The reaction of benzene with a ground state carbon atom, C(3Pj)

An experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction with implications for the photochemical models of Titan

We report on a combined experimental and theoretical investigation of the N(2D) + C6H6 (benzene) reaction, which is of relevance in the aromatic chemistry of the atmosphere of Titan. Experimentally, the reaction was studied (i) under single-collision conditions by the crossed molecular beams (CMB) scattering method with mass spectrometric detection and time-of-flight analysis at the collision energy (Ec) of 31.8 kJ mol-1 to determine the primary products, their branching fractions (BFs), and the reaction micromechanism, and (ii) in a continuous supersonic flow reactor to determine the rate constant as a function of temperature from 50 K to 296 K. Theoretically, electronic structure calculations of the doublet C6H6N potential energy surface (PES) were performed to assist the interpretation of the experimental results and characterize the overall reaction mechanism. The reaction is found to proceed via barrierless addition of N(2D) to the aromatic ring of C6H6, followed by formation of several cyclic (five-, six-, and seven-membered ring) and linear isomeric C6H6N intermediates that can undergo unimolecular decomposition to bimolecular products. Statistical estimates of product BFs on the theoretical PES were carried out under the conditions of the CMB experiments and at the temperatures relevant for Titan's atmosphere. In all conditions the ring-contraction channel leading to C5H5 (cyclopentadienyl) + HCN is dominant, while minor contributions come from the channels leading to o-C6H5N (o-N-cycloheptatriene radical) + H, C4H4N (pyrrolyl) + C2H2 (acetylene), C5H5CN (cyano-cyclopentadiene) + H, and p-C6H5N + H. Rate constants (which are close to the gas kinetic limit at all temperatures, with the recommended value of 2.19 ± 0.30 × 10-10 cm3 s-1 over the 50-296 K range) and BFs have been used in a photochemical model of Titan's atmosphere to simulate the effect of the title reaction on the species abundances as a function of the altitude.

Chemistry and physics of dopants embedded in helium droplets

Helium droplets represent a cold inert matrix, free of walls with outstanding properties to grow complexes and clusters at conditions that are perfect to simulate cold and dense regions of the interstellar medium. At sub-Kelvin temperatures, barrierless reactions triggered by radicals or ions have been observed and studied by optical spectroscopy and mass spectrometry. The present review summarizes developments of experimental techniques and methods and recent results they enabled.

Gas-Phase Infrared Spectra of the C7H5 Radical and Its Bimolecular Reaction Products

Resonance-stabilized radicals are considered as possible intermediates in the formation of polycyclic aromatic hydrocarbons (PAHs) in interstellar space. Here, we investigate the fulvenallenyl radical, the most stable C₇H₅ isomer by IR/UV ion dip spectroscopy employing free electron laser radiation in the mid-infrared region between 550 and 1750 cm^-1. The radical is generated by pyrolysis from phthalide. Various jet-cooled reaction products are identified by their mass-selective IR spectra in the fingerprint region, based on a comparison with computed spectra. Interestingly, benzyl is present as a second resonance-stabilized radical. It is connected to fulvenallenyl by a sequence of two H atom losses or additions. Among the identified aromatic hydrocarbons are toluene and styrene, as well as polycyclic molecules, such as indene, naphthalene, fluorene and phenanthrene. Mechanisms for the formation of PAH from C₇H₅ have already been suggested in previous computational work. In particular, the radical/radical reaction of two fulvenallenyl radicals provides an efficient route to phenanthrene in one bimolecular step and might be relevant for PAH formation under astrochemical conditions.

Untangling the methane chemistry in interstellar and solar system ices toward ionizing radiation: a combined infrared and reflectron time-of-flight analysis

