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We explored the elementary reaction of atomic carbon (C; 3Pj) with pyridine (C5H5; X1A1) at a collision energy of 34 ± 4 kJ mol-1 utilizing the crossed molecular beams technique. Forward-convolution fitting of the data was combined with high-level electronic structure calculations and statistical (RRKM) calculations on the triplet C6H5N potential energy surface (PES). These investigations reveal that the reaction dynamics are indirect and dominated by large range reactive impact parameters leading via barrier-less addition to the nitrogen atom and to two chemically nonequivalent "aromatic" carbon-carbon bonds forming three distinct collision complexes. At least two reaction pathways through atomic hydrogen loss were identified on the triplet surface. These channels involve multiple isomerization steps of the initial collision complexes via ring-opening and ring expansion forming an acyclic 1-ethynyl-3-isocyanoallyl radical (P1; 2A") and a hitherto unreported seven-membered 1-aza-2-dehydrocyclohepta-2,4,6-trien-4-yl radical isomer (P3; 2A), respectively. For RRKM calculations at zero collision energy, representing conditions in cold molecular clouds, the ring expansion product P3 is formed nearly exclusively for the atomic hydrogen loss channel, but based on these computations, the molecular fragmentation channel forming acetylene (C2H2) plus 3-cyano-2-propen-1-ylidene (P6; 3A") accounts for nearly all of the degradation products of the reaction of atomic carbon with pyridine, proposing a destruction pathway of interstellar pyridine, which may account for the absence in the detection of pyridine in the interstellar medium. These results are also discussed in light of the isoelectronic carbon-benzene (C6H6; X1A1) system with important implications to the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons (NPAHs) in the interstellar medium compared to mass growth processes of PAH counterparts through ring expansion. [ABSTRACT FROM AUTHOR]

We carried out crossed molecular beam experiments and electronic structure calculations to unravel the chemical dynamics of the reaction of the silylidyne(-d1) radical (SiH/SiD; X²Π) with dimethylacetylene (CH3CCCH3; X¹A1g). The chemical dynamics were indirect and initiated by the barrierless addition of the silylidyne radical to both carbon atoms of dimethylacetylene forming a cyclic collision complex 2,3-dimethyl-1-silacyclopropenyl. This complex underwent unimolecular decomposition by atomic hydrogen loss from the silicon atom via a loose exit transition state to form the novel 2,3-dimethyl-1-silacycloprop-2-enylidene isomer in an overall exoergic reaction (experimentally: −29 ± 21 kJ mol-1; computationally: −10 ± 8 kJ mol-1). An evaluation of the scattering dynamics of silylidyne with alkynes indicates that in each system, the silylidyne radical adds barrierlessly to one or to both carbon atoms of the acetylene moiety, yielding an acyclic or a cyclic collision complex, which can also be accessed via cyclization of the acyclic structures. The cyclic intermediate portrays the central decomposing complex, which fragments via hydrogen loss almost perpendicularly to the rotational plane of the decomposing complex exclusively from the silylidyne moiety via a loose exit transition state in overall weakly exoergic reaction leading to ((di)methyl-substituted) 1-silacycloprop-2-enylidenes (−1 to −13 kJ mol-1 computationally; −12 ± 11 to −29 ± 21 kJ mol-1 experimentally). Most strikingly, the reaction dynamics of the silylidyne radical with alkynes are very different from those of C1-C4 alkanes and C2-C4 alkenes, which do not react with the silylidyne radical at the collision energies under our crossed molecular beam apparatus, due to either excessive entrance barriers to reaction (alkanes) or overall highly endoergic reaction processes (alkenes). Nevertheless, molecules carrying carbon-carbon double bonds could react, if the carbon-carbon double bond is either consecutive like in allene (H2CCCH2) or in conjugation with another carbon-carbon double bond (conjugated dienes) as found, for instance, in 1,3-butadiene (H2CCHCHCH2). [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of astrophysically relevant propyl derivatives (C 3 H 7 X; X = CN, OH, HCO) at 157 nm exploiting an ultracompact velocity map imaging (UVMIS) setup has been reported. The successful operation of UVMIS allowed the exploration of the 157 nm photodissociation of six (iso)propyl systems─ n/i -propyl cyanide (C 3 H 7 CN), n/i -propyl alcohol (C 3 H 7 OH), and (iso)butanal (C 3 H 7 CHO)─to explore the C 3 H 7 loss channel. The distinct center-of-mass translational energy distributions for the i -C 3 H 7 X (X= CN, OH, HCO) could be explained through preferential excitation of the low frequency C-H bending modes of the formyl moiety compared to the higher frequency stretching of the cyano and hydroxy moieties. Although the ionization energy of the n -C 3 H 7 radical exceeds the energy of a 157 nm photon, C 3 H 7 + was observed in the n -C 3 H 7 X (X = CN, OH, HCO) systems as a result of photoionization of vibrationally "hot" n -C 3 H 7 fragments, photoionization of i -C 3 H 7 after a hydrogen shift in vibrationally "hot" n -C 3 H 7 radicals, and/or two-photon ionization. Our experiments reveal that at least the isopropyl radical ( i -C 3 H 7 ) and possibly the normal propyl radical ( n -C 3 H 7 ) should be present in the interstellar medium and hence searched for by radio telescopes.

