Spectroscopic Identification and Photochemistry of Astrochemically Relevant Phosphorus-bearing Molecules [O, C, N, P] and [2O, C, N, P]

Diatomic molecules phosphorus monoxide (PO) and phosphorus mononitride (PN) are the main reservoirs of gas-phase phosphorus in interstellar and circumstellar environments, indicating the possibility of forming new phosphorus-bearing molecules through reactions with other interstellar species. To explore the astrochemistry of PO and PN, new simple phosphorus-bearing molecules [O, C, N, P] and [2O, C, N, P] were generated in the gas phase and isolated in cryogenic matrices for characterization with matrix-isolation IR and UV/vis spectroscopy in combination with calculations at the CCSD(T)-F12a/VTZ-F12 level of theory. In an inert argon matrix, OPCN isomerizes to OPNC upon UV-light irradiation at 365 nm, followed by successive isomerizations to PNCO and POCN with concomitant dissociation to diatomic PN and CO under further irradiation at 193 nm. By analogy, the isomerization of O2PCN to O2PNC and OPNCO followed by fragmentation to OPN/CO and PN/CO2 occurs in the matrix upon irradiation at 193 nm. In a chemically active CO ice, the photolytic reaction of OPCN with CO yields CO2 and PCN, and O2PCN reacts with CO by forming OPCN and CO2, in which the photochemical networks for these P-bearing species linking the astrochemically important PN and PO have been proposed. The experimental identification of these phosphorus-bearing molecules is supported by quantum chemical calculations, and the spectroscopic data may aid in their detection in the interstellar and circumstellar medium.


Introduction
Phosphorus (P) is one of the essential biogenic elements, and P-bearing compounds perform many fundamental biochemical functions in living organisms like metabolism, replication, and energy transfer (Westheimer 1987;Maciá 2005).Despite its importance for life, the astrochemistry of P-bearing molecules remains poorly understood.Yet, to date, only seven covalent P-bearing species have been observed in the interstellar medium (ISM) or circumstellar envelopes, namely, phosphorus mononitride (PN; Ziurys 1987), phosphorus monoxide (PO; Tenenbaum et al. 2007), PO + (Rivilla et al. 2022), CP (Guélin et al. 1990), CCP (Halfen et al. 2008), HCP (Agúndez et al. 2007), and PH 3 (Agúndez et al. 2014), and the phosphorus astrochemistry remains highly debatable (Jiménez-Serra et al. 2018;Chantzos et al. 2020).In contrast, the chemical networks for the evolution of the interstellar H-, C-, N-, O-, and S-bearing compounds are better understood (Willacy & Millar 1997;Agúndez & Wakelam 2013;Gong et al. 2017;Holdship et al. 2019;Bulut et al. 2021).As the main reservoirs of gas-phase phosphorus in the interstellar and circumstellar environments (de la Concepción et al. 2021;Souza et al. 2021;Wurmser & Bergner 2022), diatomic compounds PO and PN have attracted numerous areas of interest, including their formation, destruction, and interconversion (Figure 1).Particularly, PO and PN have been detected in several oxygen-rich circumstellar envelopes, and it has also been proposed that P-bearing molecules are common in O-rich envelopes with a significant amount of phosphorus (>20%) remaining in the gas phase (Tenenbaum et al. 2007;Ziurys et al. 2007Ziurys et al. , 2018)).Very recently, both PO and PN have been observed in the outer Galaxy, where supernovae are not present, suggesting another source of phosphorus at the edge of the Galaxy (Koelemay et al. 2023).In many cases, PO is found to be slightly more abundant than PN (Rivilla et al. 2016), and the PO/PN ratio can potentially be used as a length indicator of the pre-stellar collapse phase (Aota & Aikawa 2012;Lefloch et al. 2016).Recent astronomical observations have suggested that the formation of PO and PN is related; however, the mechanism is unclear.According to the theoretical study on the mechanism for the interconversion between PO and PN, the depletion of PO by N atom (PO + N → PN + O) occurs spontaneously, whereas the collision between PN and O atom contains a small barrier of 6-10 kJ mol −1 (Souza et al. 2021).It has been proposed that both PO and PN are key prebiotic compounds, particularly, PO is the building block of biomolecules for the backbone of ribonucleic acids DNA and RNA (de la Concepción et al. 2021;Plane et al. 2021), and PN acts as a modification site for the molecular switch in the process of protein arginine phosphorylation (pArg; Elsholz et al. 2012).Chemically, phosphorus mononitride (PN) is much more reactive than the nitrogen congener N 2 and it polymerizes very rapidly at ambient conditions.Recently, the synthesis and reactions of PN have been extensively explored (Kinjo et al. 2010;Eckhardt et al. 2022;Qian et al. 2023).By analogy, PO is a transient heavy analog of nitrogen monoxide (NO), and its reactivity as a free species remains barely explored (Sterenberg et al. 2002).
