Functional Relationships between Kinetic, Flow, and Geometrical Parameters in a High-Temperature Chemical Microreactor

Computational fluid dynamics (CFD) simulations and isothermal approximation were applied for the interpretation of experimental measurements of the C₁₀H₇Br pyrolysis efficiency in the high-temperature microreactor and of the pressure drop in the flow tube of the reactor. Applying isothermal approximation allows the derivation of analytical relationships between the kinetic, gas flow, and geometrical parameters of the microreactor, which, along with CFD simulations, accurately predict the experimental observations. On the basis of the obtained analytical relationships, a clear strategy for measuring rate coefficients of (pseudo) first-order bimolecular and unimolecular reactions using the microreactor was proposed. The pressure- and temperature-dependent rate coefficients for the C₁₀H₇Br pyrolysis calculated using variable reaction coordinate transition state theory were invoked to interpret the experimental data on the pyrolysis efficiency.

Synthesis of urea in cometary model ices and implications for Comet 67P/Churyumov–Gerasimenko

Formamide and urea, two building blocks of life, form easily in astrophysical ices made of very simple, inorganic materials.

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’.

Laboratory simulation of Kuiper belt object volatile ices under ionizing radiation: CO-N2 ices as a case study

The exposure of icy Kuiper belt objects (KBOs) by ionizing radiation was simulated in this case of exposing carbon monoxide-nitrogen (CO-N(2)) ices by energetic electrons. The radiation-induced chemical processing was monitored on-line and in situ via FTIR spectroscopy and quadrupole mass spectrometry. Besides the array of carbon oxides being reproduced as in neat irradiated carbon monoxide (CO) ices studied previously, the radiation exposure at 10 K resulted in the formation of nitrogen-bearing species of isocyanato radical (OCN), linear (l-NCN), nitric oxide (NO), nitrogen dioxide (NO(2)), plus diazirinone (N(2)CO). The infrared assignments of these species were further confirmed by isotopic shifts. The temporal evolution of individual species was found to fit in first-order reaction schemes, prepping up the underlying non-equilibrium chemistry on the formation of OCN, l-NCN, and NO radicals in particular. Also unique to the binary KBO model ices and viable for the future remote detection is diazirinone (N(2)CO) at 1860 cm(-1) (2ν(5)) formed at lower radiation exposure.

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.

Reaction Dynamics on the Formation of Styrene: A Crossed Molecular Beam Study of the Reaction of Phenyl Radicals with Ethylene

The reaction dynamics of phenyl radicals (C6H5) with ethylene (C2H4) and D4-ethylene (C2D4) were investigated at two collision energies of 83.6 and 105.3 kJ mol-1 utilizing a crossed molecular beam setup. The experiments suggested that the reaction followed indirect scattering dynamics via complex formation and was initiated by an addition of the phenyl radical to the carbon-carbon double bond of the ethylene molecule forming a C6H5CH2CH2 radical intermediate. Under single collision conditions, this short-lived transient species was found to undergo unimolecular decomposition via atomic hydrogen loss through a tight exit transitions state to synthesize the styrene molecule (C6H5C2H3). Experiments with D4-ethylene verified that in the corresponding reaction with ethylene the hydrogen atom was truly emitted from the ethylene unit but not from the phenyl moiety. The overall reaction to form styrene plus atomic hydrogen from the reactants was found to be exoergic by 25 +/- 12 kJ mol(-1). This study provides solid evidence that in combustion flames the styrene molecule, a crucial precursor to form polycyclic aromatic hydrocarbons (PAHs), can be formed within a single neutral-neutral collision, a long-standing theoretical prediction which has remained to be confirmed by laboratory experiments under well-defined single collision conditions for the last 50 years.

