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A high temperature phenyl-mediated addition-cyclization-dehydrogenation mechanism to form peri-fused polycyclic aromatic hydrocarbon (PAH) derivatives-illustrated through the formation of dibenzo[e,l]pyrene (C24H14)-is explored through a gas-phase reaction of the phenyl radical (C6H5Ë) with triphenylene (C18H12) utilizing photoelectron photoion coincidence spectroscopy (PEPICO) combined with electronic structure calculations. Low-lying vibrational modes of dibenzo[e,l]pyrene exhibit out-of-plane bending and are easily populated in high temperature environments such as combustion flames and circumstellar envelopes of carbon stars, thus stressing dibenzo[e,l]pyrene as a strong target for far-IR astronomical surveys.
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The bimolecular gas-phase reaction of ground-state atomic carbon (C(3Pj)) with disilane (Si2H6, X1A1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol-1. Two channels were observed: a molecular hydrogen elimination plus Si2CH4 (reaction 1) pathway and a silane loss channel along with the formation of SiCH2 (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si2CH6 reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si2CH4 and SiCH2 to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si2C) observed recently in the circumstellar shell of IRC + 10216.
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1-Germavinylidene (H2CGe; X1A1), the germanium analogue of vinylidene (H2CC; X1A1), was prepared via a directed gas-phase synthesis through the bimolecular reaction of ground state atomic carbon (C; 3P) with germane (GeH4; X1A1) under single-collision conditions. The reaction commences with the barrierless insertion of carbon into the Ge-H bond followed by intersystem crossing from the triplet to singlet surface and migration of atomic hydrogen to germylene (H2GeCH2), which predominantly decomposes via molecular hydrogen loss to 1-germavinylidene (H2CGe; X1A1). Therefore, the replacement of a single carbon atom in the acetylene-vinylidene system by germanium critically impacts the chemical bonding, molecular structure, and thermodynamic stability of the carbene-type structures favoring 1-germavinylidene (H2CGe) over germyne (HGeCH) by 160 kJ mol-1. Hence, the carbon-germane system represents a benchmark in the exploration of the chemistries of main group 14 elements with germanium-bearing systems showing few similarities with the isovalent carbon system.
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The strong tendency to stack in the solid state and rich luminescence for the Pt(II) complexes makes them potential candidates as new mechanochromic materials and sensing applications. Six mononuclear complexes [Pt(ppy)(O4NCS2)] (1), [Pt(bpy)(O4NCS2)]ClO4 (2), [Pt(ppy)(O5NCS2)] (3), [Pt(phen)(O4NCS2)]ClO4·CH3OH (5a), [Pt(phen)(O4NCS2)]ClO4 (5b), and [Pt(phen)(O5NCS2)]ClO4 (6a), one dinuclear complex [Pt2(phen)2(NaO5NCS2)2(ClO4)3]ClO4 (6b), and one one-dimensional (1-D) coordination polymer {[Pt2(bpy)2(NaO5NCS2)2(ClO4)2](ClO4)2}n (4) were synthesized by reacting [Pt(ppy)Cl]2, Pt(bpy)Cl2, and Pt(phen)Cl2 (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline) with (1-aza-15-crown-5)dithiocarbamate (O4NCS2) or (1-aza-18-crown-6)dithiocarbamate (O5NCS2), respectively, which have been isolated and structurally characterized by X-ray diffraction. Neutral complexes 1 and 3 contain no intermolecular Pt(II)···Pt(II) contact, whereas cationic complexes 2, 5a, 5b, and 6a with ClO4- as counteranions show alternative intermolecular Pt(II)···Pt(II) contacts of 3.535/4.091, 3.480/5.001, 3.527/4.571, and 3.446/4.987 Å in the solid state, respectively. Interestingly, complex 4 forms a 1-D coordination polymer through coordination between the encapsulated Na+ ions inside the azacrown ether rings of O5NCS2 and ClO4- anions with respective intra- and intermolecular Pt(II)···Pt(II) contacts of 3.402 and 3.847 Å in crystal lattices, whereas a dinuclear complex 6b was surprisingly formed and also connected by the encapsulated Na+ ions and ClO4- anions with alternative intra- and intermolecular Pt(II)···Pt(II) contacts of 3.650 and 3.677/4.4.372 Å, respectively. Upon excitation, complexes 1 and 3 showed similar vibronic luminescence at 507, 534, and 502, 532 nm, respectively, and the other complexes 2 and 4-6 showed broad luminescence with maxima at 537-567 nm. The B3LYP/LanL2DZ calculation was carried out and used to clarify their excited-state properties. In addition, the powder samples for complexes 1-4 almost showed no energy shift for the luminescence and significantly those of complexes 5-6 exhibited the mechanochromic luminescence upon grinding. It is noted that complexes 5a and 6a only showed minor red shifts (i.e., from 544 to 556 nm for complex 5a and from 551 to 565 nm for complex 6a), whereas complex 6b exhibited a remarkable red shift from 558 to 603 nm upon grinding. Besides, their luminescence reversibility was also examined toward various solvents.
