitrenes,41 H NP features as\n2\na fundamentally new and highly reactive aminophosphinidene that might be used as a\nbuilding block in synthetic phosphorus chemistry. The photoreactions of H NP with small\n2\nmolecules CO and O to the formation of H NPCO and H NPO underscore its role as a\n2 2 2 2\npotentially useful intermediate in constructing novel N−P−C and N−P−O motifs. Considering\nthe possible involvement of the [2H, N, P] isomers in the astrochemical network of the\nimportant phosphorus-bearing species PN in the interstellar medium (ISM), the facile\ngeneration of HPNH in the gas phase will stimulate further studies on its role in interstellar\nphosphorus chemistry. Particularly, the disclosed photochemistry of the [2H, N, P] isomers\nincluding the reactions of H NP with CO and O could be of interest in understanding the\n2 2\nphosphorus chemistry in cold dense molecular clouds, where photochemistry has been\nproposed as an important driving force for the formation of complex organic molecules.69,70”.\n----------------------------------------End of responses #1---------------------------------------\nReviewer 2:\nComment: This article reports an excellent work on very unique molecules. The article is very\nwell written. First report of unique molecule HPNH has been included in this work.\nPhotoisomerization of the species is also impressive. The experimental evidence provided for\nthe formation is based on matrix isolation IR and UV-vis. Photoisomerization details are on\nthe cryogenic matrices. The chemical reactivity was reported with reference to reactions with\nCO and O . Whereas all these evidences are quite worthy, they are related to transient\n2\nexistence of the species. What is the half-life of the cis and trans isomers and what are their\nstability parameters. What are the experimental structural parameters. What is the half-life of\nthe trapping products H NPCO and H2NPO ? How the products are characterized -- any\n2 2\ncrystal structure or NMR? This work will be of interest to astrophysics/astrochemistry\nscientists. But not to broad spectrum chemistry community.\nAuthor reply: We are grateful to the Reviewer for the positive and insightful comments on\nour work.\nThe cis and trans isomers of HPNH were generated by pyrolysis of (t-Bu) PNH /matrix\n2 2\ngas in an Al O tube furnace (at ca. 950 K) and subsequently condensed onto the cold matrix\n2 3\nstage (10 K). The distance between the tube and the matrix stage is about 5 cm, and the\ndimension of the furnace has been specified in the section of Methods: “Al O tube furnace\n2 3\n(o.d. 2.0 mm, i.d. 1.0 mm), which can be heated over a length of ca. 30 mm by a tantalum\nwire (resistance 2.9 Ω, voltage 10.0 V, and current 3.50 A)”. The absence of PN and H NP\n2\namong the pyrolysis products indicates that HPNH is thermally persistent in the gas phase\nunder the pyrolysis conditions. Once isolated in the cryogenic matrix (10 K), all species,\nincluding the HPNH isomers and the reaction products H NPCO and H NPO , are stable\n2 2 2\nunder the matrix isolation conditions.\nThe structures of H PN, H NP, and HPNH isomers have been studied before by many\n2 2\ntheoretical calculations at various theoretical levels. In the present study, their structures were\nfully optimized at the CCSD(T)-F12b/VTZ-F12 level of theory and depicted in the\nSupplementary Information as Fig. S10. The experimental determination of the molecular\nstructures is highly challenging due to the high reactivity of these multiply bonded species.\nConsidering the facile generation of HPNH in the gas phase, further studies with other\nspectroscopic techniques such as microwave spectroscopy should be possible to obtain the\nstructural parameters in the future.\nFig. S10. Calculated structures for [2H, N, P] isomers (bond lengths in Å, angles in °) at the\nCCSD(T)-F12b/VTZ-F12 level of theory.\nAs for the importance of the HPNH isomers, the fundamental importance of HPNH and H NP\n2\nin main-group chemistry has been demonstrated by the novel bonding/electronic properties\nand reactivity associated with the prototype phosphorus-nitrogen multiple bonds, as\nevidenced by numerous theoretical studies (for examples, see references 27-33), including the\nfirst theoretical study by Trinquier in 1982 [reference 32: Phosphinonitrene, phosphazene, and\naminophosphinidene. Structures and stabilities. J. Am. Chem. Soc. 104, 6969−6977 (1982)]\nand also the very recently predicted singlet ground state nature for H NP using the high-level\npnictinidenes. Chem. Eur. J. 27, 14461−14471 (2021)]. Although phosphazenes RP=NR and\naminophosphinidenes R N≡P are versatile building blocks in synthetic phosphorus chemistry,\n2\nall the early attempts to prepare the parent [2H, N, P] isomers were unsuccessful due to the\nlack of synthetic methods. Therefore, the first time synthesis of the long sought-after HPNH\nand H NP contributes to the fundamental knowledge of main-group chemistry.\nmolecule PN thanks to the newly discovered methods for its preparation [for examples, see\nreference 22: Eckhardt, A. K. et al. Taming phosphorus mononitride. Nat. Chem. 14, 928−934\n(2022) and reference 23: Edin, S. et al. Unleashing phosphorus mononitride. Nat. Commun.