We investigated the formation of small organosilicon molecules─potential precursors to silicon-carbide dust grains ejected by dying carbon-rich asymptotic giant branch stars─in the gas phase via the reaction of atomic carbon (C) in its 3 P electronic ground state with silane (SiH 4 ; X 1 A 1 ) 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 (HCSiH 2 ; X 2 B 2 ) via the unimolecular decomposition of triplet silaethylene (H 2 CSiH 2 ; a 3 A″). The silaethylene radical was formed via hydrogen migration of the triplet silylmethylene (HCSiH 3 ; X 3 A″) 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 (CH 3 Si; X 2 A″) and methylenesilyl (CH 2 SiH; X 2 A') 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.

The bimolecular gas-phase reaction of the methylidyne radical (CH; X 2 Π) with 1,2-butadiene (CH 2 CCHCH 3 ; X 1 A') was investigated at a collision energy of 20.6 kJ mol -1 under single collision conditions. Combining our laboratory data with high-level electronic structure calculations, we reveal that this bimolecular reaction proceeds through the barrierless addition of the methylidyne radical to the carbon-carbon double bonds of 1,2-butadiene leading to doublet C 5 H 7 intermediates. These collision adducts undergo a nonstatistical unimolecular decomposition through atomic hydrogen elimination to at least the cyclic 1-vinyl-cyclopropene ( p5 / p26 ), 1-methyl-3-methylenecyclopropene ( p28 ), and 1,2-bis(methylene)cyclopropane ( p29 ) in overall exoergic reactions. The barrierless nature of this bimolecular reaction suggests that these cyclic C 5 H 6 isomers might be viable targets to be searched for in cold molecular clouds like TMC-1.

The gas phase reaction of the simplest silicon-bearing radical silylidyne (SiH; X 2 Π) with disilane (Si 2 H 6 ; X 1 A 1g ) was investigated in a crossed molecular beams machine. Combined with electronic structure calculations, our data reveal the synthesis of the previously elusive trisilacyclopropyl radical (Si 3 H 5 )-the isovalent counterpart of the cyclopropyl radical (C 3 H 5 )-along with molecular hydrogen via indirect scattering dynamics through long-lived, acyclic trisilapropyl (i-Si 3 H 7 ) 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 (C 2 H 6 ) reaction, which leads to propylene (C 3 H 6 ) radical plus atomic hydrogen but not to cyclopropyl (C 3 H 5 ) radical plus molecular hydrogen. The identification of the trisilacyclopropyl radical (Si 3 H 5 ) 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.

