S ince the first report by Curtius in 1890, 1,2 hydrazoic acid (HN 3 , 1, Figure 1), the simplest covalent azide, has received extensive attention from the synthetic (in)organic, 3,4 combustion chemistry, 5,6 physical chemistry, 7,8 and theoretical chemistry 9 communities due to its high detonation energy and potential application of liquid energy density materials. 10,11 Photodissociation experiments of hydrazoic acid at wavelengths shorter than 220 nm found that the cyclic isomer 1H-triazirine (c-HN 3 , 2, Figure 1)as a potential reaction intermediate to the cyclic-trinitrogen (c-N 3 ) radical. 12,13 This molecule, which has been inferred as a transient radical in the reaction between nitrous acid (HNO 2 ) and protonated hydrazine (N 2 H 5 + ), 14,15 represents a benchmark of a strained cyclic compound with a ring strain energy of 198 kJ mol -1 . 16 Theoretical computations predicted that 1H-triazirine (c-HN 3 , 2) is kinetically stable but thermodynamically less favorable by 158 kJ mol -1 with respect to hydrazoic acid (HN 3 , 1). The ring strain energy results in an energy release of up to 20 kJ g -1 , which is 1 order of magnitude higher than 2.18 kJ g -1 for trinitrotoluene (TNT). 17,18 However, 1H-triazirine (c-HN 3 , 2) has remained elusive to date due to the inherent difficulty in the synthesis and isolation of highly strained and explosive molecules.

In conjunction with Langmuir's concept of isovalency, 19 in which molecular entities with the same number of valence Figure 1. Molecular structures of HN 3 and HP 3 isomers. Bond lengths are given in picometers (pm) and bond angles in degrees; point groups, electronic ground states, computed adiabatic ionization energies corrected for the electric field effect (blue), and relative energies (red) are also shown. The energies were computed at the CCSD(T)/CBS//B3LYP/cc-pVTZ plus zero-point vibrational energies level of theory. The atoms are color coded in white (hydrogen), blue (nitrogen), and orange (phosphorus). Coordinates and normal modes are provided in Tables S5 andS6.

Letter pubs.acs.org/JPCL electrons are predicted to have similar chemistries and structures, particular attention has been devoted to the preparation of the isovalent counterparts of hydrazoic acid (HN 3 , 1) and 1H-triazirine (c-HN 3 , 2), in which all three nitrogen atoms (N) are replaced by isovalent phosphorus (P) atoms: 1H-triphosphirene (c-HP 3 , 3) 20,21 and 2-triphosphenylidene (HP 3 , 4, Figure 1). 1H-Triphosphirene (c-HP 3 , 3) has a ring strain energy of only 35 kJ mol -1 , which is significantly lower compared to 1H-triazirine (c-HN 3 , 2) of 198 kJ mol -1 16 and the tetrahedral phosphorus molecule (P 4 ) of 74 kJ mol -1 . This indicates that 1H-triphosphirene (c-HP 3 , 3) should be easier to prepare than its isovalent 1H-triazirine (c-HN 3 , 2) counterpart. However, although white phosphorus (P 4 ) can be transformed into the cyclo-P 3 ligand in the process of generating nickel(II) complexes, 22,23 the preparation of 1Htriphosphirene (c-HP 3 , 3) has remained a fundamental synthetic challenge due to the lack of preparative synthetic chemistry routes to isolate the cyclo-P 3 as σ-donor ligand. Consequently, 1H-triphosphirene (c-HP 3 , 3) along with its acyclic 2-triphosphenylidene (HP 3 , 4) isomer exemplifies one of the least explored classes of inorganic molecules.