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Reactions in interstellar ices are shown to be capable of producing key prebiotic molecules without energetic radiation that are necessary for the origins of life. When present in interstellar ices, carbamic acid (H2NCOOH) can serve as a condensed-phase source of the molecular building blocks for more complex proteinogenic amino acids. Here, Fourier transform infrared spectroscopy during heating of analogue interstellar ices composed of carbon dioxide and ammonia identifies the lower limit for thermal synthesis to be 62 ± 3 K for carbamic acid and 39 ± 4 K for its salt ammonium carbamate ([H2NCOO-][NH4+]). While solvation increases the rates of formation and decomposition of carbamic acid in ice, the absence of solvent effects after sublimation results in a significant barrier to dissociation and a stable gas-phase molecule. Photoionization reflectron time-of-flight mass spectrometry permits an unprecedented degree of sensitivity toward gaseous carbamic acid and demonstrates sublimation of carbamic acid from decomposition of ammonium carbamate and again at higher temperatures from carbamic acid dimers. Since the dimer is observed at temperatures up to 290 K, similar to the environment of a protoplanetary disk, this dimer is a promising reservoir of amino acids during the formation of stars and planets.
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We investigated the formation of small organosilicon moleculesâpotential precursors to silicon-carbide dust grains ejected by dying carbon-rich asymptotic giant branch starsâin the gas phase via the reaction of atomic carbon (C) in its 3P electronic ground state with silane (SiH4; X1A1) using the crossed molecular beams technique. The reactants collided under single collision conditions at a collision energy of 13.0 ± 0.2 kJ mol-1, leading to the formation of the silylenemethyl radical (HCSiH2; X2B2) via the unimolecular decomposition of triplet silaethylene (H2CSiH2; a3Aâ³). The silaethylene radical was formed via hydrogen migration of the triplet silylmethylene (HCSiH3; X3Aâ³) radical, which in turn was identified as the initial collision complex accessed via the barrierless insertion of atomic carbon into the silicon-hydrogen bond of silane. Our results mark the first observation of the silylenemethyl radical, where previously only its thermodynamically more stable methylsilylidyne (CH3Si; X2Aâ³) and methylenesilyl (CH2SiH; X2A') isomers were observed in low-temperature matrices. Considering the abundance of silane and the availability of atomic carbon in carbon-rich circumstellar environments, our results suggest that future astrochemical models should be updated to include contributions from small saturated organosilicon molecules as potential precursors to pure gaseous silicon-carbides and ultimately to silicon-carbide dust.
