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A novel heterosubstituted carbene spontaneously binds CO2 in a tunneling reaction under cryogenic conditions.

Simple imines and diimines are common building blocks in organic synthesis, but the compound class is spectroscopically not well characterized. Herein we report the formation of the simplest diimine, namely 1,2-diiminoethane, as well as spectroscopic characterization by cryogenic matrix isolation IR and UV/Vis spectroscopy. Three conformers of 1,2-diiminoethane form after UV irradiation of 1,2-diazidoethane by N more...

Oxirenes—highly strained 4π Hückel antiaromatic organics—have been recognized as key reactive intermediates in the Wolff rearrangement and in interstellar environments. Predicting short lifetimes and tendency toward ring opening, oxirenes are one of the most mysterious classes of organic transients, with the isolation of oxirene ( c -C 2 H 2 O) having remained elusive. Here, we report on the preparation of oxirene in low-temperature methanol-acetaldehyde matrices upon energetic processing through isomerization of ketene (H 2 CCO) followed by resonant energy transfer of the internal energy of oxirene to the vibrational modes (hydroxyl stretching and bending, methyl deformation) of methanol. Oxirene was detected upon sublimation in the gas phase exploiting soft photoionization coupled with a reflectron time-of-flight mass spectrometry. These findings advance our fundamental understanding of the chemical bonding and stability of cyclic, strained molecules and afford a versatile strategy for the synthesis of highly ring-strained transients in extreme environments. more...

Glycolaldehyde is considered the entry point in the aqueous prebiotic formose (Butlerow) reaction although it mainly exists in its unreactive hydrated form in aqueous solution. The characterization of the more reactive nucleophilic enol form under interstellar conditions has remained elusive to date. Here we report on the identification of glycolaldehyde enol (1,2-ethenediol, HOHC═CHOH) in low temperature methanol-bearing ices at temperatures as low as 5 K. Exploiting isotope labeling and isomer-selective photoionization coupled with reflectron time-of-flight mass spectrometry, our results unravel distinct reaction pathways to 1,2-ethenediol, thus demonstrating the kinetic stability, availability for prebiotic sugar formation, and potential detectability in deep space. more...

Tri-tert-butylphosphatetrahedrane (1) is shown here to act as a synthon of isomeric tri-tert-butylphosphacyclobutadiene in the presence of a Lewis acid or transition-metal complex. When it is combined with a substoichiometric amount of triphenylborane, compound 1 forms a ladderane-type dimer of tri-tert-butylphosphacyclobutadiene in 72% isolated yield. Trapping of a generated intermediate was achieved by repeating the experiment in the presence of excess styrene (20 equiv) or ethylene (1 atm), and the corresponding [4 + 2] cycloadducts of tri-tert-butylphosphacyclobutadiene were isolated in 88% and 74% yields, respectively. The platinum complex (Ph3P)2Pt(C2H4) also reacts with 1 to form an orange η2 complex of tri-tert-butylphosphacyclobutadiene in 80% isolated yield. Additionally, we report a novel method for generating a phosphinidenoid species via fluoride-induced trimethylsilyl fluoride elimination, leading to an improved preparative procedure for 1 (182 mg, 33% isolated yield). more...

The search for methods to bind CO

A non-ideal contact at the electrode/solid electrolyte interface of a solid-state battery arising due to pores (voids) or inclusions results in a geometric constriction effect that severely deteriorates the electric transport properties of the battery cell. The lack of understanding of this phenomenon hinders the optimization process of novel components, such as reversible and high-rate metal anodes. Deeper insight into the constriction phenomenon is necessary to correctly monitor interface degradation and to accelerate the successful use of metal anodes in solid-state batteries. Here, we use a 3D electric network model to study the fundamentals of the constriction effect. Our findings suggest that dynamic constriction as a non-local effect cannot be captured by conventional 1D equivalent circuit models and that its electric behavior is not ad hoc predictable. It strongly depends on the interplay of the geometry of the interface causing the constriction and the microscopic transport processes in the adjacent phases. In the presence of constriction, the contribution from the non-ideal electrode/solid electrolyte interface to the impedance spectrum may exhibit two signals that cannot be explained when the porous interface is described by a physical-based (effective medium theory) 1D equivalent circuit model. In consequence, the widespread assumption of a single interface contribution to the experimental impedance spectrum may be entirely misleading and can cause serious misinterpretation. more...