Here, we report the first preparation of 1H-triphosphirene (c-HP 3 , 3) together with the 2-triphosphenylidene (HP 3 , 4) isomer in cryogenic phosphine (PH 3 )-dinitrogen (N 2 ) matrices exposed to energetic electrons at 5 K. Combined with electronic structure calculations, both isomers are unambiguously identified upon the temperature-programmed desorption (TPD) phase of the irradiated ices. This is achieved through isomer-selective photoionization in the gas phase accounting for the computed adiabatic ionization energies (IEs) of 3 and 4 (Figure 1) by taking advantage of vacuumultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS). Electronic structure calculations disclose that 1H-triphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4) can be prepared through decomposition of two triphosphirane (c-P 3 H 3 , 5, Figure S3) and 1-triphosphene (P 3 H 3 , 6, Figure S3) transients, respectively, via molecular dihydrogen loss. The very first preparation and detection of 1H-triphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4) document their gas-phase stability over at least 10 ± 1 μs. These findings not only revolutionize our fundamental knowledge on the chemical bonding and electronic structure of the strained, cyclic group XV molecule 1H-triphosphirene (c-HP 3 , 3) along with its acyclic isomer 2-triphosphenylidene (HP 3 , 4) but also afford an unconventional approach to prepare highly strained, still elusive molecules such as 1H-triazirine (c-HN 3 , 2) and possibly tetrahedral tetranitrogen (N 4 ) in the near future involving progressive nonequilibrium chemistries.

Fourier-transform infrared spectroscopy (FTIR) was applied to monitor the chemical evolution of the ices during the radiation exposure at 5.0 ± 0.1 K. The absorptions of phosphine were identified in the spectrum of the pristine ice with prominent fundamentals visible at, e.g., 2314 cm -1 (v 3 ), 1097 cm -1 (v 4 ), and 983 cm -1 (v 2 ). 24 The irradiation process produced two shoulders at 2270 and 1063 cm -1 and a distinct absorption at 788 cm -1 , which are associated with P-H stretching modes, PH 2 scissoring modes, and the deformation mode of phosphorus (P) and nitrogen (N) containing rings, 25 respectively. The substitution of N 2 by 15 N 2 shifted the 788 cm -1 peak to 784 cm -1 ; this suggests that the structural moiety associated with this absorption contains nitrogen (Table S4). However, since energetic electron irradiation can produce a wide inventory of new species, whose absorptions of the functional groups often overlap in the infrared regime, 26,27 infrared spectroscopy can determine newly formed f unctional groups but does not always allow identification of individual molecules in the case of complex mixtures. Therefore, an alternative method is required to probe discrete isomers selectively. This is accomplished by photoionizing the subliming molecules in the temperature-programmed desorption (TPD) phase by tunable vacuum-ultraviolet (VUV) photoionization and detecting the ions in a reflectron time-of-flight mass spectrometer (PI-ReTOF-MS) based on their arrival times on a multichannel plate. By tuning the photoionization energies (PEs) 28 above or below the isomer(s) of interest, specific isomers can be selectively ionized and hence identified based on their IEs (Figure 1). Considering the computed IEs of 1H-triphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4) of 8.81-8.96 and 8.33-8.48 eV, respectively, three PEs of 10.49, 8.53, and 8.20 eV are required. Photons at 10.49 eV can ionize both isomers 3 and 4; 8.53 eV photons can ionize only 4, whereas 8.20 eV photons ionize neither 3 nor 4. The temperature-dependent mass spectra collected at distinct photon energies along with the blank experiment are compiled in Figure 2, whereas the corresponding TPD profiles of the ionized target molecules at mass-to-charge ratios equal to 94 (m/z = 94, HP 3 + ) are visualized in Figure 3. At a photon energy of 10.49 eV (Figure 3A, black line; Figure S1), a broad sublimation event extending from 185 to 250 K can be observed in the TPD profile at m/z = 94; this profile has a maximum at 215 K. A control experiment replacing dinitrogen by 15-dinitrogen in the ices reveals no mass shift of this TPD profile (Figure S1), demonstrating that the carrier of the signal at m/z = 94 does not contain a nitrogen atom and, hence, can only be assigned to a molecule with the molecular formula HP 3 . When the photon energy was lowered to 8.53 eV, a photon energy that only allows 4 to be ionized, the sublimation profile changed significantly. Here, the TPD profile revealed low ion counts from 210 to 250 K peaking at 229 K (Figure 3A, red line). Tuning the photon energy even lower to 8.20 eV, no sublimation event was found at m/z = 94 (Figure 3A, blue line). This reveals that the sublimation event with 8.53 eV photons can be associated with isomer 4, which cannot be ionized at a photon energy of 8.20 eV. Deconvolution of the TPD profile with a bimodal Gaussian functional (Figure 3B) [29][30][31] reveals the integrated ratio of the ion counts for both sublimation events at 215 and 229 K to be (7.4 ± 0.1):1 at a photon energy of 10.49 eV. These findings provide compelling evidence for the formation and detection of both 1Htriphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4). It is critical to highlight that control experiments were also conducted in a fashion similar to that of the actual experiments, but without exposing the ices to energetic electrons in the TPD phase. No ion counts were observed at the m/z = 94 (Figure 3A, green line), revealing that the observed ion counts are linked to the energetic processing of the ice mixtures but not the result of potential ion-molecule chemistries in the subliming ices.

