Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

Gas-phase formation of the resonantly stabilized 1-indenyl (C9H7•) radical in the interstellar medium

The 1-indenyl (C9H7•) radical, a prototype aromatic and resonantly stabilized free radical carrying a six- and a five-membered ring, has emerged as a fundamental molecular building block of nonplanar polycyclic aromatic hydrocarbons (PAHs) and carbonaceous nanostructures in deep space and combustion systems. However, the underlying formation mechanisms have remained elusive. Here, we reveal an unconventional low-temperature gas-phase formation of 1-indenyl via barrierless ring annulation involving reactions of atomic carbon [C(3P)] with styrene (C6H5C2H3) and propargyl (C3H3•) with phenyl (C6H5•). Macroscopic environments like molecular clouds act as natural low-temperature laboratories, where rapid molecular mass growth to 1-indenyl and subsequently complex PAHs involving vinyl side-chained aromatics and aryl radicals can occur. These reactions may account for the formation of PAHs and their derivatives in the interstellar medium and carbonaceous chondrites and could close the gap of timescales of their production and destruction in our carbonaceous universe.

Reaction Dynamics Study of the Molecular Hydrogen Loss Channel in the Elementary Reactions of Ground-State Silicon Atoms (Si(3P)) With 1- and 2-Methyl-1,3-Butadiene (C5H8)

The bimolecular gas-phase reactions involving ground-state atomic silicon (Si; ³P) and 1- and 2-methyl-1,3-butadiene were studied via crossed molecular beam experiments. Our data revealed indirect scattering dynamics through long-lived SiC₅H₈ collision complex(es) along with molecular hydrogen loss pathways, leading to facile formation of SiC₅H₆ isomer(s). We propose that the reactions of silicon with 1- and 2-methyl-1,3-butadiene possess reaction dynamics in an analogy to the silicon-1,3-butadiene system. This leads to cyclic methyl-substituted 2-methylene-1-silacyclobutene isomers via nonadiabatic reaction dynamics through intersystem crossing (ISC) from the triplet to the singlet surface in overall exoergic reactions through tight exit transition states and molecular hydrogen loss. Our study also suggests that the methyl group-although a spectator from the chemical viewpoint-can influence the disposal of the angular momentum into the rotational excitation of the final product.

Discovery of the propargyl radical (CH2CCH) in TMC-1: one of the most abundant radicals ever found and a key species for cyclization to benzene in cold dark clouds

We present the first identification in interstellar space of the propargyl radical (CH2CCH). This species was observed in the cold dark cloud TMC-1 using the Yebes 40m telescope. The six strongest hyperfine components of the 20,2-10,1 rotational transition, lying at 37.46 GHz, were detected with signal-to-noise ratios in the range 4.6-12.3 σ. We derive a column density of 8.7 × 1013 cm-2 for CH2CCH, which translates to a fractional abundance relative to H2 of 8.7 × 10-9. This radical has a similar abundance to methyl acetylene, with an abundance ratio CH2CCH/CH3CCH close to one. The propargyl radical is thus one of the most abundant radicals detected in TMC-1, and it is probably the most abundant organic radical with a certain chemical complexity ever found in a cold dark cloud. We constructed a gas-phase chemical model and find calculated abundances that agree with, or fall two orders of magnitude below, the observed value depending on the poorly constrained low-temperature reactivity of CH2CCH with neutral atoms. According to the chemical model, the propargyl radical is essentially formed by the C + C2H4 reaction and by the dissociative recombination of C3Hn + ions with n = 4-6. The propargyl radical is believed to control the synthesis of the first aromatic ring in combustion processes, and it probably plays a key role in the synthesis of large organic molecules and cyclization processes to benzene in cold dark clouds.