In an all-solid-state battery, the electrical contact between its individual components is of key relevance in addition to the electrochemical stability of its interfaces. Impedance spectroscopy is particularly suited for the non-destructive investigation of interfaces and of their stability under load. Establishing a valid correlation between microscopic processes and the macroscopic impedance signal, however, is challenging and prone to errors. Here, we use a 3D electric network model to systematically investigate the effect of various electrode/sample interface morphologies on the impedance spectrum. It is demonstrated that the interface impedance generally results from a charge transfer step and a geometric constriction contribution. The weights of both signals depend strongly on the material parameters as well as on the interface morphology. Dynamic constriction results from a non-ideal local contact, e.g., from pores or voids, which reduce the electrochemical active surface area only in a certain frequency range. Constriction effects dominate the interface behavior for systems with small charge transfer resistance like garnet-type solid electrolytes in contact with a lithium metal electrode. An in-depth analysis of the origin and the characteristics of the constriction phenomenon and their dependence on the interface morphology is conducted. The discussion of the constriction effect provides further insight into the processes at the microscopic level, which are, e.g., relevant in the case of reversible metal anodes. more...

The species on the C3H2O potential energy surface have long been known to play a vital role in extraterrestrial chemistry. Here we report on the hitherto uncharacterized isomer ethynylhydroxycarbene (H-C≡C-C-OH, 1) generated by high-vacuum flash pyrolysis of ethynylglyoxylic acid ethyl ester and trapped in solid argon matrices at 3 and 20 K. Upon irradiation at 436 nm trans-1 rearranges to its higher lying cis-conformer. Prolonged irradiation leads to the formation of propynal. When the matrix is kept in the dark, 1 reacts within a half-life of ca. 70 h to propynal in a conformer-specific [1,2]H-tunneling process. Our results are fully consistent with computations at the CCSD(T)/cc-pVTZ and the B3LYP/def2-QZVPP levels of theory. more...

The identification of silicon-substituted, complex organics carrying multiple functional groups via classical infrared spectroscopy is challenging because the group frequencies of functional groups often overlap. Photoionization (PI) reflectron time-of-fight mass spectrometry (ReTOF-MS) in combination with temperature programmed desorption (TPD) holds certain advantages since molecules are identified after sublimation from the matrix into in the gas phase based on distinct ionization energies and sublimation temperatures. In this study, we reveal the detection of 1-sila glycolaldehyde (HSiOCH 2 OH), 2-sila acetic acid (SiH 3 COOH), and 1,2-disilaacetaldehyde (SiH 3 SiHO) - the silicon analogues of the well-known glycolaldehyde (HCOCH 2 OH), acetic acid (CH 3 COOH), and acetaldehyde (CH 3 CHO), in the gas phase after preparation in silane (SiH 4 ) - carbon dioxide (CO 2 ) ices exposed to energetic electrons and subliming the neutral reaction products formed within the ices into the gas phase. more...

We present the first formation of the previously elusive phosphine imide (HNPH3) along with its phosphinous amide (H2N–PH2) isomer via exposure of phosphine (PH3) and ammonia (NH3) ices to ionizing radiation. Our approach may be extended to prepare, separate, and detect highly reactive compounds such as intermediates of Wittig reactions. more...

The simplest α,β-unsaturated sulfinyl radical CH2[double bond, length as m-dash]C(H)SO˙ has been generated in the gas phase by high-vacuum flash pyrolysis (HVFP) of sulfoxide CH2[double bond, length as m-dash]C(H)S(O)CF3 at ca. 800 °C. Two planar cis and trans conformers of CH2[double bond, length as m-dash]C(H)SO˙ were isolated in cryogenic matrixes (N2, Ne, and Ar) and characterized with IR and UV/Vis spectroscopy. In addition to the photo-induced cis ⇋ trans conformational interconversion, CH2[double bond, length as m-dash]C(H)SO˙ displays complex photochemistry. Upon irradiation with a purple light LED (400 nm), CH2[double bond, length as m-dash]C(H)SO˙ isomerizes to novel radicals CH3SCO˙, ˙CH2SC(O)H, and ˙CH2C(O)SH with concomitant dissociation to a caged molecular complex CH3S˙CO. Subsequent UV-laser (266 nm) irradiation causes fragmentation to ˙CH3/OCS and additional formation of an elusive carbonyl radical CH3C(O)S˙, which rearranges to ˙CH2C(O)SH upon further UV-light irradiation (365 nm). The vibrational data and bonding analysis of the two conformers of CH2[double bond, length as m-dash]C(H)SO˙ suggest that both are floppy radicals in which the unpaired electron conjugates with the vicinal π(C[double bond, length as m-dash]C) bond, leading to significant contribution of the canonical resonance form of ˙CH2-C(H)SO. The mechanism for the isomerization of CH2[double bond, length as m-dash]C(H)SO˙ is discussed based on the observed intermediates along with a computed potential energy profile at the CCSD(T)-F12a/aug-cc-pVTZ//B3LYP/6-311++G(3df,3pd) level of theory. more...