Crossed molecular beam studies of the reactions of allyl radicals, C3H5(X2A2), with methylacetylene (CH3CCH(X1A1)), allene (H2CCCH2(X1A1)), and their isotopomers

The chemical dynamics of the reaction of allyl radicals, C(3)H(5)(X(2)A(2)), with two C(3)H(4) isomers, methylacetylene (CH(3)CCH(X(1)A(1))) and allene (H(2)CCCH(2)(X(1)A(1))) together with their (partially) deuterated counterparts, were unraveled under single-collision conditions at collision energies of about 125 kJ mol(-1) utilizing a crossed molecular beam setup. The experiments indicate that the reactions are indirect via complex formation and proceed via an addition of the allyl radical with its terminal carbon atom to the terminal carbon atom of the allene and of methylacetylene (alpha-carbon atom) to form the intermediates H(2)CCHCH(2)CH(2)CCH(2) and H(2)CCHCH(2)CHCCH(3), respectively. The lifetimes of these intermediates are similar to their rotational periods but too short for a complete energy randomization to occur. Experiments with D4-allene and D4-methylacetylene verify explicitly that the allyl group stays intact: no hydrogen emission was observed but only the release of deuterium atoms from the perdeuterated reactants. Further isotopic substitution experiments with D3-methylacetylene combined with the nonstatistical nature of the reaction suggest that the intermediates decompose via hydrogen atom elimination to 1,3,5-hexatriene, H(2)CCHCH(2)CHCCH(2), and 1-hexen-4-yne, H(2)CCHCH(2)CCCH(3), respectively, via tight exit transition states located about 10-15 kJ mol(-1) above the separated products. The overall reactions were found to be endoergic by 98 +/- 4 kJ mol(-1) and have characteristic threshold energies to reaction between 105 and 110 kJ mol(-1). Implications of these findings to combustion and interstellar chemistry are discussed.

Unraveling the formation of HCPH(X2A') molecules in extraterrestrial environments: crossed molecular beam study of the reaction of carbon atoms, C(3Pj), with phosphine, PH3(X1A1)

The reaction between ground state carbon atoms, C(3P(j)), and phosphine, PH3(X(1)A1), was investigated at two collision energies of 21.1 and 42.5 kJ mol(-1) using the crossed molecular beam technique. The chemical dynamics extracted from the time-of-flight spectra and laboratory angular distributions combined with ab initio calculations propose that the reaction proceeds on the triplet surface via an addition of atomic carbon to the phosphorus atom. This leads to a triplet CPH3 complex. A successive hydrogen shift forms an HCPH2 intermediate. The latter was found to decompose through atomic hydrogen emission leading to the cis/trans-HCPH(X(2)A') reaction products. The identification of cis/trans-HCPH(X(2)A') molecules under single collision conditions presents a potential pathway to form the very first carbon-phosphorus bond in extraterrestrial environments like molecular clouds and circumstellar envelopes, and even in the postplume chemistry of the collision of comet Shoemaker-Levy 9 with Jupiter.

Ab initio/Rice-Ramsperger-Kassel-Marcus study of the singlet C4H4 potential energy surface and of the reactions of C2(XΣg+1) with C4H4(XA1g+1) and C(D1) with C3H4 (allene and methylacetylene)

Ab initio modified Gaussian-2 G2M(RCC,MP2) calculations have been performed for various isomers and transition states on the singlet C4H4 potential energy surface. The computed relative energies and molecular parameters have then been used to calculate energy-dependent rate constants for different isomerization and dissociation processes in the C4H4 system employing Rice-Ramsperger-Kassel-Marcus theory and to predict branching ratios of possible products of the C2(1Sigmag+)+C2H4, C(1D)+H2CCCH2, and C(1D)+H3CCCH reactions under single-collision conditions. The results show that C2 adds to the double C=C bond of ethylene without a barrier to form carbenecyclopropane, which then isomerizes to butatriene by a formal C2 "insertion" into the C-C bond of the C2H4 fragment. Butatriene can rearrange to the other isomers of C4H4, including allenylcarbene, methylenecyclopropene, vinylacetylene, methylpropargylene, cyclobutadiene, tetrahedrane, methylcyclopropenylidene, and bicyclobutene. The major decomposition products of the chemically activated C4H4 molecule formed in the C2(1Sigmag+)+C2H4 reaction are calculated to be acetylene+vinylidene (48.6% at Ecol = 0) and 1-buten-3-yne-2-yl radical [i-C4H3(X2A'), H2C=C=C=CH*]+H (41.3%). As the collision energy increases from 0 to 10 kcal/mol, the relative yield of i-C4H3+H grows to 52.6% and that of C2H2+CCH2 decreases to 35.5%. For the C(1D)+allene reaction, the most important products are also i-C4H3+H (55.2%) and C2H2+CCH2 (30.1%), but for C(1D)+methylacetylene, which accesses a different region of the C4H4 singlet potential energy surface, the calculated product branching ratios differ significantly: 65%-69% for i-C4H3+H, 18%-14% for C2H2+CCH2, and approximately 8% for diacetylene+H2.