The fulvenallene molecule (C 7 H 6 ) has been synthesized via the elementary gas-phase reaction of the methylidyne radical (CH) with the benzene molecule (C 6 H 6 ) on the doublet C 7 H 7 surface under single collision conditions. The barrier-less route to the cyclic fulvenallene molecule involves the addition of the methylidyne radical to the π-electron density of benzene leading eventually to a Jahn-Teller distorted tropyl (C 7 H 7 ) radical intermediate and exotic ring opening-ring contraction sequences terminated by atomic hydrogen elimination. The methylidyne-benzene system represents a benchmark to probe the outcome of the elementary reaction of the simplest hydrocarbon radical-methylidyne-with the prototype of a closed-shell aromatic molecule-benzene-yielding nonbenzenoid fulvenallene. Combined with electronic structure and statistical calculations, this bimolecular reaction sheds light on the unusual reaction dynamics of Hückel aromatic systems and remarkable (polycyclic) reaction intermediates, which cannot be studied via classical organic, synthetic methods, thus opening up a versatile path to access this previously largely obscure class of fulvenallenes.

The crossed molecular beam reactions of the methylidyne radical (CH; X 2 Π) with propylene (CH 3 CHCH 2 ; X 1 A') along with (partially) substituted reactants were conducted at collision energies of 19.3 kJ mol -1 . Combining our experimental data with ab initio electronic structure and statistical calculations, the methylidyne radical is revealed to add barrierlessly to the carbon-carbon double bond of propylene reactant resulting in a cyclic doublet C 4 H 7 intermediate with a lifetime longer than its rotation period. These adducts undergo a nonstatistical unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming the cyclic products 1-methylcyclopropene and 3-methylcyclopropene with overall reaction exoergicities of 168 ± 25 kJ mol -1 . These C 4 H 6 isomers are predicted to exist even in low-temperature environments such as cold molecular clouds like TMC-1, since the reaction is barrierless and exoergic, all transition states are below the energy of the separated reactants, and both the methylidyne radical (CH; X 2 Π) and propylene reactant were detected in cold molecular clouds such as TMC-1.

The reactions of the 1-propynyl radical (CH 3 CC; X 2 A 1 ) with two C 3 H 4 isomers, methylacetylene (H 3 CCCH; X 1 A 1 ) and allene (H 2 CCCH 2 ; X 1 A 1 ), along with their (partially) deuterated counterparts were explored at collision energies of 37 kJ mol -1 , exploiting crossed molecular beams to unravel the chemical reaction dynamics to synthesize distinct C 6 H 6 isomers under single collision conditions. The forward convolution fitting of the laboratory data along with ab initio and statistical calculations revealed that both reactions have no entrance barrier, proceed via indirect (complex-forming) reaction dynamics involving C 6 H 7 intermediates with life times longer than their rotation period(s), and are initiated by the addition of the 1-propynyl radical with its radical center to the π-electron density of the unsaturated hydrocarbon at the terminal carbon atoms of methylacetylene (C1) and allene (C1/C3). In the methylacetylene system, the initial collision complexes undergo unimolecular decomposition via tight exit transition states by atomic hydrogen loss, forming dimethyldiacetylene (CH 3 CCCCCH 3 ) and 1-propynylallene (H 3 CCCHCCCH 2 ) in overall exoergic reactions (123 and 98 kJ mol -1 ) with a branching ratio of 9.4 ± 0.1; the methyl group of the 1-propynyl reactant acts solely as a spectator. On the other hand, in the allene system, our experimental data exhibit the formation of the fulvene (c-C 5 H 4 CH 2 ) isomer via a six-step reaction sequence with two higher energy isomers-hexa-1,2-dien-4-yne (H 2 CCCHCCCH 3 ) and hexa-1,4-diyne (HCCCH 2 CCCH 3 )-also predicted to be formed based on our statistical calculations. The pathway to fulvene advocates that, in the allene-1-propynyl system, the methyl group of the 1-propynyl reactant is actively engaged in the reaction mechanism to form fulvene. Because both reactions are barrierless and exoergic and all transition states are located below the energy of the separated reactants, the hydrogen-deficient C 6 H 6 isomers identified in our investigation are predicted to be synthesized in low-temperature environments, such as in hydrocarbon-rich atmospheres of planets and their moons such as Titan along with cold molecular clouds such as Taurus Molecular Cloud-1.