Due to the limited astronomical observation of P-bearing molecules in the ISM, quantum chemical calculations and laboratory synthesis of new P-bearing candidate species may provide important insights into the chemical evolution of the detected ones.Recently, seminal achievements have been made in the generation and characterization of the supposed extraterrestrial small P-bearing molecules, such as HPO (Ar matrix; Withnall & Andrews 1987), NPO (Ar matrix and solution; Ahlrichs et al. 1988;Zeng et al. 2011b;Eckhardt et al. 2021) Zhu et al. 2020Zhu et al. , 2021)), and biorevelant phosphorus oxoacids (gas phase; Turner et al. 2018Turner et al. , 2019)).The fundamental properties including their formation from the interstellar P-bearing precursors and complex isomerization reactions have been extensively explored.For instance, as the adducts of PN and O atom, triatomic species NPO and PNO were generated and found to undergo isomerization under irradiation conditions (Ar matrix; Ahlrichs et al. 1988;Zeng et al. 2011b).It is noteworthy that PN has been discovered in the oxygen-rich shell of the red supergiant star VY Canis Majoris (VY CMa; Ziurys et al. 2007), implying the possible formation of the two NPO isomers through collisional reactions.Recently, the structures, spectroscopic data, and reaction network for the novel tetratomic species consisting of H, P, S, and O have been comprehensively explored by using quantum chemical calculations, and it was proposed that the stable HSPO isomers might be formed through the barrierless radical-radical association reactions between the detected interstellar species SH and PO in cold molecular clouds (Esposito et al. 2022).Among the theoretically predicted P-bearing candidates in the ISM, molecules consisting of [O, C, N, P] and [2O, C, N, P] are of particular interest, since the CN group plays a key role in the generation of biorelevant precursors (Sutherland 2016;Liu et al. 2020;Zhao et al. 2023).The [O, C, N, P] isomers including OPCN and PNCO have been theoretically studied, and they have been proposed to be potent interstellar species that could be formed through the association of the reactive PO and CN radicals in the ISM or circumstellar envelopes (Finney et al. 2017).In sharp contrast to the well-characterized structure and reactivity of the nitrogen analogs [O, C, 2N] and [2O, C, 2N] (Ar matrix and gas phase; Dorko & Buelow 1975;Maier et al. 1997;de Petris et al. 2005;Zeng et al. 2011a;Rahm et al. 2014), the experimental knowledge about the phosphorus analogs [O, C, N, P] and [2O, C, N, P] including OPCN and O 2 PCN remains barely known due to the paucity of suitable synthetic methods.
In this paper, we report the gas-phase synthesis (Figure 2) and characterization of the elusive [O, C, N, P] and [2O, C, N, P] isomers for the first time.In addition to the experimental and theoretical spectroscopic data, the photochemical networks for these P-bearing species linking the astrochemically important PN and PO have been disclosed (Figure 1).