Quantum Behavior of Spin-Orbit Inelastic Scattering of C-Atoms by D2 at Low Energy

Fine-structure populations and collision–induced energy transfer in atoms are of interest for many fields, from combustion to astrophysics. In particular, neutral carbon atoms are known to play a role in interstellar media, either as probes of physical conditions (ground state ³P_j spin-orbit populations), or as cooling agent (collisional excitation followed by radiative decay). This work aims at investigating the spin-orbit excitation of atomic carbon in its ground electronic state due to collisions with molecular deuterium, an isotopic variant of H₂, the most abundant molecule in the interstellar medium. Spin-orbit excitations of C(³P_j) by H₂ or D₂ are governed by non-adiabatic and spin-orbit couplings, which make the theoretical treatment challenging, since the Born-Oppenheimer approximation no longer holds. Inelastic collisional cross-sections were determined for the C(³P₀) + D₂ → C(³P_j) + D₂ (with j = 1 and 2) excitation process. Experimental data were acquired in a crossed beam experiment at low collision energies, down to the excitation thresholds (at 16.42 and 43.41 cm⁻¹, respectively). C-atoms were produced mainly in their ground spin-orbit state, ³P₀, by dissociation of CO in a dielectric discharge through an Even-Lavie pulsed valve. The C-atom beam was crossed with a D₂ beam from a second valve. The state-to-state cross-sections were derived from the C(³P_j) (j = 1 or 2) signal measured as a function of the beam crossing angle, i.e., as a function of the collision energy. The results show different quantum behaviors for excitation to C(³P₁) or C(³P₂) when C(³P₀) collides with ortho-D₂ or normal-D₂. These experimental results are analyzed and discussed in the light of highly accurate quantum calculations. A good agreement between experimental and theoretical results is found. The present data are compared with those obtained for the C-He and C-H₂ collisional systems to get new insights into the dynamics of collision induced spin-orbit excitation/relaxation of atomic carbon.

Probing Nonadiabatic Effects in Low-Energy C(3 P j) + H2 Collisions

Nonadiabatic effects are of fundamental interest in collision dynamics. In particular, inelastic collisions between open-shell atoms and molecules, such as the collisional excitation of C(³ P _j) by H₂, are governed by nonadiabatic and spin-orbit couplings that are the sole responsible of collisional energy transfer. Here, we study collisions between carbon in its ground state C(³ P _j=0) and molecular hydrogen (H₂) at low collision energies that result in spin-orbit excitation to C(³ P _j=1) and C(³ P _j=2). State-to-state integral cross sections are obtained experimentally from crossed-beam experiments with a source of almost pure beam of C(³ P _j=0) and theoretically from highly accurate quantum calculations. We observe very good agreement between experimental and theoretical data that demonstrates our ability to model nonadiabatic dynamics. New rate coefficients at temperatures relevant to astrochemical modeling are also provided. They should lead to an increase of the abundance of atomic C(³ P) derived from the observations of interstellar clouds and a decrease of the efficiency of the cooling of the interstellar gas due to carbon atoms.

Understanding the quantum nature of low-energy C(3P j ) + He inelastic collisions

Inelastic collisions that occur between open-shell atoms and other atoms or molecules, and that promote a spin-orbit transition, involve multiple interaction potentials. They are non-adiabatic by nature and cannot be described within the Born-Oppenheimer approximation; in particular, their theoretical modelling becomes very challenging when the collision energies have values comparable to the spin-orbit splitting. Here we study inelastic collisions between carbon in its ground state C(³P_j=0) and helium atoms-at collision energies in the vicinity of spin-orbit excitation thresholds (~0.2 and 0.5 kJ mol^-1)-that result in spin-orbit excitation to C(³P_j=1) and C(³P_j=2). State-to-state integral cross-sections are obtained from crossed-beam experiments with a beam source that provides an almost pure beam of C(³P_j=0) . We observe very good agreement between experimental and theoretical results (acquired using newly calculated potential energy curves), which validates our characterization of the quantum dynamical resonances that are observed. Rate coefficients at very low temperatures suitable for chemical modelling of the interstellar medium are also calculated.