Summary Pyruvic acid represents a key molecule in prebiotic chemistry to form metabolites and amino acids. Without liquid water on the early Earth, endogenous formation of pyruvic acid is unlikely, and an exogenous delivery constitutes an appealing alternative. However, despite the detection of more than 200 molecules in space, pyruvic acid is elusive. Here, we describe its formation by barrierless recombination of hydroxycarbonyl (HOCO⋅) and acetyl (CH3CO⋅) radicals in ices of acetaldehyde (CH3CHO) and carbon dioxide (CO2) modeling interstellar conditions driven by cosmic rays. Exploiting isotopically labeled ices and photoionization reflectron time-of-flight mass spectrometry, the reaction products were selectively photoionized in the temperature-programmed desorption phase and isomers discriminated based on their ionization energies. This reveals a key reaction pathway for pyruvic acid synthesis through non-equilibrium reactions in interstellar cold molecular clouds and star-forming regions, thus offering a unique entry point to abiotic organic synthesis in deep space. more...

The tetrahedral 1,2,3-triphospha-4-azatricyclo [1.1.0.0 2,4 ] butane (P 3 N) molecule—an isovalent species of phosphorus (P 4 )—was prepared in low-temperature (5 K) phosphine-nitrogen ices and was identified in the gas phase through isomer-selective, tunable, soft photoionization reflectron time-of-flight mass spectrometry. Theoretical calculations reveal that the substitution of a single phosphorus atom by nitrogen in the P 4 molecule results in enhanced spherical aromaticity while simultaneously increasing the strain energy from 74 to 195 kJ mol −1 . In P 3 N, the P─P bond is shortened compared to those in P 4 by 3.6 pm, while the P─N─P bond angle of 73.0° is larger by 13.0° compared to the P─P─P bond angle of 60.0° in P 4 . The identification of tetrahedral P 3 N enhances our fundamental understanding of the chemical bonding, electronic structure, and stability of binary, interpnictide tetrahedral molecules and reveals a universal route to prepare ring strained cage molecules in extreme environments. more...

Cage-opening reactions of the highly strained tri

The hitherto elusive 1

11-Azido-9,10-dihydro-9,10-phosphanoanthracene (N

We present the first spectroscopic identification of hitherto unknown 1,1‐ethenediol, the enol tautomer of acetic acid. The title compound was generated in the gas phase through flash vacuum pyrolysis of malonic acid at 400 °C. The pyrolysis products were subsequently trapped in argon matrices at 10 K and characterized spectroscopically by means of IR and UV/Vis spectroscopy together with matching its spectral data with computations at the CCSD(T)/cc‐pCVTZ and B3LYP/6–311++G(2d,2p) levels of theory. Upon photolysis at λ=254 nm, the enol rearranges to acetic acid and ketene., Finally made: The hitherto unreported 1,1‐ethenediol, the higher energy tautomer of acetic acid, was synthesized and spectroscopically characterized. It can be readily prepared through thermal decarboxylation of malonic acid, as has generally often been implied in the synthetically highly valuable reactions of 1,3‐dicarboxylic acids. more...

Wir berichten über die erste spektroskopische Identifizierung des bisher unbekannten 1,1-Ethendiol, dem Enol-Tautomer der Essigsäure. Das Enol wurde durch Hochvakuumblitzpyrolyse bei 400 °C aus Malonsäure in der Gasphase dargestellt. Alle Zersetzungsprodukte wurden anschließend in kryogenen Argonmatrices bei 10 K isoliert und mittels IR- und UV/Vis-Spektroskopie charakterisiert. Alle experimentell gewonnenen Spektren wurden mit berechneten AE-CCSD(T)/cc-pCVTZ und B3LYP/6–311++G(2d,2p) Spektren abgeglichen. Nach Belichtung mit λ=254 nm lagert das Enol zur Essigsäure um oder wird zu Keten und Wasser zersetzt. more...