Reaction of cyanoacetylene HCCCN(XΣ+1) with ground-state carbon atoms C(P3) in cold molecular clouds

The reaction of the simplest cyanopolyyne, cyanoacetylene [HCCCN(X (1)Sigma(+))], with ground-state atomic carbon C((3)P) is investigated theoretically to explore the probable routes for the depletion of the famed interstellar molecule HCCCN, and the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Six collision complexes (c1-c6) without entrance barrier as a result of the carbon atom addition to the pi systems of HCCCN are located. The optimized geometries and harmonic frequencies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex are obtained by utilizing the unrestricted B3YLP6-311G(d,p) level of theory, and the corresponding CCSD(T)/cc-pVTZ energies are calculated. Subsequently, with the facilitation of Rice-Ramsperger-Kassel-Marcus (RRKM) and variational RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for the titled reaction are determined, and the product yields are estimated. Five collision complexes (c1-c3, c5, and c6) are predicted to give the same products, a chained CCCCN (p2)+H, via the linear and most stable intermediate, HCCCCN (i2), while collision complex c4 is likely to dissociate back to C+HCCCN. The study suggests that this class of reaction is an important route to the destruction of cyanoacetylene and cyanopolyynes in general, and to the synthesis of linear carbon-chained nitriles at the temperature as low as 10 K to be incorporated in future chemical models of interstellar clouds.

A crossed beams study of the reaction of carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X1A′)—investigating the formation of cyano propargyl radicals

The chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A'), were examined under single collision conditions at collision energies of 29.9 and 43.9 kJ mol(-1) using the crossed molecular beams approach. The experimental studies were combined with electronic structure calculations on the triplet C4H3N potential energy surface (H. F. Su, R. I. Kaiser, A. H. H. Chang, J. Chem. Phys., 2005, 122, 074320). Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less; also, no molecular hydrogen elimination channel was observed experimentally. These results are in agreement with the computational studies predicting solely the existence of a carbon versus hydrogen atom exchange pathway and the dominance of the 1-cyano propargyl radical product. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical.

Potential Energy Surface and Product Branching Ratios for the Reaction of Dicarbon, C2(XΣg+), with Methylacetylene, CH3CCH(X1A1): An Ab Initio/RRKM Study

Ab initio calculations of the potential energy surface for the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction have been carried at the G2M level of theory. The calculations show that the dicarbon molecule in the ground singlet electronic state can add to methylacetylene without a barrier producing a three-member or a four-member ring intermediate, which can rapidly rearrange to the most stable H(3)CCCCCH isomer on the C(5)H(4) singlet surface. This isomer can then lose a hydrogen atom (H) or molecular hydrogen (H(2)) from the CH(3) group with the formation of H(2)CCCCCH and HCCCCCH, respectively. Alternatively, H atom migrations and three-member-ring closure/opening rearrangements followed by H and H(2) losses can lead to other isomers of the C(5)H(3) and C(5)H(2) species. According to the calculated energetics, the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH reaction is likely to be a major source of the C(5)H(3) radicals (in particular, the most stable H(2)CCCCCH and HCCCHCCH isomers, which are relevant to the formation of benzene through the reactions with CH(3)). Among heavy-fragment product channels, only C(3)H(3) + C(2)H and c-C(3)H(2) + C(2)H(2) might compete with C(5)H(3) + H and C(5)H(2) + H(2). RRKM calculations of reaction rate constants and product branching ratios depending on the reactive collision energy showed that the major reaction products are expected to be H(2)CCCCCH + H (64-66%) and HCCCHCCH + H (34-30%), with minor contributions from HCCCCCH + H(2) (1-2%), HCCCHCC + H(2) (up to 1%), C(3)H(3) + C(2)H (up to 1%), and c-C(3)H(2) + C(2)H(2) (up to 0.1%) if the energy randomization is complete. The calculations also indicate that the C(2)(X(1)Sigma(g)(+)) + CH(3)CCH(X(1)A(1)) reaction can proceed by direct H-abstraction of a methyl hydrogen to form C(3)H(3) + C(2)H almost without a barrier.