Pure methane (CH₄/CD₄) ices were exposed to three ionizing radiation sources at 5.5 K under ultrahigh vacuum conditions to compare the complex hydrocarbon spectrum produced across several interstellar environments. These irradiation sources consisted of energetic electrons to simulate secondary electrons formed in the track of galactic cosmic rays (GCRs), Lyman α (10.2 eV; 121.6 nm) photons simulated the internal VUV field in a dense cloud, and broadband (112.7-169.8 nm; 11.0-7.3 eV) photons which mimic the interstellar ultra-violet field. The in situ chemical evolution of the ices was monitored via Fourier transform infrared spectroscopy (FTIR) and during heating via mass spectrometry utilizing a quadrupole mass spectrometer with an electron impact ionization source (EI-QMS) and a reflectron time-of-flight mass spectrometer with a photoionization source (PI-ReTOF-MS). The FTIR analysis detected six small hydrocarbon products from the three different irradiation sources: propane [C₃H₈(C₃D₈)], ethane [C₂H₆(C₂D₆)], the ethyl radical [C₂H₅(C₂D₅)], ethylene [C₂H₄(C₂D₄)], acetylene [C₂H₂(C₂D₂)], and the methyl radical [CH₃(CD₃)]. The sensitive PI-ReTOF-MS analysis identified a complex array of products with different products being detected between experiments with general formulae: C_nH_2n+2 (n = 4-8), C_nH_2n (n = 3-9), C_nH_2n-2 (n = 3-9), C_nH_2n-4 (n = 4-9), and C_nH_2n-6 (n = 6-7) from electron irradiation and C_nH_2n+2 (n = 4-8), C_nH_2n (n = 3-10), C_nH_2n-2 (n = 3-11), C_nH_2n-4 (n = 4-11), C_nH_2n-6 (n = 5-11), and C_nH_2n-8 (n = 6-11) from broadband photolysis and Lyman α photolysis. These experiments show that even the simplest hydrocarbon can produce important complex hydrocarbons such as C₃H₄ and C₄H₆ isomers. Distinct isomers from these groups have been shown to be important reactants in the synthesis of polycyclic aromatic hydrocarbons like indene (C₉H₈) and naphthalene (C₁₀H₈) under interstellar conditions.

Gas-Phase Synthesis of the Elusive Cyclooctatetraenyl Radical (C8 H7 ) via Triplet Aromatic Cyclooctatetraene (C8 H8 ) and Non-Aromatic Cyclooctatriene (C8 H8 ) Intermediates

The 1,2,4,7-cyclooctatetraenyl radical (C₈ H₇ ) has been synthesized for the very first time via the bimolecular gas-phase reaction of ground-state carbon atoms with 1,3,5-cycloheptatriene (C₇ H₈ ) on the triplet surface under single-collision conditions. The barrier-less route to the cyclic 1,2,4,7-cyclooctatetraenyl radical accesses exotic reaction intermediates on the triplet surface, which cannot be synthesized via classical organic chemistry methods: the triplet non-aromatic 2,4,6-cyclooctatriene (C₈ H₈ ) and the triplet aromatic 1,3,5,7-cyclooctatetraene (C₈ H₈ ). Our approach provides a clean gas-phase synthesis of this hitherto elusive cyclic radical species 1,2,4,7-cyclooctatetraenyl via a single-collision event and opens up a versatile, unconventional path to access this previously largely obscure class of cyclooctatetraenyl radicals, which have been impossible to access through classical synthetic methods.

Gas Phase Detection of Benzocyclopropenyl

The gas phase detection of benzocyclopropenyl is reported. In this aromatic resonance stabilized radical, a large angular strain is present due to a three-membered ring annelated to a benzene. The resonant two-color two-photon ionization technique is used to record the D1((2)A2) ← D0((2)B1) electronic transition of this radical after the in situ synthesis in a discharge source. The spectrum features absorptions up to 3300 cm(-1) above the origin band at 19,305 cm(-1). Benzocyclopropenyl is possibly the major product of the bimolecular reaction of benzene and an atomic carbon at low temperatures.