The crossed beams reactions of the 1-propynyl radical (CH 3 CC; X 2 A 1 ) with 1,3-butadiene (CH 2 CHCHCH 2 ; X 1 A g ), 1,3-butadiene- d 6 (CD 2 CDCDCD 2 ; X 1 A g ), 1,3-butadiene- d 4 (CD 2 CHCHCD 2 ; X 1 A g ), and 1,3-butadiene- d 2 (CH 2 CDCDCH 2 ; X 1 A g ) were performed under single collision conditions at collision energies of about 40 kJ mol -1 . The underlying reaction mechanisms were unraveled through the combination of the experimental data with electronic structure calculations at the CCSD(T)-F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) + ZPE(B3LYP/6-311G(d,p) level of theory along with statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations. Together, these data suggest the formation of the thermodynamically most stable C 7 H 8 isomer-toluene (C 6 H 5 CH 3 )-via the barrierless addition of 1-propynyl to the 1,3-butadiene terminal carbon atom, forming a low-lying C 7 H 9 intermediate that undergoes multiple isomerization steps resulting in cyclization and ultimately aromatization following hydrogen atom elimination. RRKM calculations predict that the thermodynamically less stable isomers 1,3-heptadien-5-yne, 5-methylene-1,3-cyclohexadiene, and 3-methylene-1-hexen-4-yne are also synthesized. Since the 1-propynyl radical may be present in cold molecular clouds such as TMC-1, this pathway could potentially serve as a carrier of the methyl group incorporating itself into methyl-substituted (poly)acetylenes or aromatic systems such as toluene via overall exoergic reaction mechanisms that are uninhibited by an entrance barrier. Such pathways are a necessary alternative to existing high energy reactions leading to toluene that are formally closed in the cold regions of space and are an important step toward understanding the synthesis of polycyclic aromatic hydrocarbons (PAHs) in space's harsh extremes.

The hitherto elusive dibridged germaniumsilylene molecule (Ge(μ-H 2 )Si) has been formed for the first time via the bimolecular gas-phase reaction of ground-state germanium atoms (Ge) with silane (SiH 4 ) 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.

We present a combined experimental and theoretical investigation of the bimolecular gas-phase reaction of the phenyl radical (C 6 H 5 ) with silane (SiH 4 ) under single collision conditions to investigate the chemical dynamics of forming phenylsilane (C 6 H 5 SiH 3 ) via a bimolecular radical substitution mechanism at a tetracoordinated silicon atom. Verified by electronic structure and quasiclassical trajectory calculations, the replacement of a single carbon atom in methane by silicon lowers the barrier to substitution, thus defying conventional wisdom that tetracoordinated hydrides undergo preferentially hydrogen abstraction. This reaction mechanism provides fundamental insights into the hitherto unexplored gas-phase chemical dynamics of radical substitution reactions of mononuclear main group hydrides under single collision conditions and highlights the distinct reactivity of silicon compared to its isovalent carbon. This mechanism might be also involved in the synthesis of cyanosilane (SiH 3 CN) and methylsilane (CH 3 SiH 3 ) probed in the circumstellar envelope of the carbon star IRC+10216.