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We report the results on the combustion of single, levitated droplets of exo-tetrahydrodicyclopentadiene (JP-10) doped with titanium-aluminum-boron (Ti-Al-B) reactive metal nanopowders (RMNPs) in an oxygen (60%)-argon (40%) atmosphere by exploiting an ultrasonic levitator with droplets ignited by a carbon dioxide laser. Ultraviolet-visible (UV-vis) emission spectroscopy revealed the presence of gas-phase aluminum (Al) and titanium (Ti) atoms. These atoms can be oxidized in the gas phase by molecular oxygen to form spectroscopically detected aluminum monoxide (AlO) and titanium monoxide (TiO) transients. Analysis of the optical ignition videos supports that the nanoparticles are ignited before JP-10. The detection of boron monoxide (BO) further proposes an active surface chemistry through the oxidation of the RMNPs and the release of at least BO into the gas phase. The oxidation of gas-phase BO by molecular oxygen to boron dioxide (BO2) plus atomic oxygen might operate in the gas phase, although the involvement of surface oxidation processes of RMNPs to BO2 cannot be discounted. The UV-vis emission spectra also revealed the key reactive intermediates (OH, CH, C2, and HCO) of the oxidation of JP-10. Electronic structure calculations reveal that the presence of reactive radicals has a profound impact on the oxidation of JP-10. Although titanium monoxide (TiO) reacts to produce titanium dioxide (TiO2), it does not engage in an active JP-10 chemistry as all abstraction pathways are endoergic by more than 217 kJ mol-1. This is similar for atomic aluminum and titanium, whose hydrogen abstraction reactions from JP-10 were revealed to be endoergic by at least 77 kJ mol-1. Therefore, aluminum and titanium react preferentially with molecular oxygen to produce their monoxides. However, the formation of BO, AlO, and BO2 supplies a pool of highly reactive radicals, which can abstract hydrogen from JP-10 via transition states ranging from only 1 to 5 kJ mol-1 above the separated reactants, forming JP-10 radicals along with the hydrogen abstraction products (boron hydride oxide, aluminum monohydroxide, and metaboric acid) in the overall exoergic reactions. These abstraction barriers are well below the barriers of abstractions for ground-state atomic oxygen and molecular oxygen. In this sense, gas-phase BO, AlO, and BO2 catalyze the oxidation of gas-phase JP-10 via hydrogen abstraction, forming highly reactive JP-10 radicals. Overall, the addition of RMNPs to JP-10 not only provides a higher energy density fuel but is also expected to lead to shorter ignition delays compared to pure JP-10 due to the highly reactive pool of radicals (BO, AlO, and BO2) formed in the initial stage of the oxidation process.
RESUMEN
The hitherto elusive monobridged Ge(µ-H)GeH (X1 A') molecule was prepared in the gas phase by bimolecular reaction of atomic germanium with germane (GeH4 ). Electronic structure calculations revealed that this reaction commenced on the triplet surface with the formation of a van der Waals complex, followed by insertion of germanium into a germanium-hydrogen bond over a submerged barrier to form the triplet digermanylidene intermediate (HGeGeH3 ); the latter underwent intersystem crossing from the triplet to the singlet surface. On the singlet surface, HGeGeH3 predominantly isomerized through two successive hydrogen shifts prior to unimolecular decomposition to Ge(µ-H)GeH isomer, which is in equilibrium with the vinylidene-type (H2 GeGe) and dibridged (Ge(µ-H2 )Ge) isomers. This reaction leads to the formation of cyclic dinuclear germanium molecules, which do not exist on the isovalent C2 H2 surface, thus deepening our understanding of the role of nonadiabatic reaction dynamics in preparing nonclassical, hydrogen-bridged isomers carrying main group XIV elements.
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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.
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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.