Laboratory Studies of Methane and Its Relationship to Prebiotic Chemistry

To examine how prebiotic chemical evolution took place on Earth prior to the emergence of life, laboratory experiments have been conducted since the 1950s. Methane has been one of the key molecules in these investigations. In earlier studies, strongly reducing gas mixtures containing methane and ammonia were used to simulate possible reactions in the primitive atmosphere of Earth, producing amino acids and other organic compounds. Since Earth's early atmosphere is now considered to be less reducing, the contribution of extraterrestrial organics to chemical evolution has taken on an important role. Such organic molecules may have come from molecular clouds and regions of star formation that created protoplanetary disks, planets, asteroids, and comets. The interstellar origin of organics has been examined both experimentally and theoretically, including laboratory investigations that simulate interstellar molecular reactions. Endogenous and exogenous organics could also have been supplied to the primitive ocean, making submarine hydrothermal systems plausible sites of the generation of life. Experiments that simulate such hydrothermal systems where methane played an important role have consequently been conducted. Processes that occur in other Solar System bodies offer clues to the prebiotic chemistry of Earth. Titan and other icy bodies, where methane plays significant roles, are especially good targets. In the case of Titan, methane is both in the atmosphere and in liquidospheres that are composed of methane and other hydrocarbons, and these have been studied in simulation experiments. Here, we review the wide range of experimental work in which these various terrestrial and extraterrestrial environments have been modeled, and we examine the possible role of methane in chemical evolution. Key Words: Methane-Interstellar environments-Submarine hydrothermal systems-Titan-Origin of life. Astrobiology 17, 786-812.

Flow tube studies of the C(3P) reactions with ethylene and propylene

Product detection studies of C(³P) atom reactions with ethylene, C₂H₄(X¹A_g) and propylene, C₃H₆(X¹A′) are carried out in a flow tube reactor at 332 K and 4 Torr (553.3 Pa) under multiple collision conditions.

Dynamics of carbon-hydrogen and carbon-methyl exchanges in the collision of 3P atomic carbon with propene

We investigated the dynamics of the reaction of (3)P atomic carbon with propene (C3H6) at reactant collision energy 3.8 kcal mol(-1) in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet ionization. Products C4H5, C4H4, C3H3, and CH3 were observed and attributed to exit channels C4H5 + H, C4H4 + 2H, and C3H3 + CH3; their translational-energy distributions and angular distributions were derived from the measurements of product time-of-flight spectra. Following the addition of a (3)P carbon atom to the C=C bond of propene, cyclic complex c-H2C(C)CHCH3 undergoes two separate stereoisomerization mechanisms to form intermediates E- and Z-H2CCCHCH3. Both the isomers of H2CCCHCH3 in turns decompose to C4H5 + H and C3H3 + CH3. A portion of C4H5 that has enough internal energy further decomposes to C4H4 + H. The three exit channels C4H5 + H, C4H4 + 2H, and C3H3 + CH3 have average translational energy releases 13.5, 3.2, and 15.2 kcal mol(-1), respectively, corresponding to fractions 0.26, 0.41, and 0.26 of available energy deposited to the translational degrees of freedom. The H-loss and 2H-loss channels have nearly isotropic angular distributions with a slight preference at the forward direction particularly for the 2H-loss channel. In contrast, the CH3-loss channel has a forward and backward peaked angular distribution with an enhancement at the forward direction. Comparisons with reactions of (3)P carbon atoms with ethene, vinyl fluoride, and vinyl chloride are stated.

Exploring the dynamics of C/H and C/Cl exchanges in the C(3P) + C2H3Cl reaction

The dynamics of the C((3)P) + C2H3Cl reaction at collision energy 3.8 kcal mol(-1) was investigated in a crossed molecular-beam apparatus using synchrotron vacuum-ultraviolet ionization. Time-of-flight spectra of products C3H2Cl, C3H3, and Cl were recorded at various laboratory scattering angles, from which translational-energy distributions and angular distributions of product channels C3H2Cl + H and C3H3 + Cl were derived. Cl correlates satisfactorily with C3H3 in linear momentum and angular distributions, which confirms the production of C3H3 + Cl. The H-loss (Cl-loss) channel has average translational-energy release 14.3 (8.8) kcal mol(-1) corresponding to a fraction 0.30 (0.14) of available energy into the translational degrees of freedom of product HCCCHCl + H (H2CCCH + Cl). The branching ratio of channel H to channel Cl was determined approximately as 12:88. The measurements of translational-energy releases and photoionization thresholds cannot distinguish HCCCHCl from H2CCCCl because both isomers have similar enthalpy of formation and ionization energy; nevertheless, the Rice-Ramsperger-Kassel-Marcus calculation prefers HCCCHCl. The measurement of photoionization spectra identifies product C3H3 as H2CCCH (propargyl). Both products C3H2Cl + H and C3H3 + Cl might correlate to the same triplet intermediate H2CCCHCl but have distinct angular distributions; the former is nearly isotropic whereas the latter is forward biased. A comparison with the C((3)P) + C2H3F reaction is stated.