We investigated the 1-propynyl (CH 3 CC; X 2 A 1 ) plus acetylene/acetylene- d 2 (HCCH/DCCD; X 1 Σ g + ) under single-collision conditions using the crossed molecular beams method. The reaction was found to produce C 5 H 4 plus atomic hydrogen (H) via an indirect reaction mechanism with a reaction energy of -123 ± 18 kJ mol -1 . Using the DCCD isotopologue, we confirmed that the hydrogen atom is lost from the acetylene reactant. Our computational analysis suggests the reaction proceeds by the barrierless addition of the 1-propynyl radical to acetylene, resulting in C 5 H 5 intermediate(s) that dissociate preferentially to methyldiacetylene (CH 3 CCCCH; X 1 A 1 ) via hydrogen atom emission with a computed reaction energy of -123 ± 4 kJ mol -1 . The barrierless nature of this reaction scheme suggests the 1-propynyl radical may be a key intermediate in hydrocarbon chain growth in cold molecular clouds like TMC-1, where methyl-substituted (poly)acetylenes are known to exist.

The bimolecular gas phase reaction of ground-state silicon (Si; 3 P) with dimethylacetylene (C 4 H 6 ; X 1 A 1g ) was investigated under single collision conditions in a crossed molecular beams machine. Merged with electronic structure calculations, the data propose nonadiabatic reaction dynamics leading to the formation of singlet SiC 4 H 4 isomer(s) and molecular hydrogen (H 2 ) via indirect scattering dynamics along with intersystem crossing (ISC) from the triplet to the singlet surface. The reaction may lead to distinct energetically accessible singlet SiC 4 H 4 isomers ( 1 p8- 1 p24) in overall exoergic reaction(s) (-107 -20 +12 kJ mol -1 ). All feasible reaction products are either cyclic, carry carbene analogous silylene moieties, or carry C-Si-H or C-Si-C bonds that would require extensive isomerization from the initial collision complex(es) to the fragmenting singlet intermediate(s). The present study demonstrates the first successful crossed beams study of an exoergic reaction channel arising from bimolecular collisions of silicon, Si( 3 P), with a hydrocarbon molecule.

We explored the elementary reaction of atomic carbon (C; 3 P j ) with pyridine (C 5 H 5 N; X 1 A 1 ) at a collision energy of 34 ± 4 kJ mol -1 utilizing the crossed molecular beams technique. Forward-convolution fitting of the data was combined with high-level electronic structure calculations and statistical (RRKM) calculations on the triplet C 6 H 5 N potential energy surface (PES). These investigations reveal that the reaction dynamics are indirect and dominated by large range reactive impact parameters leading via barrier-less addition to the nitrogen atom and to two chemically nonequivalent "aromatic" carbon-carbon bonds forming three distinct collision complexes. At least two reaction pathways through atomic hydrogen loss were identified on the triplet surface. These channels involve multiple isomerization steps of the initial collision complexes via ring-opening and ring expansion forming an acyclic 1-ethynyl-3-isocyanoallyl radical (P1; 2 A″) and a hitherto unreported seven-membered 1-aza-2-dehydrocyclohepta-2,4,6-trien-4-yl radical isomer (P3; 2 A), respectively. For RRKM calculations at zero collision energy, representing conditions in cold molecular clouds, the ring expansion product P3 is formed nearly exclusively for the atomic hydrogen loss channel, but based on these computations, the molecular fragmentation channel forming acetylene (C 2 H 2 ) plus 3-cyano-2-propen-1-ylidene (P6; 3 A″) accounts for nearly all of the degradation products of the reaction of atomic carbon with pyridine, proposing a destruction pathway of interstellar pyridine, which may account for the absence in the detection of pyridine in the interstellar medium. These results are also discussed in light of the isoelectronic carbon-benzene (C 6 H 6 ; X 1 A 1 ) system with important implications to the rapid degradation of nitrogen-bearing polycyclic aromatic hydrocarbons (NPAHs) in the interstellar medium compared to mass growth processes of PAH counterparts through ring expansion.