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Small silicon hydrides have attracted extensive interest because of their role in the chemical evolution of circumstellar envelopes of evolved carbon stars and applications in surface growth processes and as transients in semiconductor manufacturing. Combined with electronic structure calculations, we demonstrate that monobridged silylidynesilylenes [(Si(µ-D)SiH2, Si(µ-H)SiHD, Si(µ-H)SiH2] and silylsilylidyne [H3SiSi, H2DSiSi], which are nearly isoenergetic, can be prepared via molecular hydrogen loss channels in the crossed molecular beam study of the reaction of D1-silylidyne (SiD; X2Π) with silane (SiH4; X1A1) in a crossed molecular beams machine. Compared to the dynamics of the isovalent methylidyne (CH) - methane (CH4) system, our study delivers a unique view at the intriguing isomerization processes and reaction dynamics of dinuclear silicon hydride transients, thus contributing to our knowledge on the chemical bonding of silicon hydrides at the molecular level.
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The hitherto elusive oxaziridine molecule (cyclo-H2CONH) - an optically active, high energy isomer of nitrosomethane (CH3NO) - is prepared in processed methane-nitrogen monoxide ices and detected upon sublimation in the gas phase. Electronic structure calculations reveal likely routes via addition of carbene (CH2) to the nitrogen-oxygen double bond of nitrosyl hydride (HNO). Our findings provide a fundamental framework to explore the preparation and stability of racemic oxaziridines exploited in chiral substrate-controlled diastereoselective preparation such as Sharpless asymmetric epoxidation, thus advancing our fundamental understanding of the preparation and chemical bonding of strained rings in small organic molecules.
RESUMEN
The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; X2Π) with disilane (Si2H6; X1A1g) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (Si3H5)-the isovalent counterpart of the cyclopropyl radical (C3H5)-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-Si3H7) collision complex(es). Possible hydrogen-atom roaming on the doublet surface proceeds to molecular hydrogen loss accompanied by ring closure. The chemical dynamics are quite distinct from the isovalent methylidyne (CH)-ethane (C2H6) reaction, which leads to propylene (C3H6) radical plus atomic hydrogen but not to cyclopropyl (C3H5) radical plus molecular hydrogen. The identification of the trisilacyclopropyl radical (Si3H5) opens up preparative pathways for an unusual gas phase chemistry of previously inaccessible ring-strained (inorgano)silicon molecules as a result of single-collision events.
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The hitherto elusive N-hydroxyoxaziridine molecule (c-H2CON(OH)), a chiral, high energy isomer of nitromethane (CH3NO2) and one of the simplest representatives of an oxaziridine, is detected in the gas phase. Electronic structure calculations propose an impending synthesis eventually via addition of carbene (CH2) to the nitrogen-oxygen double bond of nitrous acid (HONO). The oxaziridine ring demonstrates an unusual kinetic stability toward ring opening compared to the isoeletronic cyclopropane (C3H6) counterpart. This system defines a fundamental benchmark to explore the formation and stability of racemic derivatives of strained oxaziridines (c-H2CONH) and changes our perception how we think about fundamental decomposition and isomerization mechanisms in (model compounds of) energetic materials.
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Understanding of the fundamental mechanisms involved in the decomposition of 1,3,5-trinitro-1,3,5-triazinane (RDX) still represents a major challenge for the energetic materials and physical (organic) chemistry communities mainly because multiple competing dissociation channels are likely involved and previous detection methods of the products are not isomer selective. In this study we exploited a microsecond pulsed infrared laser to decompose thin RDX films at 5â K under mild conditions to limit the fragmentation channels. The subliming decomposition products during the temperature programed desorption phase are detected using isomer selective single photoionization time-of-flight mass spectrometry (PI-ReTOF-MS). This technique enables us to assign a product signal at m/z=42 to ketene (H2 CCO), but not to diazomethane (H2 CNN; 42 amu) as speculated previously. Electronic structure calculations support our experimental observations and unravel the decomposition mechanisms of RDX leading eventually to the elusive ketene (H2 CCO) via an exotic, four-membered ring intermediate. This study highlights the necessity to exploit isomer-selective detection schemes to probe the true decomposition products of nitramine-based energetic materials.
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Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance-stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl-addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen-abstraction/acetylene-addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.