Dynamics of the C/H and C/F exchanges in the reaction of 3P carbon atoms with vinyl fluoride

Two product channels C3H2F + H and C3H3 + F were identified in the reaction of C((3)P) atoms with vinyl fluoride (C2H3F) at collision energy 3.7 kcal mol(-1) in a crossed molecular-beam apparatus using selective photoionization. Time-of-flight (TOF) spectra of products C3H2F and C3H3 were measured at 12-16 laboratory angles as well as a TOF spectrum of atomic F, a counter part of C3H3, was recorded at single laboratory angle. From the best simulation of product TOF spectra, translational-energy distributions at seven scattering angles and a nearly isotropic (forward and backward peaked) angular distribution were derivable for exit channel C3H2F + H (C3H3 + F) that has average kinetic-energy release of 14.5 (4.9) kcal mol(-1). Products C3H2F + H and C3H3 + F were estimated to have a branching ratio of ~53:47. Furthermore, TOF spectra and photoionization spectra of products C3H2F and C3H3 were measured at laboratory angle 62° with photoionization energy ranging from 7 eV to 11.6 eV. The appearance of TOF spectra is insensitive to photon energy, implying that only single species overwhelmingly contributes to products C3H2F and C3H3. HCCCHF (H2CCCH) was identified as the dominant species based on the measured ionization threshold of 8.3 ± 0.2 (8.6 ± 0.2) eV and the maximal translational-energy release. The C/H and C/F exchange mechanisms are stated.

Elementary reactions and their role in gas-phase prebiotic chemistry

The formation of complex organic molecules in a reactor filled with gaseous mixtures possibly reproducing the primitive terrestrial atmosphere and ocean demonstrated more than 50 years ago that inorganic synthesis of prebiotic molecules is possible, provided that some form of energy is provided to the system. After that groundbreaking experiment, gas-phase prebiotic molecules have been observed in a wide variety of extraterrestrial objects (including interstellar clouds, comets and planetary atmospheres) where the physical conditions vary widely. A thorough characterization of the chemical evolution of those objects relies on a multi-disciplinary approach: 1) observations allow us to identify the molecules and their number densities as they are nowadays; 2) the chemistry which lies behind their formation starting from atoms and simple molecules is accounted for by complex reaction networks; 3) for a realistic modeling of such networks, a number of experimental parameters are needed and, therefore, the relevant molecular processes should be fully characterized in laboratory experiments. A survey of the available literature reveals, however, that much information is still lacking if it is true that only a small percentage of the elementary reactions considered in the models have been characterized in laboratory experiments. New experimental approaches to characterize the relevant elementary reactions in laboratory are presented and the implications of the results are discussed.

Probing the dynamics of polyatomic multichannel elementary reactions by crossed molecular beam experiments with soft electron-ionization mass spectrometric detection