Small molecules solely consisting of H, N, O, and S are highly relevant intermediates in atmospheric chemistry and biology. Even though several isomers of [HNO2S] have been computationally predicted, only the IR spectra for the two lowest-energy isomers HNSO2 and syn–syn HONSO have been previously reported. Herein, the photochemistry (193 nm laser) of HNSO2 in N2-, Ne-, and Ar-matrices (≤15 K) has been studied. Aside from syn–syn HONSO, several new isomers including anti–syn HONSO, gauche–syn HOSNO, syn HOS(O)N, anti HOS(O)N, syn HS(O)NO, anti HN(O)SO, gauche–syn HSONO, and an elusive caged-radical pair HOS˙⋯˙NO have been identified. Additionally, the formation of fragments HONO, HO˙, ˙NO, and ˙NO2 has also been observed. The characterization of these species with matrix-isolation IR and UV/Vis spectroscopy is supported by 15N-labeling and quantum chemical computations at the B3LYP/6-311++G(3df,3pd) level. Furthermore, the photo-induced isomerization reactions, including the conformational conversion of syn–syn HONSO → anti–syn HONSO and reversible isomerization of HOSNO ↔ anti–syn HONSO, syn–syn HONSO ↔ HN(O)SO, HSONO ↔ HS(O)NO, and HOS˙⋯˙NO ↔ HOSNO have also been observed, and the underlying mechanism is discussed. more...

We present a novel concept for the in situ control of site-selectivity of catalytic acetylations of partially protected sugars using light as external stimulus and oligopeptide catalysts equipped with an azobenzene moiety. The isomerizable azobenzene-peptide backbone defines the size and shape of the catalytic pocket, while the π-methyl-l-histidine (Pmh) moiety transfers the electrophile. Photoisomerization of the more...

We present a technique for systematically investigating electronic and ionic charge transport in single Li(Ni1/3Co1/3Mn1/3)O2 (NCM 111) secondary particles as a function of size. We perform electro... more...

Carbonyl-bearing complex organic molecules (COMs) in the interstellar medium (ISM) are of significant importance due to their role as potential precursors to biomolecules. Simple aldehydes and ketones like acetaldehyde, acetone, and propanal have been recognized as fundamental molecular building blocks and tracers of chemical processes involved in the formation of distinct COMs in molecular clouds and star-forming regions. Although previous laboratory simulation experiments and modeling established the potential formation pathways of interstellar acetaldehyde and propanal, the underlying formation routes to the simplest ketone—acetone—in the ISM are still elusive. Herein, we performed a systematic study to unravel the synthesis of acetone, its propanal and propylene oxide isomers, as well as the propenol tautomers in interstellar analog ices composed of methane and acetaldehyde along with isotopic-substitution studies to trace the reaction pathways of the reactive intermediates. Chemical processes in the ices were triggered at 5.0 K upon exposure to proxies of Galactic cosmic rays in the form of energetic electrons. The products were detected isomer-selectively via vacuum ultraviolet (VUV) photoionization reflectron time-of-flight mass spectrometry. In our experiments, the branching ratio of acetone (CH3COCH3):propylene oxide (c-CH3CHOCH2):propanal (CH3CH2CHO) was determined to be (4.82 ± 0.05):(2.86 ± 0.13):1. The radical–radical recombination reaction leading to acetone emerged as the dominant channel. The propenols appeared only at a higher radiation dose via keto–enol tautomerization. The current study provides mechanistic information on the fundamental nonequilibrium pathways that may be responsible for the formation of acetone and its (enol) isomers inside the interstellar icy grains. more...

The rotational spectra of the singly substituted carbon-13 isotopic species of six sulphur-terminated carbon chains have been detected between 5 and 40 GHz using a supersonic discharge jet source i... more...

Phosphorus mononitride (PN) only has a fleeting existence on Earth, and molecular precursors for the release of this molecule under mild conditions in solution have remained elusive. Here we report the synthesis of an anthracene-based precursor-an anthracene moiety featuring an azidophosphine bridge across its central ring-that dissociates into dinitrogen, anthracene and P≡N in solution with a first-order half-life of roughly 30 min at room temperature. Heated under reduced pressure, this azidophosphine-anthracene precursor decomposes in an explosive fashion at around 42 °C, as demonstrated in a molecular-beam mass spectrometry study. The precursor is also shown to serve as a PN transfer reagent in the synthesis of an Fe-NP coordination complex, through ligand exchange with its Fe-N more...

