Theoretical study of the reaction mechanism of ethynyl radical with benzene and related reactions on the C8H7 potential energy surface

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.

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.

Products from pyrolysis of gas-phase propionaldehyde

A hyperthermal nozzle was utilized to study the thermal decomposition of propionaldehyde, CH3CH2CHO, over a temperature range of 1073-1600 K. Products were identified with two detection methods: matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry. Evidence was observed for four reactions during the breakdown of propionaldehyde: α-C-C bond scission yielding CH3CH2, CO, and H, an elimination reaction forming methylketene and H2, an isomerization pathway leading to propyne via the elimination of H2O, and a β-C-C bond scission channel forming methyl radical and (•)CH2CHO. The products identified during this experiment were CO, HCO, CH3CH2, CH3CH═C═O, H2O, CH3C≡CH, CH3, H2C═C═O, CH2CH2, CH3CH═CH2, HC≡CH, CH2CCH, H2CO, C4H2, C4H4, and CH3CHO. The first eight products result from primary or bimolecular reactions involving propionaldehyde while the remaining products occur from reactions including the initial pyrolysis products. While the pyrolysis of propionaldehyde involves reactions similar to those observed for acetaldehyde and butyraldehyde in recent studies, there are a few unique products observed which highlight the need for further study of the pyrolysis mechanism.

The role of isovalency in the reactions of the cyano (CN), boron monoxide (BO), silicon nitride (SiN), and ethynyl (C2H) radicals with unsaturated hydrocarbons acetylene (C2H2) and ethylene (C2H4)

The classification of chemical reactions based on shared characteristics is at the heart of the chemical sciences, and is well exemplified by Langmuir's concept of isovalency, in which ‘two molecular entities with the same number of valence electrons have similar chemistries’.

Modeling reaction pathways of low energy particle deposition on polymer surfaces via first principle calculations

The chemical processes that lead to polystyrene surface modification via low energy deposition of C(2)H(+), C(2)F(+), CH(2), CH(2)(+), and H(+) radicals and ions are examined using first principles calculations. Specifically, the reaction mechanisms responsible for products identified in classical molecular dynamics with reactive empirical bond-order potentials are examined using density functional theory. In addition, these calculations consider how the presence of charges on the incident particles changes the result for the CH(2) system through the comparison of barriers, transition states, and final products for CH(2) and CH(2)(+). The structures of the reaction species and energy barriers are determined using the B3LYP hybrid functional. Finally, CCSD/6-31G(d,p) single point energy calculations are carried out to obtain optimized energy barriers. The results indicate that the large variety of reactions occurring on the polystyrene surface are a consequence of complex interactions between the substrate and the deposited particles, which can easily be identified and characterized using advanced computational methodologies, such as first principle calculations.

Crossed molecular beam study on the formation of phenylacetylene and its relevance to Titan's atmosphere

The crossed molecular beam experiment of the deuterated ethynyl radical (C(2)D; X(2)Sigma(+)) with benzene [C(6)H(6)(X(1)A(1g))] and its fully deuterated analog [C(6)D(6)(X(1)A(1g))] was conducted at a collision energy of 58.1 kJ mol(-1). Our experimental data suggest the formation of the phenylacetylene-d(6) via indirect reactive scattering dynamics through a long-lived reaction intermediate; the reaction is initiated by a barrierless addition of the ethynyl-d(1) radical to benzene-d(6). This initial collision complex was found to decompose via a tight exit transition state located about 42 kJ mol(-1) above the separated products; here, the deuterium atom is ejected almost perpendicularly to the rotational plane of the decomposing intermediate and almost parallel to the total angular momentum vector. The overall experimental exoergicity of the reaction is shown to be 121 +/- 10 kJ mol(-1); this compares nicely with the computed reaction energy of -111 kJ mol(-1). Even though the experiment was conducted at a collisional energy higher than equivalent temperatures typically found in the atmosphere of Titan (94 K and higher), the reaction may proceed in Titan's atmosphere as it involves no entrance barrier, all transition states involved are below the energy of the separated reactants, and the reaction is exoergic. Further, the phenylacetylene was found to be the sole reaction product.

A theoretical study of the reaction mechanism and product branching ratios of C2H + C2H4 and related reactions on the C4H5 potential energy surface

Ab initio and density functional RCCSD(T)/cc-pVQZ//B3LYP/6-311G** calculations of various stationary points on the C(4)H(5) global potential energy surface have been performed to resolve the C(2)H + C(2)H(4) and C(2)H(3) + C(2)H(2) reaction mechanisms under single-collision conditions. The results show vinylacetylene + H as the nearly exclusive products for both reactions, with exothermicities of 26.5 and 4.3 kcal/mol, respectively. For C(2)H + C(2)H(4), the most important mechanisms include a barrierless formation of the CH(2)CH(2)CCH adduct c6 (56.9 kcal/mol below the reactants) in the entrance channel followed either by H loss from the vicinal CH(2) group via a barrier of 35.7 kcal/mol or by 1,2-H migration to form CH(3)CHCCH c3 (69.8 kcal/mol lower in energy than C(2)H + C(2)H(4)) via a 33.8 kcal/mol barrier and H elimination from the terminal CH(3) group occurring with a barrier of 49.4 kcal/mol. RRKM calculations of energy-dependent rate constants for individual reaction steps and branching ratios for various channels indicate that 77-78% of vinylacetylene is formed from the initial adduct, whereas 22-21% is produced via the two-step mechanism involving the 1,2-H shift c6-c3, with alternative channels contributing less than 1%. The theoretical results support the experimental crossed molecular beams observations of vinylacetylene being the major product of the C(2)H + C(2)H(4) reaction and the fact that CH(2)CHCCH is formed via a tight transition state with an exit barrier of 5-6 kcal/mol and also confirm that vinylacetylene can be produced from C(2)H + C(2)H(4) under low temperature conditions of Titan's atmosphere. The prevailing mechanism for the C(2)H(3) + C(2)H(2) reaction starts from the initial formation of different n-C(4)H(5) conformers occurring with significant entrance barriers of approximately 6 kcal/mol. The n-C(4)H(5) isomers reside 35-38 kcal/mol lower in energy than C(2)H(3) + C(2)H(2) and can rapidly rearrange to one another overcoming relatively low barriers of 3-5 kcal/mol. H loss from the n-C(4)H(5) species then gives the vinylacetylene product via exit barriers of approximately 6 kcal/mol with the corresponding transition states lying 1.2-1.6 kcal/mol above the C(2)H(3) + C(2)H(2) reactants. Since the C(2)H(3) + C(2)H(2) reaction is hindered by relatively high entrance barriers, it is not expected to be important in Titan's atmospheric environments but can produce n-C(4)H(5) or vinylacetylene under high temperature and pressure combustion conditions.