Gas-Phase Synthesis of the Benzyl Radical (C6H5CH2)

Thermal Decomposition of 2- and 4-Iodobenzyl Iodide Yields Fulvenallene and Ethynylcyclopentadienes: A Joint Threshold Photoelectron and Matrix Isolation Spectroscopic Study

The thermal decomposition of 2- and 4-iodobenzyl iodide at high temperatures was investigated by mass-selective threshold photoelectron spectroscopy (ms-TPES) in the gas phase, as well as by matrix isolation infrared spectroscopy in cryogenic matrices. Scission of the benzylic C-I bond in the precursors at 850 K affords 2- and 4-iodobenzyl radicals (ortho- and para-IC6H4CH2•), respectively, in high yields. The adiabatic ionization energies of ortho-IC6H4CH2• to the X̃+(1A') and ã+(3A') cation states were determined to be 7.31 ± 0.01 and 8.78 ± 0.01 eV, whereas those of para-IC6H4CH2• were measured to be 7.17 ± 0.01 eV for X̃+(1A1) and 8.98 ± 0.01 eV for ã+(3A1). Vibrational frequencies of the ring breathing mode were measured to be 560 ± 80 and 240 ± 80 cm-1 for the X̃+(1A') and ã+(3A') cation states of ortho-IC6H4CH2•, respectively. At higher temperatures, subsequent aryl C-I cleavage takes place to form α,2- and α,4-didehydrotoluene diradicals, which rapidly undergo ring contraction to a stable product, fulvenallene. Nevertheless, the most intense vibrational bands of the elusive α,2- and α,4-didehydrotoluene diradicals were observed in the Ar matrices. In addition, high-energy and astrochemically relevant C7H6 isomers 1-, 2-, and 5-ethynylcyclopentadiene are observed at even higher pyrolysis temperatures along with fulvenallene. Complementary quantum chemical computations on the C7H6 potential energy surface predict a feasible reaction cascade at high temperatures from the diradicals to fulvenallene, supporting the experimental observations in both the gas phase and cryogenic matrices.

A unified reaction network on the formation of five-membered ringed polycyclic aromatic hydrocarbons (PAHs) and their role in ring expansion processes through radical-radical reactions

Exploiting a chemical microreactor in combination with an isomer-selective product identification through fragment-free photoionization utilizing tunable vacuum ultraviolet (VUV) light in tandem with the detection of the ionized molecules by a high resolution reflection time-of-flight mass spectrometer (Re-TOF-MS), the present investigation reveals molecular mass growth processes to four distinct polycyclic aromatic hydrocarbons carrying two six- and one five-membered ring (C₁₃H₁₀): 3H-cyclopenta[a]naphthalene, 1H-cyclopenta[b]naphthalene, 1H-cyclopenta[a]naphthalene, and fluorene in the gas phase. Temperatures of 973 and 1023 K simulating conditions in combustion settings along with circumstellar envelopes of carbon-rich stars and planetary nebulae. These reactions highlight the importance of methyl-substituted aromatic reactants (biphenyl, naphthalene) which can be converted to the methylene (-CH₂˙) motive by hydrogen abstraction or photolysis. Upon reaction with acetylene, methylene-substituted aromatics carrying a hydrogen atom at the ortho position of the ring can be then converted to cyclopentadiene-annulated aromatics thus providing a versatile pathway to five-membered ring aromatics at elevated temperatures.

The Role of Methylaryl Radicals in the Growth of Polycyclic Aromatic Hydrocarbons: The Formation of Five-Membered Rings

The regions of the C₁₃H₁₁ potential energy surface (PES) related to the unimolecular isomerization and decomposition of the 1-methylbiphenylyl radical and accessed by the 1-/2-methylnaphthyl + C₂H₂ reactions have been explored by ab initio G3(MP2,CC)//B3LYP/6-311G(d,p) calculations. The kinetics of these reactions relevant to the growth of polycyclic aromatic hydrocarbons (PAH) under high-temperature conditions in circumstellar envelopes and in combustion flames has been studied employing the RRKM-Master Equation approach. The unimolecular reaction of 1-methylbiphenylyl proceeding via a five-membered ring closure followed by H elimination is predicted to be very fast, on a submicrosecond scale above 1000 K and to result in the formation of an embedded five-membered ring in the 9H-fluorene product. The 1-/2-methylnaphthyl + C₂H₂ reaction mechanism involves acetylene addition to the radical on the methylene group followed by a six- or five-membered ring closure and aromatization via an H atom loss. Despite of the complexity of the C₁₃H₁₁ PES, these straightforward pathways are dominant in the high-temperature regime (above ∼1000 K), with the prevailing products being phenalene, with a significant contribution of 1H-cyclopenta(a)naphthalene, for 1-methylnaphthyl + C₂H₂, and 1H-cyclopenta(b)naphthalene and 3H-cyclopenta(a)naphthalene, for 2-methylnaphthyl + C₂H₂. The methylnaphthyl reactions with acetylene represent a clean source of the three-ring PAHs, but they are relatively slow owing to the high entrance barriers of ∼10 kcal/mol, with the rate constants of about an order of magnitude lower as compared to those for naphthyl + allene and σ-aryl + C₂H₂. The 1-methylnaphthyl + C₂H₂ and 2-methylnaphthyl + C₂H₂ reactions represent prototypes for PAH growth by an extra six- and five-membered ring on a zigzag edge or a corner of PAH and the generated modified Arrhenius expressions are recommended for kinetic modeling of PAH expansion by the mechanism of acetylene addition to methylaryl radicals.