Matrix-isolation IR Spectroscopy
Matrix IR spectra were recorded on a Fourier-transform infrared spectrometer (Bruker 70V) in reflectance mode using a transfer optic.A KBr beam splitter and MCT detector were used in the mid-IR region (4000-500 cm −1 ).Typically, 200 scans at a resolution of 0.5 cm −1 were co-added for each spectrum.The gaseous sample was mixed by passing through a flow of Ar or CO gas through a cold U-trap (−9 °C) containing ca. 20 mg of the freshly prepared precursor C 2 H 4 O 2 PCN.Then, the mixture (sample: dilution gas = 1:1000, estimated ratio) was passed through an aluminum oxide (Al 2 O 3 ) tube furnace (o.d.2.0 mm, i.d.1.0 mm), which can be heated over a length of ca. 25 mm using a tantalum wire (o.d.0.4 mm, resistance 0.4 Ω, voltage 11.9 V, and current 3.46 A).The temperature (1000 K) at the surface of the pyrolysis tube was roughly estimated with a NiCr-Ni thermocouple.Then, the resulting pyrolysis products were immediately deposited (2 mmol hr −1 ) in a high vacuum (10 −6 Pa) onto the gold-plated copper block matrix support (10 K for Ar matrix, 16 K for CO matrix) using a closed-cycle helium cryostat (Sumitomo Heavy Industries, SRDK-408D2-F50H) inside the vacuum chamber.Temperatures at the second stage of the cold head were controlled using an East Changing TC 290 digital cryogenic  temperature controller a silicon diode (DT-670).Photolysis experiments were performed using an ArF excimer laser (Gamlaser EX5/250, 193 nm, 3 mJ, 3 Hz) and a UV flashlight (Boyu, 365 nm, 24 W).

Matrix-isolation UV/Vis Spectroscopy
Matrix UV/vis spectra were recorded on a Perkin Elmer Lambda 850+ spectrometer (190-800 nm, scanning speed of 1 nm s −1 ).The high-vacuum flash pyrolysis products using the similar Al 2 O 3 furnace (o.d.2.0 mm, i.d.1.0 mm) were deposited onto a CaF 2 matrix support (10 K) using a closed-cycle helium cryostat (Sumitomo Heavy Industries, SRDK-408D2-F50H) inside the vacuum chamber.Temperatures at the second stage of the cold head were controlled using a Lake Shore 335 digital cryogenic temperature controller, a silicon diode (DT-670).

Quantum Chemical Calculation Methods
Structures and harmonic IR frequencies for stationary points were calculated at the B3LYP/6-311++G(3df,3pd) level of theory (Becke 1993).Local minima were confirmed by vibrational frequency analysis, and transition states were ascertained with additional intrinsic reaction coordinate calculations (Fukui 1981;Hratchian & Schlegel 2005).The time-dependent TD-B3LYP/6-311++G(3df,3pd) (Foresman et al. 1992) method was performed for the calculation of the vertical excitation energies.All these computations were performed using the Gaussian 09 software package (Frisch et al. 2016).Further calculations of the single-point energies for all the species on the potential energy profiles were carried out at the (U)CCSD(T)-F12a/aug-cc-pVTZ (Adler et al. 2007;Knizia et al. 2009;Celis Gil & Thijssen 2017)

Results and Discussion
Gas-phase generation of OPCN (3) and O 2 PCN (5) was accomplished by high-vacuum flash pyrolysis (HVFP) of a common dioxaphospholane-based precursor, C 2 H 4 O 2 PCN (1), at 1000 K (Figure 3).In the IR spectrum of the matrix-isolated pyrolysis products (Figure 3 To aid the assignment, theoretical calculations on the IR spectra of OPCN and O 2 PCN along with their isomers were performed at the CCSD(T)-F12a/VTZ-F12 level using VCI theory (Mathea & Rauhut 2021;Mathea et al. 2022).In line with experimental observations, the predicted prominent IR bands corresponding to the stretching mode of CN moiety for OPCN (Table 1) and O 2 PCN (Table 2) appear at about 2200 cm −1 .Considering the distinct lowest-energy transitions for OPCN (418 nm, oscillator strength f = 0.0064, Table A1 and Figure A1) and O 2 PCN  (201 nm, f = 0.0143, Table A2 and Figure A1), the matrix containing the pyrolysis products of 1 was irradiated first with UV light at 365 nm.The corresponding IR difference spectrum (Figure 4(A)) reflecting the photoinduced changes of the matrix shows sole depletion of OPCN (3) with concomitant formation of OPNC (8).The latter is also present among the pyrolysis products of the cyanide precursor 1, indicating that the isomerization from 3 to 8 is also thermally feasible.
