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

Low-temperature formation of polycyclic aromatic hydrocarbons in Titan’s atmosphere

The detection of benzene in Titan’s atmosphere led to the emergence of polycyclic aromatic hydrocarbons (PAHs) as potential nucleation agents triggering the growth of Titan’s orange-brownish haze layers. However, the fundamental mechanisms leading to the formation of PAHs in Titan’s low-temperature atmosphere have remained elusive. We provide persuasive evidence through laboratory experiments and computations that prototype PAHs like anthracene and phenanthrene (C14H10) are synthesized via barrierless reactions involving naphthyl radicals (C10H7•) with vinylacetylene (CH2=CH–C≡CH) in low-temperature environments. These elementary reactions are rapid, have no entrance barriers, and synthesize anthracene and phenanthrene via van der Waals complexes and submerged barriers. This facile route to anthracene and phenanthrene—potential building blocks to complex PAHs and aerosols in Titan—signifies a critical shift in the perception that PAHs can only be formed under high-temperature conditions, providing a detailed understanding of the chemistry of Titan’s atmosphere by untangling elementary reactions on the most fundamental level

Theoretical study of the structure, energetics, and the n-pi* electronic transition of the acetone plus nH(2)O (n = 1-3) complexes

The structure, energetics, and vibrational spectra of the (CH3)(2)CO .(H2O)(n) (n = 1-3) complexes have been studied using density functional and ab initio B3LYP, MP2, and CCSD(T) methods. The excitation energies and the oscillator strength for the n-pi* electronic transition in acetone and acetone-water complexes have been calculated using the CIS, CASSCF, and CASPT2 approaches. The results show that the first water molecule is coordinated to the carbonyl group of acetone, while the oxygen atom of H2O forms a weak hydrogen bond with a methyl hydrogen. The second H2O occupies a position between the first water and a methyl group, and the third H2O occupies a position between the second H2O and the methyl hydrogen of acetone. The energies of the coordination of the first, second, and third water molecules to the complexes are 3.7, 5.7, and 6.7 kcal/mol, respectively. The formation of the (CH3)(2)CO .(H2O)(n) complexes results in the shift of vibrational frequencies for acetone and water, particularly, red shifts for the OH stretching vibrations (up to 358 cm(-1)) and CO stretching vibrations, as well as a blue shift for the HOH bending vibrations. A small but noticeable red shift. (similar to 30 cm(-1)) of the C-H stretch can be observed in the (CH3)(2)CO .(H2O)(2) complex 2a. The excitation energy of the n-pi* electronic transition is blue shifted by 0.25-0.30 eV, which is in agreement with the experimental blue shift observed in acetone/H2O. The oscillator strength for the n-pi* transition increases from zero to similar to 10(-4) in (CH3)(2)CO .(H2O)(3). The effect of the coordination of water molecules on the spectral intensity is expected to be weaker than the effect due to vibronic coupling

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

Kinetics of C10H7Br Pyrolysis

© 2018, Allerton Press, Inc. The temperature and pressure-dependent rate constants for the process C10H7Br ↔ C10H7+Br were evaluated using the variable reaction coordinate transition state theory VRC-TST. The calculated rate constants and computational fluid dynamics (CFD) calculations were employed to estimate the pyrolysis efficiency of 2-bromonaphthalene in the resistively-heated SiC high-temperature “chemical reactor” at the temperature of about 1500 K. The observed 40% pyrolysis efficiency is reproduced by CFD calculations if the value of the calculated rate constant for the C10H7Br pyrolysis is increased by a factor of 2

Ab initio study of the n-pi(*) electronic transition in acetone: Symmetry-forbidden vibronic spectra

