On the Synthesis of the Astronomically Elusive 1-Ethynyl-3-Silacyclopropenylidene (c-SiC4H2) Molecule in Circumstellar Envelopes of Carbon-rich Asymptotic Giant Branch Stars and Its Potential Role in the Formation of the Silicon Tetracarbide Chain (SiC4)

Organosilicon molecules such as silicon carbide (SiC), silicon dicarbide (c-SiC2), silicon tricarbide (c-SiC3), and silicon tetracarbide (SiC4) represent basic molecular building blocks connected to the growth of silicon-carbide dust grains in the outflow of circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars. Yet, the fundamental mechanisms of the formation of silicon carbides and of the early processes that initiate the coupling of silicon-carbon bonds in circumstellar envelopes have remained obscure. Here, we reveal in a crossed molecular beam experiment contemplated with ab initio electronic calculations that the astronomically elusive 1-ethynyl-3-silacyclopropenylidene molecule (c-SiC4H2, Cs, X1A′) can be synthesized via a single-collision event through the barrierless reaction of the silylidyne radical (SiH) with diacetylene (C4H2). This system represents a benchmark of a previously overlooked class of reactions, in which the silicon-carbon bond coupling can be initiated by a barrierless and overall exoergic reaction between the simplest silicon-bearing radical (silylidyne) and a highly hydrogen-deficient hydrocarbon (diacetylene) in the inner circumstellar envelopes of evolved carbon-rich stars such as IRC+10216. Considering that organosilicon molecules like 1-ethynyl-3-silacyclopropenylidene might be ultimately photolyzed to bare carbon-silicon clusters like the linear silicon tetracarbide (SiC4), hydrogenated silicon-carbon clusters might represent the missing link eventually connecting simple molecular precursors such as silane (SiH4) to the population of silicon-carbide based interstellar grains ejected from carbon-rich AGB stars into the interstellar medium

Gas-phase synthesis of silaformaldehyde (h2sio) and hydroxysilylene (hsioh) in outflows of oxygen-rich asymptotic giant branch stars

Silicon- and oxygen-containing species such as silicon monoxide (SiO) and silicon dioxide (SiO2) represent basic molecular building blocks connected to the growth of silicate grains in outflows of oxygen-rich asymptotic giant branch (AGB) stars like R Doradus. Yet the fundamental mechanisms of the formation of silicate grains and the early processes that initiate the coupling of the silicon with the oxygen chemistries in circumstellar envelopes have remained obscure. Here, in a crossed molecular beams experiment combined with ab initio electronic structure calculations, we reveal that at least the d2-silaformaldehyde (D2SiO) and d2-hydroxysilylene (DSiOD) molecules -proxies for the astronomically elusive silaformaldehyde (H2SiO) and hydroxysilylene (HSiOH) molecules-can be synthesized via the reaction of the D1-silylidyne radical (SiD; X2π) with D2-water (D2O) under single-collision conditions. This system represents a benchmark of a previously overlooked class of reactions, in which the silicon- oxygen bond coupling can be initiated by a reaction between the simplest silicon-bearing radical (silylidyne) and one of the most abundant species in the circumstellar envelopes of evolved oxygen-rich AGB stars (water). As supported by novel astrochemical modeling, considering that silicon- and oxygen-containing species like H2SiO and HSiOH might be photolyzed easily, they ultimately connect to simple molecular precursors such as SiO that drive a chain of reactions conceivably forming higher molecular weight silicon oxides and, ultimately, a population of silicates at high temperatures

Low-temperature gas-phase formation of indene in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings—the essential building block of nonplanar PAHs—is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C9H8)—the prototype aromatic molecule with a five-membered ring—via a barrierless bimolecular reaction involving the simplest organic radical—methylidyne (CH)—and styrene (C6H5C2H3) through the hitherto elusive methylidyne addition–cyclization–aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C20H10) along with fullerenes (C60, C70), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy

Photodissociation Dynamics of Halogenated Heterocyclic Molecules in Gas Phase

Documentos apresentados no âmbito do reconhecimento de graus e diplomas estrangeirosThe process of photodissociation is the breaking of one or several bonds in a molecule through the absorption of photon. The area of photodissociation dynamics concerns with the detailed mechanisms of photodissociation process investigated on molecular level. One of the principal goal of these kind of studies is to obtain a clear picture of the various dynamic processes in the excited electronic state as the molecule leaves the Franck-Condon region, traverses the ‘transition state’ (i.e., the barrier, if there is any), and finally reaches the asymptotic channel(s), where the fragments are formed. Photodissociation experiments are generally performed state selectively i.e. molecule being prepared in well defined quantum state. Therefore, results from these kinds of experiments are readily compared with the theoretical calculations, resulting in significantly enhanced understanding of the elementary chemical processes [1]. Photodissociation dynamics of various molecules of interest are studied in molecular beam (MB) generated by supersonic expansion through a pulsed valve after seeding it in Helium gas. The isolated molecules in the MB are photodissociated using ultraviolet (UV) laser. One of the resultant photofragments, in the present case mainly atomic species, is probed using Resonance Enhanced Multiphoton Ionization Time-of- Flight Mass Spectrometry (REMPI-TOF-MS). The combination of supersonic molecular beam, REMPI and TOF mass spectrometer has potential of detecting the chemical species state and mass selectively In this thesis, the photodissociation dynamics of halogenated heterocyclic molecules has been studied. It is well known that the photodissociation of halogen containing molecules in the UV range generates halogen atoms which are detrimental for the ozone layer in the stratosphere. Also, it is fundamentally interesting to study the halogenated heterocyclic molecules because of their rich excited state dynamics due to their radiationless transitions, vibronic and spin-orbit coupling. Further, effect of fluorine atom substitution on dynamics has also been investigated. In this context, five and six member heterocyclic molecules have been chosen with one to three hetero atoms in the ring