The strong IR bands at 2155.4 and 2022.0 cm −1 belong to the CN stretching vibration of OPCN (3) and OPNC (8) with calculated VCI frequencies of 2164.7 and 2024.1 cm −1 , respectively.They are close to the same mode found in the closely related isomeric systems ONCN/ONNC (2163.0/1997.5 cm −1 , Ar matrix; Maier et al. 1997) and OSCN/OSNC (2128.0/1989.3cm −1 , Ar matrix; Wu et al. 2017), in which similar photoisomerization reactions were also observed.The PO stretching vibration in 3 locates at 1218.3 cm −1 , and it is redshifted in comparison to 8 at 1252.8 cm −1 .Both show good agreement with the calculated values of 1221.2 and 1247.1 cm −1 , and they are also close to the frequencies for the same mode in chlorophosphinidene oxide ClPO (1258 cm −1 , Ar matrix; Ahlrichs et al. 1986), methylphosphinidene oxide CH 3 PO (1249.9cm −1 , Ar matrix; Chu et al. 2018), and phenylphosphinidene oxide PhPO (1185 cm −1 , Ar matrix; Mardyukov et al. 2020).The two remaining bands at 570.2 and 611.3 cm −1 correspond to the inplane deformation modes for the PCN and PNC moieties in 3 (cal.592.1 cm −1 ) and 8 (cal.630.6 cm −1 ), respectively.It is worth mentioning that the existence of OPCN was first claimed among the reaction products of gaseous OPCl 3 with heated solid AgCN (Allaf et al. 1999).However, this identification with gas-phase IR spectroscopy was found to be highly questionable by further theoretical study (Robertson & McNaughton 2003) due to the large discrepancy in the observed IR bands (2165 and 1385 cm −1 ) with the B3LYP/aug-cc-pVTZ calculated values (2261 and 1237 cm −1 ).In contrast, the good agreement of our experimental IR data with the calculations and also the accompanied photoisomerization with OPNC provide compelling evidence for the identification of OPCN.The weak bands for the remaining IR fundamental vibrations below 450 cm −1 of OPCN and OPNC (Table 1) are out of the present spectral range.
b Observed band position (>500 cm −1 ) for the most intense matrix site and relative intensity (in parentheses) based on the integrated band area.c Not observed.The PC stretching mode in 5 strongly couples with the PO 2 scissoring mode and locates at 662.1 cm −1 , and it is lower than that in CH 3 PO 2 at 725.2 cm −1 .Among the IR bands for the newly generated species after the laser irradiation of the matrix, the strongest one at 2247.4 cm −1 associates with the NCO stretching mode for the unexpected rearrangement product OPNCO (10) (Wu et al. 2018), possibly arising from the isomerization of O 2 PCN (5) via the intermediacy of O 2 PNC (9).Additionally, another three weak but distinguishable peaks for 10 at 1410.7, 1245.8, and 674.8 cm −1 can also be identified.It is noteworthy that traces of 9 and 10 are also present among the pyrolysis products of the cyanide precursor 1, indicating the occurrence of isomerization under the pyrolysis conditions.The  A2).The formation of 18 can be assured with the IR band at 2065.9 cm −1 for the characteristic CN stretching vibration (cal.2125.0 cm −1 , Table A3).Similar to the photoisomerization between ONNC and NOCN (Maier et al. 1997), the phosphorus analog OPNC (8) isomerizes to POCN (11).The observed band positions at 2272.3 and 1092.4 cm −1 are in good agreement with the calculated frequencies of 2275.8 and 1108.0 cm −1 for the CN and CO stretching modes (Table 1), respectively.According to the calculation at the (U)CCSD(T)-F12a/aug-cc-pVTZ level of theory, POCN prefers a triplet spin multiplicity, and the closedshell singlet state is higher with an energy gap (ΔE ST ) of 19.7 kcal mol −1 .The IR band for the POC in-plane deformation mode (cal.660 cm −1 ) is masked by the band of O 2 PCN at 662.1 cm −1 .