\n16, 5596 (2025)]. By analogy, the successful preparation and characterization of HPNH and\nits isomer H NP will enable further studies on their structures and reactivity. Given the\n2\ninvolvement of [2H, N, P] isomers in interstellar phosphorus chemistry, the disclosed\nphotochemistry in this work is crucial for building the chemical network for the important\ninterstellar species PN.\nTo make it clear, the conclusion has been changed to “Similar to the broad applications of the\nclosely related singlet phosphinophosphinidenes60 and phosphinonitrenes,41 H NP features as\n2\na fundamentally new and highly reactive aminophosphinidene that might be used as a\nbuilding block in synthetic phosphorus chemistry. The photoreactions of H NP with small\n2\nmolecules CO and O to the formation of H NPCO and H NPO underscore its role as a\n2 2 2 2\npotentially useful intermediate in constructing novel N−P−C and N−P−O motifs. Considering\nthe possible involvement of the [2H, N, P] isomers in the astrochemical network of the\nimportant phosphorus-bearing species PN in the interstellar medium (ISM), the facile\ngeneration of HPNH in the gas phase will stimulate further studies on its role in interstellar\nphosphorus chemistry. Particularly, the disclosed photochemistry of the [2H, N, P] isomers\nincluding the reactions of H NP with CO and O could be of interest in understanding the\n2 2\nphosphorus chemistry in cold dense molecular clouds, where photochemistry has been\nproposed as an important driving force for the formation of complex organic molecules.69,70”.\n----------------------------------------End of responses #2---------------------------------------\nReviewer 3:\nGeneral comment: The manuscript reports on the generation and spectroscopic identification\nof the simplest iminophosphane (HPNH) and aminophosphinidene (H₂NP), as well as their\nphotochemical interconversion and their reactions with O₂ and CO molecules. The\nidentification of these species, based on their IR spectra and supported by high-level\ncalculations, is convincing. The work is interesting and, with minor revisions, could be\nsuitable for publication.\nAuthor reply: We greatly appreciate the Reviewer’s positive recommendation.\nComment 1: The authors do not explain why the experiments were carried out in an N₂ matrix.\nThe more common choices are Ar, Ne, or para-H₂ matrices. N₂ matrices interact more strongly\nwith isolated molecules, leading to larger matrix shifts, and due to site effects, many of the\nspectral features may split. I may have some ideas about why the authors chose an N₂ matrix,\nbut it is important that they provide their own explanation in the manuscript.\nAuthor reply: We fully agree with the Reviewer that noble gas matrices are frequently used\nfor the isolation of reactive intermediates. In fact, we also performed experiments with Ar and\nNe matrices. As expected, the yields of HPNH from high-vacuum flash pyrolysis of\n(t-Bu) PNH by using Ar and Ne as matrix gas are much lower due to less efficient sample\n2 2\ndeposition than using the more condensable N . Nevertheless, the photochemistry of HPNH\n2\nand H₂NP including the 410 nm-induced isomerization from trans-HPNH to cis-HPNH\nremains consistent across all matrices (see the below Fig. R1). For clarity, the spectra with Ar\nand Ne matrices have been included in the revised Supplementary Information (Fig. S4 and\nthe newly added Fig. S5). Additionally, a sentence has been added on page 5: “When using\nthe less condensable Ar or Ne as the carrier gas in the HVFP experiments, the yields of the\nmatrix-isolated HNPH become lower (Figs. S4 and S5).”.\nFig. R1. IR difference spectra (non-scaled) showing the isomerization from trans-HPNH (a)\nto cis-HPNH (b) in solid N (panel A), Ar (panel B) and Ne (panel C) matrices upon\n2\nphotoexcitation at 410 nm (10 min).\nFig. S5. (A) IR spectrum (3400−800 cm–1) of the matrix-isolated pyrolysis (ca. 950 K)\nproducts of (t-Bu) PNH in a Ne-matrix at 3 K. (B) IR difference spectrum showing the\n2 2\nisomerization from trans-HPNH (a) to cis-HPNH (b) in the matrix upon photoexcitation at\n410 nm (10 min). The IR bands for C H (c), C H (d), and HOPO (h) are also marked.\n4 8 4 10\nComment 2: The computational method employed is also not the most typical one for this\nkind of study. It would be helpful if the authors could provide references to earlier works\ndemonstrating the expected accuracy at the chosen level of theory.\nAuthor reply: We thank the reviewer for this valuable suggestion. The\nCCSD(T)-F12/cc-pVTZ-F12 level of theory combined with configuration-selective\nvibrational configuration interaction (VCI), as employed in this study, has been demonstrated\nto achieve high accuracy in predicting IR spectra for HNPH isomers. This is evidenced by the\nexcellent agreement between theoretical and experimental spectra (Tables 1 and 2), with\ndifferences of less than 10 cm⁻¹ from the experimental observations. This method has also\nbeen used for the calculations on the IR spectra of other phosphorus-containing species such\nas the P H isomers (Angew. Chem. Int. 2023, 62, e202217353) and HPN (J. Am. Chem. Soc.