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For decades, the source of phosphorus incorporated into Earth's first organisms has remained a fundamental, unsolved puzzle. Although contemporary biomolecules incorporate P(+V) in their phosphate moieties, the limited bioavailability of phosphates led to the proposal that more soluble P(+III) compounds served as the initial source of phosphorus. Here, we report via laboratory simulation experiments that the three simplest alkylphosphonic acids, soluble organic phosphorus P(+III) compounds, can be efficiently generated in interstellar, phosphine-doped ices through interaction with galactic cosmic rays. This discovery opens a previously overlooked avenue into the formation of key molecules of astrobiological significance and untangles basic mechanisms of a facile synthesis of phosphorus-containing organics in extraterrestrial ices, which can be incorporated into comets and asteroids before their delivery and detection on Earth such as in the Murchison meteorite.
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The hitherto elusive dibridged germaniumsilylene molecule (Ge(µ-H2)Si) has been formed for the first time via the bimolecular gas-phase reaction of ground-state germanium atoms (Ge) with silane (SiH4) under single-collision conditions. Merged with state-of-the-art electronic structure calculations, the reaction was found to proceed through initial formation of a van der Waals complex in the entrance channel, insertion of the germanium into a silicon-hydrogen bond, intersystem crossing from the triplet to the singlet surface, hydrogen migrations, and eventually elimination of molecular hydrogen via a tight exit transition state, leading to the germaniumsilylene "butterfly". This investigation provides an extraordinary peek at the largely unknown silicon-germanium chemistry on the molecular level and sheds light on the essential nonadiabatic reaction dynamics of germanium and silicon, which are quite distinct from those of the isovalent carbon system, thus offering crucial insights that reveal exotic chemistry and intriguing chemical bonding in the germanium-silicon system on the most fundamental, microscopic level.
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Methylamine (CH3NH2) and methanimine (CH2NH) represent essential building blocks in the formation of amino acids in interstellar and cometary ices. In our study, by exploiting isomer selective detection of the reaction products via photoionization coupled with reflectron time of flight mass spectrometry (Re-TOF-MS), we elucidate the formation of methanimine and ethylenediamine (NH2CH2CH2NH2) in methylamine ices exposed to energetic electrons as a proxy for secondary electrons generated by energetic cosmic rays penetrating interstellar and cometary ices. Interestingly, the two products methanimine and ethylenediamine are isoelectronic to formaldehyde (H2CO) and ethylene glycol (HOCH2CH2OH), respectively. Their formation has been confirmed in interstellar ice analogs consisting of methanol (CH3OH) which is ioselectronic to methylamine. Both oxygen-bearing species formed in methanol have been detected in the interstellar medium (ISM), while for methanimine and ethylenediamine only methanimine has been identified so far. In comparison with the methanol ice products and our experimental findings, we predict that ethylenediamine should be detectable in these astronomical sources, where methylamine and methanimine are present.
Asunto(s)
Etilenodiaminas/química , Iminas/química , Frío , Formaldehído/química , Radiación Ionizante , Rayos Ultravioleta , VacioRESUMEN
Phosphorus signifies an essential element in molecular biology, yet given the limited solubility of phosphates on early Earth, alternative sources like meteoritic phosphides have been proposed to incorporate phosphorus into biomolecules under prebiotic terrestrial conditions. Here, we report on a previously overlooked source of prebiotic phosphorus from interstellar phosphine (PH3) that produces key phosphorus oxoacids-phosphoric acid (H3PO4), phosphonic acid (H3PO3), and pyrophosphoric acid (H4P2O7)-in interstellar analog ices exposed to ionizing radiation at temperatures as low as 5 K. Since the processed material of molecular clouds eventually enters circumstellar disks and is partially incorporated into planetesimals like proto Earth, an understanding of the facile synthesis of oxoacids is essential to untangle the origin of water-soluble prebiotic phosphorus compounds and how they might have been incorporated into organisms not only on Earth, but potentially in our universe as well.