In this Perspective we highlight developments in the field of chemical reaction dynamics. Focus is on the advances recently made in the investigation of the dynamics of elementary multichannel radical-molecule and radical-radical reactions, as they have become possible using an improved crossed molecular beam scattering apparatus with universal electron-ionization mass spectrometric detection and time-of-flight analysis. These improvements consist in the implementation of (a) soft ionization detection by tunable low-energy electrons which has permitted us to reduce interfering signals originating from dissociative ionization processes, usually representing a major complication, (b) different beam crossing-angle set-ups which have permitted us to extend the range of collision energies over which a reaction can be studied, from very low (a few kJ mol(-1), as of interest in astrochemistry or planetary atmospheric chemistry) to quite high energies (several tens of kJ mol(-1), as of interest in high temperature combustion systems), and (c) continuous supersonic sources for producing a wide variety of atomic and molecular radical reactant beams. Exploiting these new features it has become possible to tackle the dynamics of a variety of polyatomic multichannel reactions, such as those occurring in many environments ranging from combustion and plasmas to terrestrial/planetary atmospheres and interstellar clouds. By measuring product angular and velocity distributions, after having suppressed or mitigated, when needed, the problem of dissociative ionization of interfering species (reactants, products, background gases) by soft ionization detection, essentially all primary reaction products can be identified, the dynamics of each reaction channel characterized, and the branching ratios determined as a function of collision energy. In general this information, besides being of fundamental relevance, is required for a predictive description of the chemistry of these environments via computer models. Examples are taken from recent on-going work (partly published) on the reactions of atomic oxygen with acetylene, ethylene and allyl radical, of great importance in combustion. A reaction of relevance in interstellar chemistry, as that of atomic carbon with acetylene, is also discussed briefly. Comparison with theoretical results is made wherever possible, both at the level of electronic structure calculations of the potential energy surfaces and dynamical computations. Recent complementary CMB work as well as kinetic work exploiting soft photo-ionization with synchrotron radiation are noted. The examples illustrated in this article demonstrate that the type of dynamical results now obtainable on polyatomic multichannel radical-molecule and radical-radical reactions might well complement reaction kinetics experiments and hence contribute to bridging the gap between microscopic reaction dynamics and thermal reaction kinetics, enhancing significantly our basic knowledge of chemical reactivity and understanding of the elementary reactions which occur in real-world environments.

Chemistry of energetically activated cumulenes - from allene (H2CCCH2) to hexapentaene (H2CCCCCCH2)

During the last decade, experimental and theoretical studies on the unimolecular decomposition of cumulenes (H(2)C(n)H(2)) from propadiene (H(2)CCCH(2)) to hexapentaene (H(2)CCCCCCH(2)) have received considerable attention due to the importance of these carbon-bearing molecules in combustion flames, chemical vapor deposition processes, atmospheric chemistry, and the chemistry of the interstellar medium. Cumulenes and their substituted counterparts also have significant technical potential as elements for molecular machines (nanomechanics), molecular wires (nano-electronics), nonlinear optics, and molecular sensors. In this review, we present a systematic overview of the stability, formation, and unimolecular decomposition of chemically, photo-chemically, and thermally activated small to medium-sized cumulenes in extreme environments. By concentrating on reactions under gas phase thermal conditions (pyrolysis) and on molecular beam experiments conducted under single-collision conditions (crossed beam and photodissociation studies), a comprehensive picture on the unimolecular decomposition dynamics of cumulenes transpires.

Dynamics of the C(D1)+D2 reaction: A comparison of crossed molecular-beam experiments with quasiclassical trajectory and accurate statistical calculations

In this paper we report a combined experimental and theoretical study on the dynamics of the insertion reaction C((1)D)+D(2) at 15.5 kJ mol(-1) collision energy. Product angular and velocity distributions have been obtained in crossed beam experiments and quasiclassical trajectory (QCT) and rigorous statistical calculations have been performed on the recent and accurate ab initio potential energy surface of Bussery-Honvault, Honvault, and Launay at the energy of the experiment. The molecular-beam results have been simulated using the theoretical calculations. Good agreement between experiment and both QCT and statistical predictions is found.

Chemistry of Star-Forming Regions

The space between stars is not empty but contains gas-phase and particulate matter under varying conditions. Neutral matter is found mainly in large regions of the interstellar medium known as "clouds", the largest of which, termed "giant molecular clouds", are essentially molecular in nature. Stars and planetary systems form inside these giant clouds when portions collapse and heat up. The details of the collapse can be followed by observation of the chemical changes in the molecular composition of the gas and dust particles. Moreover, an understanding of the chemical processes yields much information on the time scales and histories of the assorted stages. Among the most recent additions to our chemical knowledge of star formation are a deeper understanding of isotopic fractionation, especially involving deuterium, and a realization that the role of neutral-neutral reactions is more salient than once thought possible.