\n2 2 2\n2022, 144, 21853). Generally, the differences between experiment and theory are less than 15\ncm⁻¹. As suggested by Reviewer, the Quantum chemistry calculation section has been\nmodified: “Further vibrational frequency calculations were carried out at the\nCCSD(T)-F12B/VTZ-F1279,80 level using configuration-selective vibrational configuration\ninteraction (VCI)52 theory with Molpro 2019 package.81 This method has been shown to\nachieve high accuracy in predicting IR spectra for phosphorus-containing species such as the\nP H isomers.49”.\n2 2\nComment 3: The introduction is somewhat unfocused in its early part, but it rightly\nemphasizes the astrochemical relevance of the studied molecules toward the end. Given that\nthe identification of such species is more likely to come from radioastronomy rather than IR\nastronomy, it would be valuable to provide highly accurate calculated rotational constants\nand dipole moments, with a brief interpretation.\nAuthor reply: As suggested by the Reviewer, the rotational constants and dipole moments for\ncis/trans-HNPH, H NP and H PN were calculated at the CCSD(T)-F12b/VTZ-F12 level of\n2 2\ntheory (see the newly added Table S4). The results indicate that these molecules possess large\ndipole moments and distinct rotational constants, suggesting they are good candidates for\nastronomical detection. The following sentence has been added to the section of Quantum\nchemistry calculation: “Rotational constants and dipole moments were also calculated at the\nCCSD(T)-F12B/VTZ-F12 level”.\nTable S4. Calculated rotational constants (A, B, C in Megahertz), asymmetry parameter, and dipole\nmoments (in Debye) at the CCSD(T)-F12B/VTZ-F12 level of theory.\nSpecies A [a] B C κ[b] μ [c]\ne e e e\ntrans-HPNH 186867.4 18734.1 17027.1 −0.9799 1.5643\ncis-HPNH 188319.9 18684.0 16997.6 −0.9803 2.2763\nsinglet-H NP 351012.7 16184.9 15471.5 −0.9957 2.5539\n2\nsinglet-H PN 200587.4 19782.9 18006.9 −0.9805 4.5352\n2\n[a] Equilibrium rotational constants. [b] Asymmetry parameter κ = (2B−A−C)/(A−C), κ varies from −1 for\na prolate symmetric top to +1 for an oblate top. [c] Dipole moment of the minimum energy structure.\nComment 4: In my opinion, the calculated UV–vis spectrum shown in Figure 2 should not\nappear only as an inset, but rather below the experimental spectrum with matching axis\nscales. Similarly, displaying the calculated and experimental IR spectra in the figures may be\nworthwhile, even if the agreement is already clear from the tables.\nAuthor reply: We thank the Reviewer for the valuable suggestion. Fig. 2 has been modified\nby displaying the calculated UV–vis spectrum below the experimental spectrum with matched\naxis scales. In addition, we have included the theoretically calculated IR spectra of\ncis/trans-HPNH and H NP in Fig. 1 and Fig. 2, respectively.\nthe matrix-isolated pyrolysis (ca. 950 K) products of (t-Bu) PNH in a solid N -matrix at 10 K.\n2 2 2\nThe IR bands for C H (c) and C H (d) are also marked. (B) IR difference spectrum showing\n4 8 4 10\nthe isomerization from trans-HPNH (a) to cis-HPNH (b) in the matrix upon photoexcitation\nat 410 nm (10 min). (C) IR difference spectrum showing the reverse conversion from\ncis-HPNH (b) to trans-HPNH (a) in the matrix upon subsequent photoexcitation at 365 nm\n(30 min). (D) Calculated IR spectra for cis-HPNH (green) and trans-HPNH (violet) at the\nCCSD(T)-F12B/VTZ-F12 level of theory using configuration-selective vibrational\nconfiguration interaction theory (VCI).\nFig. 2 | Generation and characterization of H NP. (A) UV-vis spectra of the N -matrix\n2 2\nisolated high-vacuum flash pyrolysis products of (t-Bu) PNH and also the photolysis\n2 2\nproducts. The calculated vertical transitions at the TD-DSD-PBEP86/aug-cc-pVTZ level of\ntheory are also depicted. (B) IR difference spectrum (3400−700 cm-1) showing the\nisomerization from trans- and cis-HPNH (a and b) to H NP (e) upon photoexcitation at 395\n2\nnm (10 min) in the N -matrix at 10 K. (C) IR difference spectrum showing the reverse\n2\nisomerization from H NP (e) to trans- and cis-HPNH (a and b) upon subsequent\n2\nphotoexcitation at 310 nm (10 min) in the matrix at 10 K. (D) Calculated IR spectrum for\nsinglet H NP at the CCSD(T)-F12B/VTZ-F12 level of theory using configuration-selective\n2\nvibrational configuration interaction theory (VCI).\n----------------------------------------End of responses #3---------------------------------------"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have carefully addressed my comments and remarks, which I believe helped clarifying some of the questions\nthat arise naturally. I can now recommend this work for publication.", "reviewer_2": "There is no significant improvement in the manuscript compared to the original submission. This article is suitable for\nastrophysics / astrochemistry journals", "reviewer_3": "I accept the answers."}, "author_response": ""}]

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