Crossed Beam Experiments and Computational Studies of Pathways to the Preparation of Singlet Ethynylsilylene (HCCSiH; X1A'): The Silacarbene Counterpart of Triplet Propargylene (HCCCH; X3B).

Ethynylsilylene (HCCSiH; X1A') has been prepared in the gas phase through the elementary reaction of singlet dicarbon (C2) with silane (SiH4) under single-collision conditions. Electronic structure calculations reveal a barrierless reaction pathway involving 1,1-insertion of dicarbon into one of the silicon-hydrogen bonds followed by hydrogen migration to form the 3-sila-methylacetylene (HCCSiH3) intermediate. The intermediate undergoes unimolecular decomposition through molecular hydrogen loss to ethynylsilylene (HCCSiH; Cs; X1A'). The dicarbon-silane system defines a benchmark to explore the consequence of a single collision between the simplest "only carbon" molecule (dicarbon) with the prototype of a closed-shell silicon hydride (silane) yielding a nonclassical silacarbene, whose molecular geometry and electronic structure are quite distinct from the isovalent triplet propargylene (HCCCH; C2; 3B) carbon-counterpart. These organosilicon transients cannot be prepared through traditional organic, synthetic methods, thus opening up a versatile path to access the previously largely elusive class of silacarbenes

Gas-phase Synthesis of Silaformaldehyde (H₂SiO) and Hydroxysilylene (HSiOH) in Outflows of Oxygen-rich Asymptotic Giant Branch Stars

Silicon- and oxygen-containing species such as silicon monoxide (SiO) and silicon dioxide (SiO2) represent basic molecular building blocks connected to the growth of silicate grains in outflows of oxygen-rich asymptotic giant branch (AGB) stars like R Doradus. Yet the fundamental mechanisms of the formation of silicate grains and the early processes that initiate the coupling of the silicon with the oxygen chemistries in circumstellar envelopes have remained obscure. Here, in a crossed molecular beams experiment combined with ab initio electronic structure calculations, we reveal that at least the d2-silaformaldehyde (D2SiO) and d2-hydroxysilylene (DSiOD) molecules -proxies for the astronomically elusive silaformaldehyde (H2SiO) and hydroxysilylene (HSiOH) molecules-can be synthesized via the reaction of the D1-silylidyne radical (SiD; X2π) with D2-water (D2O) under single-collision conditions. This system represents a benchmark of a previously overlooked class of reactions, in which the silicon- oxygen bond coupling can be initiated by a reaction between the simplest silicon-bearing radical (silylidyne) and one of the most abundant species in the circumstellar envelopes of evolved oxygen-rich AGB stars (water). As supported by novel astrochemical modeling, considering that silicon- and oxygen-containing species like H2SiO and HSiOH might be photolyzed easily, they ultimately connect to simple molecular precursors such as SiO that drive a chain of reactions conceivably forming higher molecular weight silicon oxides and, ultimately, a population of silicates at high temperatures

Gas-phase synthesis of corannulene - a molecular building block of fullerenes.

Fullerenes (C60, C70) detected in planetary nebulae and carbonaceous chondrites have been implicated to play a key role in the astrochemical evolution of the interstellar medium. However, the formation mechanism of even their simplest molecular building block-the corannulene molecule (C20H10)-has remained elusive. Here we demonstrate via a combined molecular beams and ab initio investigation that corannulene can be synthesized in the gas phase through the reactions of 7-fluoranthenyl (C16H9˙) and benzo[ghi]fluoranthen-5-yl (C18H9˙) radicals with acetylene (C2H2) mimicking conditions in carbon-rich circumstellar envelopes. This reaction sequence reveals a reaction class in which a polycyclic aromatic hydrocarbon (PAH) radical undergoes ring expansion while simultaneously forming an out-of-plane carbon backbone central to 3D nanostructures such as buckybowls and buckyballs. These fundamental reaction mechanisms are critical in facilitating an intimate understanding of the origin and evolution of the molecular universe and, in particular, of carbon in our galaxy