A theoretical study for the reaction of vinyl cyanide C2H3CN(XA′1) with the ground state carbon atom C(P3) in cold molecular clouds

The reaction of the ground state atomic carbon, C(3P), with simple unsaturated nitrile, C2H3CN(X1A' (vinyl cyanide), is investigated theoretically to explore the probable routes for the formation of carbon-nitrogen-bearing species in extraterrestrial environments particularly of ultralow temperature. Five collision complexes without entrance barrier as a result of the carbon atom addition to the pi systems of C2H3CN are characterized. The B3YLP/6-311G(d,p) level of theory is utilized in obtaining the optimized geometries, harmonic frequencies, and energies of the intermediates, transition states, and products along the isomerization and dissociation pathways of each collision complex. Subsequently, with the facilitation of computed RRKM rate constants at collision energy of 0-10 kcal/mol, the most probable paths for each collision complexes are determined, of which the CCSD(T)/6-311G(d,p) energies are calculated. The major products predicted are exclusively due to the hydrogen atom dissociations, while the products of H2, CN, and CH2 decompositions are found negligible. Among many possible H-elimination products, cyano propargyl (p4) and 3-cyano propargyl (p5) are the most probable, in which p5 can be formed via two intermediates, cyano allene (i8) and cyano vinylmethylene (i6), while p4 is yielded from i8. The study suggests this class of reaction is an important route to the synthesis of unsaturated nitriles at the temperature as low as 10 K, and the results are valuable for future chemical models of interstellar clouds.

A combined crossed molecular beam and ab initio investigation of C2 and C3 elementary reactions with unsaturated hydrocarbons--pathways to hydrogen deficient hydrocarbon radicals in combustion flames

Crossed molecular beam experiments on dicarbon and tricarbon reactions with unsaturated hydrocarbons acetylene, methylacetylene, and ethylene were performed to investigate the dynamics of channels leading to hydrogen-deficient hydrocarbon radicals. In the light of the results of new ab initio calculations, the experimental data suggest that these reactions are governed by an initial addition of C2/C3 to the pi molecular orbitals forming highly unsaturated cyclic structures. These intermediates are connected via various transition states and are suggested to ring open to chain isomers which decompose predominantly by displacement of atomic hydrogen, forming C4H, C5H, HCCCCCH2, HCCCCCCH3, H2CCCCH and H2CCCCCH. The C2(1 sigma g+) + C2H4 reaction has no entrance barrier and the channel leading to the H2CCCCH product is strongly exothermic. This is in strong contrast with the C3(1 sigma g+) + C2H4 reaction as this is characterized by a 26.4 kJ mol-1 threshold to form a HCCCCCH2 isomer. Analogous to the behavior with ethylene, preliminary results on the reactions of C2 and C3 with C2H2 and CH3CCH showed the H-displacement channels of these systems to share many similarities such as the absence/presence of an entrance barrier and the reaction mechanism. The explicit identification of the C2/C3 vs. hydrogen displacement demonstrates that hydrogen-deficient hydrocarbon radicals can be formed easily in environments like those of combustion processes. Our work is a first step towards a systematic database of the intermediates and the reaction products which are involved in this important class of reactions. These findings should be included in future models of PAH and soot formation in combustion flames.

The formation of nitriles in hydrocarbon-rich atmospheres of planets and their satellites: laboratory investigations by the crossed molecular beam technique

Crossed molecular beam experiments of cyano radicals, CN(X(2)Sigma(+)), reacting with unsaturated hydrocarbons have been performed to investigate synthetic routes to nitriles formation in hydrocarbon-rich atmospheres of planets and their moons. We have verified that all cyano radical reactions with acetylene, ethylene, methylacetylene, allene, benzene, and dimethylacetylene proceed without entrance barrier, have exit barriers well below the energy of the reactant molecules, and are strongly exothermic. The identification of the CN versus H atom exchange channel makes these reactions compelling candidates to synthesize unsaturated nitriles in solar system environments. Some of these nitriles, hitherto unobserved in our solar system, now represent an ideal target to be detected in the future Cassini-Huygens mission to Titan.