Phosphorus mononitride (PN) only has a fleeting existence on Earth and molecular precursors for a mild release of the molecule to enable chemical synthesis in solution do not exist. Here we report the synthesis of an anthracene (A) based molecular precursor (N3PA) that dissociates into dinitrogen (N2), A and PN in solution with a first order half-life of roughly t 1/2 = 30 min at room temperature associated with an activation enthalpy of ΔH ⧧ = 19.5 ± 1.7 kcal mol –1 and an activation entropy of ΔS ⧧ = −8.8 ± 0.8 cal mol –1 K –1 . Heated under vacuum N3PA decomposes in an explosive fashion at around 42 °C as demonstrated in a molecular beam mass spectrometry (MBMS) study. N3PA serves as a PN transfer reagent as demonstrated in the synthesis of [(dppe)Fe(Cp*)(NP)][BArF24]. Surprisingly, the terminal N-bonded linkage isomer is energetically preferred due to significant covalent iron pnictogen bond character and associated less unfavorable Pauli repulsion in the metal-ligand interaction. more...

Enols have emerged as critical reactive intermediates in combustion processes and in fundamental molecular mass growth processes in the interstellar medium, but the elementary reaction pathways to enols in extreme environments, such as during the decomposition of molecular energetic materials, are still elusive. Here, we report on the original identification of the enol and keto isomers of oxy-s-triazine, as well as its deoxygenated derivative 1,3,5-triazine, formed in the photodecomposition processes of 1,3,5-trinitro-1,3,5-triazinane (RDX)-a molecular energetic material. The identification was facilitated by exploiting isomer-selective tunable photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS) in conjunction with quantum chemical calculations. The present study reports the first experimental evidence of an enol intermediate in the dissociation domain of a nitramine-based energetic material. Our investigations suggest that the enols like 1,3,5-triazine-2-ol could be the source of hydroxyl radicals, and their inclusion in the theoretical models is important to understand the unprecedented chemistry of explosive materials. more...

We present the first formation of the previously elusive phosphine imide (HN[double bond, length as m-dash]PH3) along with its phosphinous amide (H2N-PH2) isomer via exposure of phosphine (PH3) and ammonia (NH3) ices to ionizing radiation. Our approach may be extended to prepare, separate, and detect highly reactive compounds such as intermediates of Wittig reactions. more...

We present a comprehensive experimental study of a di-t-butyl-substituted cyclooctatetraene-based molecular balance to measure the effect of 16 different solvents on the equilibrium of folded versus unfolded isomers. In the folded 1,6-isomer, the two t-butyl groups are in close proximity (H···H distance ≈ 2.5 A), but they are far apart in the unfolded 1,4-isomer (H···H distance ≈ 7 A). We determined the relative strengths of these noncovalent intramolecular σ-σ interactions via temperature-dependent nuclear magnetic resonance measurements. The origins of the interactions were elucidated with energy decomposition analysis at the density functional and ab initio levels of theory, pinpointing the predominance of London dispersion interactions enthalpically favoring the folded state in any solvent measured. more...

Since the postulation of carbenes by Buchner (1903) and Staudinger (1912) as electron-deficient transient species carrying a divalent carbon atom, carbenes have emerged as key reactive intermediates in organic synthesis and in molecular mass growth processes leading eventually to carbonaceous nanostructures in the interstellar medium and in combustion systems. Contemplating the short lifetimes of these transient molecules and their tendency for dimerization, free carbenes represent one of the foremost obscured classes of organic reactive intermediates. Here, we afford an exceptional glance into the fundamentally unknown gas-phase chemistry of preparing two prototype carbenes with distinct multiplicities—triplet pentadiynylidene (HCCCCCH) and singlet ethynylcyclopropenylidene (c-C(5)H(2)) carbene—via the elementary reaction of the simplest organic radical—methylidyne (CH)—with diacetylene (HCCCCH) under single-collision conditions. Our combination of crossed molecular beam data with electronic structure calculations and quasi-classical trajectory simulations reveals fundamental reaction mechanisms and facilitates an intimate understanding of bond-breaking processes and isomerization processes of highly reactive hydrocarbon intermediates. The agreement between experimental chemical dynamics studies under single-collision conditions and the outcome of trajectory simulations discloses that molecular beam studies merged with dynamics simulations have advanced to such a level that polyatomic reactions with relevance to extreme astrochemical and combustion chemistry conditions can be elucidated at the molecular level and expanded to higher-order homolog carbenes such as butadiynylcyclopropenylidene and triplet heptatriynylidene, thus offering a versatile strategy to explore the exotic chemistry of novel higher-order carbenes in the gas phase. more...