Given the intense absorption at around 300 nm for the openshell triplet POCN (Table A1 and Figure A1), the matrix containing the 193 nm laser photolysis products was subjected to further irradiation at 365 nm.The resulting IR difference spectrum (Figure 4(C)) shows the complete depletion of the two bands for POCN (11) with the concomitant appearance of a new sharp band at 2297.3 cm −1 for a new isomer PNCO (12), in which the calculated strongest IR band at 2319.3 cm −1 (Table 1) corresponds to the characteristic NCO asymmetric stretching mode (Figure A3).PNCO (12) is also a triplet species, and its singlet state is higher in energy by 21.7 kcal mol −1 .The remaining IR bands for PNCO (Table 1) are too weak to be detected in the spectrum.The The photochemistry of the [O, C, N, P] and [2O, C, N, P] isomers in the chemically inert Ar matrix reveals the predominant isomerization reactions with minor dissociation channels.In order to trap the transient intermediates in the process of these transformations, the photochemistry was also explored in the chemically active carbon monoxide (CO) matrix at 16 K (Figure 5), which has been frequently used to trap the initially generated reactive intermediates during the dissociation-recombination reactions.Additionally, CO is the second most abundant molecule (after molecular hydrogen) in the gas phase of the ISM, and solid CO is a major constituent of interstellar ices covering dust grains in dense cold molecular clouds (<30 K; He et al. 2021).The photochemistry of astrochemically relevant species in solid CO ice provides important information for understanding the prebiotic chemistry of complex organic molecules (COMs) such as pyruvic acid, lactic acid, and glyoxylic acid (Eckhardt et al. 2019;Kleimeier et al. 2020;Turner & Kaiser 2020).For instance, an organophosphorus compound formylphosphine HC(O)PH 2 has been detected in carbon monoxide (CO)-phosphine ices (PH 3 ) after ionizing radiation with energetic electrons (Frigge et al. 2018).
According to the IR spectrum of the pyrolysis products of the C 2 H 4 O 2 PCN/CO mixture at 16 K (Figure 5(A)), O 2 PCN (5) and C 2 H 4 (6) remain as the major fragments.The IR bands of 5 in CO ice appear at 2211.4, 1470.8, 1162.8, and 650.3 cm −1 , slightly shifted in comparison with those observed in Ar matrix (2207.9,1472.1, 1150.9, and 662.1 cm −1 ) due to its weak interactions with CO.The irradiation of the CO matrix at 365 nm (Figure 5(B)) causes isomerization of OPCN (3) to OPNC (8) with additional formation of CO 2 (13), suggesting the photoreaction of 3 with CO by producing CO 2 and PCN, which also implies that the photoisomerization of OPCN in the matrix is most likely an intramolecular process.Note that CO 2 was not observed during the 3 → 8 isomerization in the Ar matrix (Figure 4(A)).The IR band for the CN stretching mode in PCN (Table A4) at around 2000 cm −1 is masked by the strong band at 2022.0 cm −1 for 8, whereas, a weak but distinguishable band at 659.4 cm −1 for the PC stretching mode can be observed for PCN.It is noteworthy that PCN has been proposed as a potent P-bearing species in the ISM, and it was observed by microwave spectroscopy among the reaction products of phosphorus vapor and cyanogen in the presence of an AC discharge (Halfen et al. 2012).In contrast to the photochemistry in the Ar matrix (Figure 4  solid CO ice suggests that the photoisomerization reactions for both species most likely occur through intramolecular rearrangement, and no dissociation-recombination process is involved.The similar photochemistry of O 2 PCN and OPCN observed in the Ar matrix and CO ice also confirms that the chemical network might also exist in interstellar CO ice analogs.Interestingly, in contrast to the known insertion of CO into astrochemically relevant species, the new photolytic reduction reactions of the two P-bearing molecules with CO (O 2 PCN + CO → OPCN + CO 2 ; OPCN + CO → PCN + CO 2 ) imply the possible involvement of CO molecules in the astrochemical network of the P-bearing molecules such as PO and PN, which have been very recently observed at the edge of the Galaxy.