Having identified isomers 1H-triphosphirene (c-HP 3 , 3) together with the 2-triphosphenylidene (HP 3 , 4), we shift our attention to exploring possible formation pathways. Experiments performed with pure phosphine ices in the same experimental setup under identical experimental conditions prepared neither 3 nor 4. 24 However, both studies detected ion counts at m/z = 96 (P 3 H 3 + ), which can be linked to the triphosphirane (c-P 3 H 3 , 5) and 1-triphosphene (P 3 H 3 , 6) molecules. 24 Decomposition of these isomers via molecular dihydrogen loss could in principle lead via dehydrogenation to 1H-triphosphirene (c-HP 3 , 3) together with the 2-triphosphenylidene (HP 3 , 4). These reaction pathways are also supported by electronic structure calculations (Figure 4). Triphosphirane (c-P 3 H 3 , 5) and 1-triphosphene (P 3 H 3 , 6) exist in two conformers, namely syn and anti with the anti conformers as the energetically preferred form (Figure S3). All conformers are connected through five transition states located between 274.7 and 365.5 kJ mol -1 above the reactant to 1Htriphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4), respectively (Figure 4). The transition state can be overcome through the transfer of kinetic energy from the impinging electrons to triphosphirane (c-P 3 H 3 , 5) and 1triphosphene (P 3 H 3 , 6). This one-step molecular hydrogen loss pathway could be also replaced through two successive atomic hydrogen losses yielding eventually also 1H-triphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4). In the present experiments, the nitrogen molecules within the matrix represent an unconventional oxidizing agent being reduced to hydrazine (N 2 H 4 ); the formation of hydrazine (N 2 H 4 ) is evident from the detection of m/z = 32 (N 2 H 4 ) in the TPD phase of the experiments. Therefore, one of the fundamental differences of the phosphine 24 and phosphine-dinitrogen systems is the capability of molecular nitrogen to remove hydrogen via four successive addition steps 32 eventually forming hydrazine (N 2 H 4 ); this "hydrogen depleted" system supports the formation of 1H-triphosphirene (c-HP 3 , 3) together with 2-triphosphenylidene (HP 3 , 4), which were not detected in pure phosphine ices due to the "hydrogen-rich" environments in these matrices. 24 We are discussing now the geometric structures and chemical bonding of the newly detected 1H-triphosphirene (c-HP 3 , 3) and 2-triphosphenylidene (HP 3 , 4) isomers and comparing these to the isovalent to hydrazoic acid (HN 3 , 1) and 1H-triazirine (c-HN 3 , 2). The 1H-triphosphirene molecule has a C s point group and a 1 A′ electronic state (Figure 1). At the B3LYP/cc-pVTZ level of theory, the P-P bond lengths are computed to be 225.3 and 201.6 pm, and the P-H bond The Journal of Physical Chemistry Letters pubs.acs.org/JPCL Letter length is computed to be 143.0 pm, longer than the N-N and N-H bond lengths 154.5, 118.5, and 102.6 pm of the 1Htriazirine. The longer P-P bonds are assigned to single bonds while the shorter P-P bond is assigned to a double bond. 33 The bond angles of ∠P-P-P are 63.4°and 53.2°. The bond angle of ∠H-P-P is 97.8°, smaller than the bond angle ∠H-N-N 102.1°of 1H-triazirine which indicates the preference of phosphorus for a stronger pyramidalization due to a favorable orbital energy splitting, i.e., an energetically low σ out orbital, as well as more spatially diffuse orbitals. Our calculated strain energy of 1H-triphosphirene by using a series of homodesmotic equations at the CBS-QB3 level of theory agree with previously reported results from the literature (Figure 5A). 