A combined crossed molecular beams and computational study on the formation of distinct resonantly stabilized C5H3 radicals via chemically activated C5H4 and C6H6 intermediates

The crossed molecular beams technique was utilized to explore the formation of three isomers of resonantly stabilized (C5H3) radicals along with their d2-substituted counterparts via the bimolecular reactions of singlet/triplet dicarbon [C2(X1Σ+g/a3Πu)] with methylacetylene [CH3CCH(X1A1)], d3-methylacetylene [CD3CCH(X1A1)], and 1-butyne [C2H5CCH(X1A')] at collision energies up to 26 kJ mol-1via chemically activated singlet/triplet C5H4/C5D3H and C6H6 intermediates. These studies exploit a newly developed supersonic dicarbon [C2(X1Σ+g/a3Πu)] beam generated via photolysis of tetrachloroethylene [C2Cl4(X1Ag)] by excluding interference from carbon atoms, which represent the dominating (interfering) species in ablation-based dicarbon sources. We evaluated the performance of the dicarbon [C2(X1Σ+g/a3Πu)] beam in reactions with methylacetylene [CH3CCH(X1A1)] and d3-methylacetylene [CD3CCH(X1A1)]; the investigations demonstrate that the reaction dynamics match previous studies in our laboratory utilizing ablation-based dicarbon sources involving the synthesis of 1,4-pentadiynyl-3 [HCCCHCCH(X2B1)] and 2,4-pentadiynyl-1 [H2CCCCCH(X2B1)] radicals via hydrogen (deuterium) atom elimination. Considering the C2(X1Σ+g/a3Πu)-1-butyne [C2H5CCH(X1A')] reaction, the hitherto elusive methyl-loss pathway was detected. This channel forms the previously unknown resonantly stabilized penta-1-yn-3,4-dienyl-1 [H2CCCHCC(X2A)] radical along with the methyl radical [CH3(X2A2'')] and is open exclusively on the triplet surface with an overall reaction energy of -86 ± 10 kJ mol-1. The preferred reaction pathways proceed first by barrierless addition of triplet dicarbon to the π-electronic system of 1-butyne, either to both acetylenic carbon atoms or to the sterically more accessible carbon atom, to form the methyl-bearing triplet C6H6 intermediates [i41b] and [i81b], respectively, with the latter decomposing via a tight exit transition state to penta-1-yn-3,4-dienyl-1 [(H2CCCHCC(X2A)] plus the methyl radical [CH3(X2A2'')]. The successful unraveling of this methyl-loss channel - through collaborative experimental and computational efforts - underscores the viability of the photolytically generated dicarbon beam as an unprecedented tool to access reaction dynamics underlying the formation of resonantly stabilized free radicals (RSFR) that are vital to molecular mass growth processes that ultimately lead to polycyclic aromatic hydrocarbons (PAHs).

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.

On the formation of nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) in circumstellar and interstellar environments

The chemical evolution of extraterrestrial environments leads to the formation of nitrogen substituted polycyclic aromatic hydrocarbons (NPAHs) via gas phase radical mediated aromatization reactions.

Oxidation of the para-Tolyl Radical by Molecular Oxygen under Single-Collison Conditions: Formation of the para-Toloxy Radical

Crossed molecular beam experiments were performed to elucidate the chemical dynamics of the para-tolyl (CH₃C₆H₄) radical reaction with molecular oxygen (O₂) 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 (CH₃C₆H₄O) 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 (C₆H₅) 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.

A Combined Experimental and Theoretical Study on the Formation of the 2-Methyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne Radical (SiH; X(2)Π) with Methylacetylene (CH3CCH; X(1)A1) and D4-Methylacetylene (CD3CCD; X(1)A1)

The bimolecular gas-phase reactions of the ground-state silylidyne radical (SiH; X(2)Π) with methylacetylene (CH3CCH; X(1)A1) and D4-methylacetylene (CD3CCD; X(1)A1) were explored at collision energies of 30 kJ mol(-1) under single-collision conditions exploiting the crossed molecular beam technique and complemented by electronic structure calculations. These studies reveal that the reactions follow indirect scattering dynamics, have no entrance barriers, and are initiated by the addition of the silylidyne radical to the carbon-carbon triple bond of the methylacetylene molecule either to one carbon atom (C1; [i1]/[i2]) or to both carbon atoms concurrently (C1-C2; [i3]). The collision complexes [i1]/[i2] eventually isomerize via ring-closure to the c-SiC3H5 doublet radical intermediate [i3], which is identified as the decomposing reaction intermediate. The hydrogen atom is emitted almost perpendicularly to the rotational plane of the fragmenting complex resulting in a sideways scattering dynamics with the reaction being overall exoergic by -12 ± 11 kJ mol(-1) (experimental) and -1 ± 3 kJ mol(-1) (computational) to form the cyclic 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC3H4; p1). In line with computational data, experiments of silylidyne with D4-methylacetylene (CD3CCD; X(1)A1) depict that the hydrogen is emitted solely from the silylidyne moiety but not from methylacetylene. The dynamics are compared to those of the related D1-silylidyne (SiD; X(2)Π)-acetylene (HCCH; X(1)Σg(+)) reaction studied previously in our group, and from there, we discovered that the methyl group acts primarily as a spectator in the title reaction. The formation of 2-methyl-1-silacycloprop-2-enylidene under single-collision conditions via a bimolecular gas-phase reaction augments our knowledge of the hitherto poorly understood silylidyne (SiH; X(2)Π) radical reactions with small hydrocarbon molecules leading to the synthesis of organosilicon molecules in cold molecular clouds and in carbon-rich circumstellar envelopes.