Ab initio calculations of geometry and vibrational frequencies of the first singlet excited (1)A(2)((1)A ") state of acetone corresponding to the n-pi* electronic transition have been carried out at the CASSCF/6-311G** level. The major geometry changes in this state as compared to the ground state involve CO out-of-plane wagging, CO stretch and torsion of the methyl groups, and the molecular symmetry changes from C-2v to C-s. The most pronounced frequency changes in the (1)A " state are the decrease of the CO stretch frequency v(3) by almost 500 cm(-1) and the increase of the CH3 torsion frequency v(12) from 22 to 170 cm(-1). The optimized geometries and normal modes are used to compute the normal mode displacements which are applied for calculations of Franck-Condon factors. Transition matrix elements over the one-electron electric field operator at various atomic centers calculated at the state-average CASSCF/6-311+G** level are used to compute vibronic couplings between the ground (1)A(1), (1)A(2), and Rydberg B-1(2)(n-3s), 2 (1)A(1)(n-3p(y)), 2 (1)A(2)(n-3p(x)), 2 B-1(2)(n-3p(z)), and B-1(1)(n-3d(xy)) electronic states, and the Herzberg-Teller expansion of the electronic wave function is applied to derive the transition dipole moment for (1)A(1)-->(1)A(2) as a function of normal coordinates. The results show that the intensity for this transition is mostly borrowed from the allowed (1)A(1)-B-1(2)(n-3s) transition due to vibronic coupling between (1)A(2) and B-1(2) through normal modes Q(20), Q(22), and Q(23) and, to some extent, from the (1)A(1)-B-1(1) transition due to Q(19) (CO in-plane bend) which couples (1)A(2) with B-1(1)(n-3d(xy)). The calculated total oscillator strength for the n-pi(*) transition through the intensity-borrowing mechanism, 3.62x10(-4), is in close agreement with the experimental value of 4.14x10(-4). Ninety-four percent of the oscillator strength comes from the perpendicular component (b(1) inducing modes) and 6% from the parallel component (b(2) modes). Calculated spectral origin, 30 115 cm(-1) at the MRCI/6-311G** level, underestimates the experimental value by similar to 300 cm(-1). Calculated positions of the most intense peaks in the spectra also reasonably agree with the experimental band maximum. The presence of numerous weak vibronic peaks densely covering a broad energy range (similar to 12 000 cm(-1)) explains the diffuse character of the experimental n-pi(*) band. Most of the bands observed in fluorescence excitation spectra [Baba and Hanazaki, Chem. Phys. Lett. 103, 93 (1983); Baba, Hanazaki, and Nagashima, J. Chem. Phys. 82, 3938 (1985)] can be assigned based on the computed spectrum. (C) 1999 American Institute of Physics. [S0021-9606(99)30324-X]

Pyrene synthesis in circumstellar envelopes and its role in the formation of 2D nanostructures

For the past decades, the hydrogen-abstraction/acetylene-addition (HACA) mechanism has been instrumental in attempting to untangle the origin of polycyclic aromatic hydrocarbons (PAHs) as identified in carbonaceous meteorites such as Allende and Murchison. However, the fundamental reaction mechanisms leading to the synthesis of PAHs beyond phenanthrene (C14H10) are still unknown. By exploring the reaction of the 4-phenanthrenyl radical (C14H9• ) with acetylene (C2H2) under conditions prevalent in carbon-rich circumstellar environments, we show evidence of a facile, isomer-selective formation of pyrene (C16H10). Along with the hydrogen-abstraction/vinylacetylene-addition (HAVA) mechanism, molecular mass growth processes from pyrene may lead through systematic ring expansions not only to more complex PAHs, but ultimately to 2D graphene-type structures. These fundamental reaction mechanisms are crucial to facilitate an understanding of the origin and evolution of the molecular universe and, in particular, of carbon in our Galaxy

A Free-Radical Prompted Barrierless Gas-Phase Synthesis of Pentacene

A representative, low-temperature gas-phase reaction mechanism synthesizing polyacenes via ring annulation exemplified by the formation of pentacene (C22H14) along with its benzo[a]tetracene isomer (C22H14) is unraveled by probing the elementary reaction of the 2-tetracenyl radical (C18H.11) with vinylacetylene (C4H4). The pathway to pentacene—a prototype polyacene and a fundamental molecular building block in graphenes, fullerenes, and carbon nanotubes—is facilitated by a barrierless, vinylacetylene mediated gas-phase process thus disputing conventional hypotheses that synthesis of polycyclic aromatic hydrocarbons (PAHs) solely proceeds at elevated temperatures. This low-temperature pathway can launch isomer-selective routes to aromatic structures through submerged reaction barriers, resonantly stabilized free-radical intermediates, and methodical ring annulation in deep space eventually changing our perception about the chemistry of carbon in our universe