16 In a hypothetical reaction one molecule of c-HP 3 reacts with one molecule of H 2 P 2 and two molecules of H 4 P 2 to form one molecule of H 5 P 3 and two molecules of H 3 P 3 . As all bond types are retained during the reaction, the absolute value of the reaction enthalpy of -34.5 kJ mol -1 can be attributed to the strain energy in c-HP 3 (Figure 5A). This is significantly lower than the calculated strain energy in 1H-triazirine (192.0 kJ mol S7 andS8. ReTOF-MS). The enhanced stability of the cyclic 1Htriphosphirene (c-HP 3 , 3) isomer compared to 2-triphosphenylidene (HP 3 , 4) is also in line with evolution when substituting second-row atoms by third-row atoms of triatomic molecules carrying group XIV elements. This is best reflected in the (quasi)linear tricarbon molecule (C 3 , X 1 Σ g + ), 34 whereas the silicon dicarbide molecule (SiC 2 , X 1 A 1 ) is cyclic and partially aromatic. 35 Accounting for the distance between the photoionization laser and the ice surface of 2 mm and the average velocity of the isomers subliming in the range 185-215 K of 195 m s -1 , the lifetime of the neutral molecules has to exceed 10 ± 1 μs, while the corresponding molecular ions have to "live" for at least 44 ± 1 μs to survive the flight time from the ionization region to the detector of the ReTOF-MS. These discoveries not only transform our fundamental understanding of the electronic structure and chemical bonding of the cyclic, strained main group molecules but also provide an original strategy to "make" highly strained, still obscure molecules such as 1H-triazirine (c-HN 3 , 2) and tetrahedral tetranitrogen (N 4 ) in the near future through nonequilibrium chemistries.

■ ASSOCIATED CONTENT * sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.2c00639.

Experimental and computational methods details; Figure S1: temperature-programmed desorption (TPD) profile recorded at m/z = 94 from the electron processed phosphine (PH 3 )-dinitrogen (N 2 ) and phosphine (PH 3 )-15-dinitrogen ( 15 N 2 ) ices via photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) at a photon energy of 10.49 eV; Figure S2: temperature-programmed desorption (TPD) profile of HP 3 at a photo energy of 10.49 eV; Figure S3: molecular structures of P 3 H 3 isomers; Table S1: data were used to calculate the average irradiation dose per molecule; Table S2: parameters for the vacuum-ultraviolet light generation used in the present experiments; Table S3: statistical branching ratios for the reaction of the silicon atom with phosphine; Table S3: computed adiabatic ionization energies of distinct HN 3 and HP 3 isomers along with error limits; Table S4: infrared absorption peaks before and after irradiation for phosphine (PH 3 )dinitrogen (N 2 )/15-dinitrogen ( 15 N 2 ) ices; and Tables S5-S8: Cartesian coordinates, vibrational frequencies, and intensity for selected structures of HN 3 , HP 3 , P 3 H 3 , and transition state (TS) structures of P 3 H 3 (PDF)

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