Chemical dynamics of the formation of the 1,3-butadiynyl radical (C4H(X2Sigma+)) and its isotopomers

The reaction of dicarbon molecules in their electronic ground, C2(X1Sigma(g)+), and first excited state, C2(a3Pi(u)), with acetylene, C2H2(X1Sigma(g)+), to synthesize the 1,3-butadiynyl radical, C4H(X2Sigma+), plus a hydrogen atom was investigated at six different collision energies between 10.6 and 47.5 kJ mol(-1) under single collision conditions. These studies were contemplated by crossed molecular beam experiments of dicarbon with three acetylene isotopomers C2D2(X1Sigma(g)+), C2HD (X1Sigma+), and 13C2H2(X1Sigma(g)+) to elucidate the role of intersystem crossing (ISC) and of the symmetry of the reaction intermediate(s) on the center-of-mass functions. On the singlet surface, dicarbon was found to react with acetylene through an indirect reaction mechanism involving a diacetylene intermediate. The latter fragmented via a loose exit transition state via an emission of a hydrogen atom to form the 1,3-butadiynyl radical C4H(X2Sigma+). The D(infinity)(h) symmetry of the decomposing diacetylene intermediate results in collision-energy invariant, isotropic (flat) center-of-mass angular distributions of this microchannel. Isotopic substitution experiments suggested that at least at a collision energy of 29 kJ mol(-1), the diacetylene isotopomers are long-lived with respect to their rotational periods. On the triplet surface, the reaction involved three feasible addition complexes located in shallower potential energy wells as compared to singlet diacetylene. The involvement of the triplet surface accounted for the asymmetry of the center-of-mass angular distributions. The detection of the 1,3-butadiynyl radical, C4H(X2Sigma+), in the crossed beam reaction of dicarbon molecules with acetylene presents compelling evidence that the 1,3-butadiynyl radical can be formed via bimolecular reactions involving carbon clusters in extreme environments such as circumstellar envelopes of dying carbon stars and combustion flames.

The “Non-Reaction” of Ground-State Triplet Carbon Atoms with Water Revisited

We introduce a novel experimental setup for the generation of carbon atoms by means of pulsed laser ablation with a pulse rate optimized to avoid warming of the matrix. The combination of this technique with annealing of the matrix, recooling, and spectral recording (e.g. IR) allowed us to differentiate between the reactivity of triplet and singlet carbon atoms towards water under matrix-isolation conditions. Our experimental procedure assures the relaxation of all unreacted carbon atoms to their triplet ground state in the 10 K matrix before spectral recording. In agreement with CCSD(T)/cc-pVTZ+ZPVE computational data and earlier lower level results, we find that triplet carbon atoms indeed do not react with water, despite their high initial energy. Intersystem crossing of the triplet to singlet states of hydroxy carbene are less important, as the barriers for rearrangement of the initial complex of triplet carbon atoms and water to covalently bound species are too high, and dissociation is more likely. We found no evidence for a direct O--H bond-insertion path for triplet carbon atoms. Self-condensation reactions of triplet carbon atoms are clearly favored and yield carbon clusters that show reactivity of their own. The proposed formation of aldoses in extraterrestrial environments can therefore only derive from "hot" carbon atoms or through photoreactions.

An experimental and computational evaluation of the energetics of the isomeric methoxyphenylcarbenes generated in carbon atom reactions

Carbon atom reactions with anisole and methoxybenzaldehyde demonstrate the reversible ring expansion of methoxyphenylcarbene (CH(3)O-C(6)H(4)-C-H). Trapping with HBF(4) yields the methoxytropylium ion, analogous to the well-known reactions of phenylcarbene. For instance, in the reaction of carbon atom with p-methoxybenzaldehyde, which proceeds by deoxygenation of the carbonyl group and formation of the corresponding arylcarbene, the products formed are methoxytropylium fluoroborate, p-methoxytoluene and m-methoxytoluene in yields of 69.4, 7.6, and 22.9%, respectively. Gas-phase density functional theory calculations were also carried out. The observed product yields from the uniquely generated p- and m-methoxyphenylcarbenes are in good agreement with the calculations. In the case of o-methoxyphenylcarbene, however, the calculations indicate that formation of dihydrobenzofuran is the most facile rearrangement on the ground-state singlet surface. In contrast, ring expansion is observed to be the major reaction path experimentally. The exothermicity of the deoxygenation step for carbon atom reaction with methoxybenzaldehyde ( approximately 100 kcal/mol) can allow for an excited singlet state to be formed initially. This excited singlet state of the methoxyphenylcarbene will resemble open-shell phenylnitrene, which is known to undergo ring expansion much more readily than phenylcarbene. On the basis of this analogy, we reconcile the difference in reactivity of the arylcarbene predicted by density functional theory calculations with the reactivity observed experimentally.