To probe the chemical connections between PO and PN via the intermediacy of the [O, C, N, P] isomers (Figure A4 and Table A5), the potential energy profile (Figure 6) for the isomerization and decomposition of OPCN was computed at the (U)CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G (3df,3pd) level of theory.The global minimum is OPCN, and its isomerization to the energetically equivalent OPNC through the transition state TS1 requires surmounting a moderate activation barrier of 22.0 kcal mol −1 , which is significantly lower than the applied photon energy from the irradiation at 365 nm (78.2 kcal mol −1 ).This is fully consistent with the experimental observation of both species among the pyrolysis products of the precursor (Figure 3(B)).Both OPCN and OPNC are thermally persistent since they are separated from the higher-energy isomers POCN and PNCO by formidable barriers (TS2 and TS3) of about 80 kcal mol −1 , but it can be overcome by the irradiation at 193 nm (148.1 kcal mol −1 ).On the other hand, the observed photoisomerization might be caused by the difference in the photon fluxes used for the irradiations and consequently two-photon process.Therefore, the presence of both species among the pyrolysis products in the gas phase is probably caused by the catalytic isomerization at the furnace surface, whereas the observed isomerization in the cryogenic matrix proceeds in the photolytically excited state.
The P-C and P-N dissociation energies in OPCN and OPNC are 91.5 and 90.1 kcal mol −1 , respectively, ruling out its dissociation even at the pyrolysis temperature (ca.1000 K).Conversely, the combination of OP and CN radicals in forming OPCN and OPNC is highly exothermic.The triplet isomer PNCO is higher than OPCN by 14.2 kcal mol −1 , but it is more stable than the putative three-membered ring isomer OC(PN) by 15.8 kcal mol −1 .This is in sharp contrast to the relative stability between triplet NNCO (de Petris et al. 2005) and cyclic OC(N 2 ) (Zeng et al. 2011a) since the latter is relatively more stable and it can be isolated as a pure substance with a lifetime of 30 hr in the gas phase at room temperature.However, triplet NNCO can be only detected by mass spectrometry with an estimated lifetime of about 0.8 μs.Similar to the decomposition of cyclic OC(N 2 ) (→ CO + N 2 ), the cyclic phosphorus analog favors exothermic fragmentation to PN and CO.Recently, the photoreaction between PN and benzoquinone has been observed in Ar matrix at 10 K, and the activation barrier for this exothermic process is about 27 kcal mol −1 (Qian et al. 2023).In contrast, the reverse reaction between PN and CO to the formation of PNCO is endothermic with a higher barrier (TS4).
By analogy, the potential energy profile for the reaction between PO 2 and CN via the intermediacy of the [2O, C, N, P] isomers (Figure A5 and Table A6) has also been studied (Figure 7).The association of these two radicals through P-C bond formation is highly exothermic, and the product O 2 PCN is kinetically stable due to the moderate barrier (TS5, 30 kcal mol −1 ) for its isomerization to the less stable O 2 PNC.The higher stability of O 2 PCN than O 2 PNC coincides with the predominant presence of the former isomer among the decomposition product of C 2 H 4 O 2 PCN (Figure 3(B)).The barrier for the rearrangement of O 2 PNC to the global minimum OPNCO amounts to 49.9 kcal mol −1 (TS6), and the subsequent dissociation of the latter to PN and CO 2 is endothermic, for which a barrier of about 50 kcal mol −1 involving a fourmembered ring transition state has been proposed (Wu et al. 2018).
The energy profiles for the [O, C, N, P] and [2O, C, N, P] isomers suggest that the cyanide/isocyanide species OPCN/ OPNC and O 2 PCN/O 2 PNC are stable enough to serve as potent intermediates in the recombination of the interstellar species PO, PO 2 , and CN, as similar barrierless radical-radical association reactions have been proposed as an efficient pathway for the generation of COMs in cold molecular clouds and in star-forming regions (Turner & Kaiser 2020).To facilitate the gas-phase detection of these novel P-bearing species in the laboratory and astronomical observation by rotational spectroscopy, the ground vibrational state rotational constants were computed at the (U)CCSD(T)-F12a/VTZ-F12 level of theory and listed in Table 3, which has been proved as a reliable method for the theoretical prediction of rotational constants of small molecules.The large differences in the    (Withnall & Andrews 1987;Ahlrichs et al. 1988;Wu et al. 2018).
rotational constants between the nitriles (OPCN and O 2 PCN) and isocyanides (OPNC and O 2 PNC) should aid in distinguishing these isomers in a spectrum.