The bimolecular gas-phase reaction of ground-state atomic silicon (Si; 3 P) with disilane (Si 2 H 6 ; 1 A 1g ) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 21 kJ mol -1 . Combined with electronic structure calculations, the results suggest the formation of Si 3 H 4 isomer(s) along with molecular hydrogen via indirect scattering dynamics through Si 3 H 6 collision complex(es) and intersystem crossing from the triplet to the singlet surface. The nonadiabatic reaction dynamics can synthesize the energetically accessible singlet Si 3 H 4 isomers in overall exoergic reaction(s) (-93 ± 21 kJ mol -1 ). All reasonable reaction products are either cyclic or hydrogen-bridged suggesting extensive isomerization processes from the reactants via the initially formed collision complex(es) to the fragmenting singlet intermediate(s). The underlying chemical dynamics of the silicon-disilane reaction are quite distinct from the isovalent carbon-ethane system that does not depict any reactivity at all, and open the door for an unconventional gas phase synthesis of hitherto elusive organosilicon molecules under single-collision conditions.

Crossed molecular beam experiments were performed to elucidate the chemical dynamics of the para-tolyl (CH 3 C 6 H 4 ) radical reaction with molecular oxygen (O 2 ) at an average collision energy of 35.3 ± 1.4 kJ mol -1 . Combined with theoretical calculations, the results show that para-tolyl is efficiently oxidized by molecular oxygen to para-toloxy (CH 3 C 6 H 4 O) plus ground-state atomic oxygen via a complex forming, overall exoergic reaction (experimental, -33 ± 16 kJ mol -1 ; computational, -42 ± 8 kJ mol -1 ). The reaction dynamics are analogous to those observed for the phenyl (C 6 H 5 ) plus molecular oxygen system which suggests the methyl group is a spectator during para-tolyl oxidation and that application of phenyl thermochemistry and reaction rates to para-substituted aryls is likely a suitable approximation.

We carried out crossed molecular beam experiments and electronic structure calculations to unravel the chemical dynamics of the reaction of the silylidyne(-d1) radical (SiH/SiD; X(2)Π) with dimethylacetylene (CH3CCCH3; X(1)A1g). The chemical dynamics were indirect and initiated by the barrierless addition of the silylidyne radical to both carbon atoms of dimethylacetylene forming a cyclic collision complex 2,3-dimethyl-1-silacyclopropenyl. This complex underwent unimolecular decomposition by atomic hydrogen loss from the silicon atom via a loose exit transition state to form the novel 2,3-dimethyl-1-silacycloprop-2-enylidene isomer in an overall exoergic reaction (experimentally: -29 ± 21 kJ mol(-1); computationally: -10 ± 8 kJ mol(-1)). An evaluation of the scattering dynamics of silylidyne with alkynes indicates that in each system, the silylidyne radical adds barrierlessly to one or to both carbon atoms of the acetylene moiety, yielding an acyclic or a cyclic collision complex, which can also be accessed via cyclization of the acyclic structures. The cyclic intermediate portrays the central decomposing complex, which fragments via hydrogen loss almost perpendicularly to the rotational plane of the decomposing complex exclusively from the silylidyne moiety via a loose exit transition state in overall weakly exoergic reaction leading to ((di)methyl-substituted) 1-silacycloprop-2-enylidenes (-1 to -13 kJ mol(-1) computationally; -12 ± 11 to -29 ± 21 kJ mol(-1) experimentally). Most strikingly, the reaction dynamics of the silylidyne radical with alkynes are very different from those of C1-C4 alkanes and C2-C4 alkenes, which do not react with the silylidyne radical at the collision energies under our crossed molecular beam apparatus, due to either excessive entrance barriers to reaction (alkanes) or overall highly endoergic reaction processes (alkenes). Nevertheless, molecules carrying carbon-carbon double bonds could react, if the carbon-carbon double bond is either consecutive like in allene (H2CCCH2) or in conjugation with another carbon-carbon double bond (conjugated dienes) as found, for instance, in 1,3-butadiene (H2CCHCHCH2).