The identification of silicon-substituted, complex organics carrying multiple functional groups by classical infrared spectroscopy is challenging because the group frequencies of functional groups often overlap. Photoionization (PI) reflectron time-of-fight mass spectrometry (ReTOF-MS) in combination with temperature-programmed desorption (TPD) holds certain advantages because molecules are identified after sublimation from the matrix into in the gas phase based on distinct ionization energies and sublimation temperatures. In this study, we reveal the detection of 1-silaglycolaldehyde (HSiOCH more...

With nearly 200 molecules detected in interstellar and circumstellar environments, the identification of the biologically relevant α-keto carboxylic acid, glyoxylic acid (HCOCOOH), is still elusive. Herein, the formation of glyoxylic acid via cosmic-ray driven, non-equilibrium chemistry in polar interstellar ices of carbon monoxide (CO) and water (H2 O) at 5 K via barrierless recombination of formyl (HCO) and hydroxycarbonyl radicals (HOCO) is reported. In temperature-programmed desorption experiments, the subliming neutral molecules were selectively photoionized and identified based on the ionization energy and distinct mass-to-charge ratios in combination with isotopically labeled experiments exploiting reflectron time-of-flight mass spectrometry. These studies unravel a key reaction path to glyoxylic acid, an organic molecule formed in interstellar ices before subliming in star-forming regions like SgrB2(N), thus providing a critical entry point to prebiotic organic synthesis. more...

S-Nitrosothiols (RSNO) are exogenous and endogenous sources of nitric oxide in biological systems due to facile homolytic cleavage of the S-N bonds. By following the photolytic decomposition of prototypical RSNO (R = Me and Et) in Ne, Ar, and N2 matrixes ( more...

We report the gas-phase preparation of cyanohydroxycarbene by high-vacuum flash pyrolysis of ethyl 2-cyano-2-oxoacetate and subsequent trapping of the pyrolysate in an inert argon matrix at 3 K. After irradiation of the matrix with green light for a few seconds singlet trans-cyanohydroxycarbene rearranges to its cis-conformer. Prolonged irradiation leads to the formation of cyanoformaldehyde and isomeric isocyanoformaldehyde. Cis- and trans-cyanohydroxycarbene were characterized by matching matrix IR and UV/Vis spectroscopic data with ab initio coupled cluster and TD-DFT computations. Trans-cyanohydroxycarbene undergoes a conformer-specific [1,2]H-tunnelling reaction through a 33.3 kcal mol−1 barrier (the highest penetrated barrier of all H-tunnelling reactions observed to date) to cyanoformaldehyde with a half-life of 23.5 ± 0.5 d; this is the longest half-life reported for an H-tunnelling process to date. During the tunnelling reaction the cis-conformer remains unchanged over the same period of time and the Curtin–Hammett principle does not apply. NIR irradiation of the O–H stretching overtone does not enhance the tunnelling rate via vibrational activation. Push–pull stabilisation of hydroxycarbenes through σ- and π-withdrawing groups therefore is even more stabilizing than push–push substitution. more...

Small molecules solely consisting of H, N, O, and S are highly relevant intermediates in atmospheric chemistry and biology. Even though several isomers of [HNO2S] have been computationally predicted, only the IR spectra for the two lowest-energy isomers HNSO2 and syn-syn HONSO have been previously reported. Herein, the photochemistry (193 nm laser) of HNSO2 in N2-, Ne-, and Ar-matrices (≤15 K) has been studied. Aside from syn-syn HONSO, several new isomers including anti-syn HONSO, gauche-syn HOSNO, syn HOS(O)N, anti HOS(O)N, syn HS(O)NO, anti HN(O)SO, gauche-syn HSONO, and an elusive caged-radical pair HOS˙˙NO have been identified. Additionally, the formation of fragments HONO, HO˙, ˙NO, and ˙NO2 has also been observed. The characterization of these species with matrix-isolation IR and UV/Vis spectroscopy is supported by 15N-labeling and quantum chemical computations at the B3LYP/6-311++G(3df,3pd) level. Furthermore, the photo-induced isomerization reactions, including the conformational conversion of syn-syn HONSO → anti-syn HONSO and reversible isomerization of HOSNO ↔ anti-syn HONSO, syn-syn HONSO ↔ HN(O)SO, HSONO ↔ HS(O)NO, and HOS˙˙NO ↔ HOSNO have also been observed, and the underlying mechanism is discussed. more...