Conclusions
Seven P-bearing small molecules, OPCN, OPNC, PNCO, POCN, O 2 PCN, O 2 PNC, and OPNCO, have been generated and spectroscopically characterized.The photochemistry of these molecules in Ar matrix (10 K) and CO ice (16 K) including the photoisomerization reactions and the fragmentation to the astrochemically important smaller P-bearing compounds PN, PO, PCN, and C-bearing compounds CO, CN, and CO 2 have been disclosed (Figure 8).The potential role of these elusive P-bearing small molecules in the interstellar phosphorus chemistry by linking the two major gas-phase carriers of phosphorus (PN and PO) through radical-radical recombination and subsequent photodecomposition reactions in the condensed ice grains has been discussed.The reaction network of these P-bearing molecules may help in building the yet poorly understood chemical models of interstellar phosphorus chemistry in the ISM.It should be noted that the laser chemistry of the matrix-isolated molecules might involve a two-photon process, which does not exist in astrophysical environments.Thus, the observed photochemical network in the matrix could be different from that in natural environments.Furthermore, the experimental and theoretical spectroscopic data for the [O, C, N, P] and [2O, C, N, P] isomers may aid their detection in the dense star-forming regions where only PN and PO have been observed as P-bearing molecules so far (De Beck et al. 2013;Ziurys et al. 2018;Rivilla et al. 2020;Tofan & Velian 2020;Sil et al. 2021).

Figure 1 .
Figure 1.Summary of the reaction networks for PO and PN in the interstellar and circumstellar medium.
level based on the density functional theory (DFT) optimized molecular structures.Calculations of the anharmonic IR frequencies and intensities with configuration-selective vibrational configuration interaction theory (VCI; Neff & Rauhut 2009) were performed at the CCSD(T)-F12a/VTZ-F12 level of theory with the MOLPRO 2023 package (Werner et al. 2012).

Figure 3 .
Figure 3. IR spectra of Ar matrix-isolated C 2 H 4 O 2 PCN (panel (A)) and its pyrolysis products (panel (B)) at 10 K.The IR bands of unknown species ( * ) are labeled.
photoconversion from O 2 PCN to O 2 PNC is inefficient due to the further isomerization to OPNCO via oxygen-atom transfer reaction in O 2 PNC, and OPNCO undergoes CO elimination by furnishing OPN (16) with subsequent rearrangement to PNO (17) as the 193 nm laser photolysis products (Figure 4(B)).The strongest band at 2062.6 cm −1 for 9 belongs to the CN stretching mode (cal.2063.3 cm −1 ), which is largely blueshifted in comparison with the same mode in OPNC (2022.0cm −1 ).Two weak bands at 1159.8 and 718.3 cm −1 for the PO 2 symmetric and PN stretching modes in O 2 PNC are observable in the experiment, and they agree with the calculations of 1165.1 and 720.9 cm −1 , respectively.The other weaker band of O 2 PNC with calculated data of 1477.3 cm −1 is masked by the band of O 2 PCN at 1472.1 cm −1 .Additionally, the rearrangement of C 2 H 4 O 2 PCN (1) to C 2 H 4 O 2 PNC (18) also occurs upon the laser irradiation at 193 nm (Figure 4(B) and Figure
(B)), a significant amount of CO 2 (Figure 5(C)) forms with simultaneous depletion of O 2 PCN (5) in CO ice upon further 193 nm laser irradiation, implying photolytic reduction of O 2 PCN by CO (O 2 PCN + CO → OPCN + CO 2 ).A similar transformation has been recently found for the structurally related chlorine compound O 2 PCl (O 2 PCl + CO → OPCl + CO 2 ) in solid CO (Jiang et al. 2022).The absence of any CO-trapping products during the photochemistry of O 2 PCN and OPCN in

Figure 5 .
Figure 5. IR spectrum of CO-matrix-isolated pyrolysis products of C 2 H 4 O 2 PCN at 16 K (panel (A)).IR difference spectra reflecting the sequential changes of CO matrix containing the pyrolysis products of C 2 H 4 O 2 PCN upon irradiation at 365 nm (12 minutes, 24 W, panel (B)) and 193 nm (19 minutes, 3 mJ, 3 Hz, panel (C)) at 16 K.Expanded spectrum B in the range of 663-655 cm −1 (panel (D)).The IR bands of unknown species ( * ) are labeled.

Table 1
Observed and Calculated IR Data of [O, C, N, P] Isomers

Table 2
Observed and Calculated IR Data for [2O, C, N, P] Isomers