Gas-phase pyrolysis of trans 3-pentenenitrile: competition between direct and isomerization-mediated dissociation.

The signal levels observed from mass spectrometers coupled by molecular beam sampling to shock tubes are impacted by dynamic pressures in the spectrometer due to rapid pressure changes in the shock tube. Accounting for the impact of the pressure changes is essential if absolute concentrations of species are to be measured. Obtaining such a correction for spectrometers operated with vacuum ultra violet photoionization has been challenging. We present here a new external calibration method which uses VUV-photoionization of CO2 to develop time-dependent corrections to species concentration/time profiles from which kinetic data can be extracted. The experiments were performed with the ICARE–HRRST (high repetition rate shock tube) at the DESIRS beamline of synchrotron SOLEIL. The calibration experiments were performed at temperatures and pressures behind reflected shock waves of 1376 ± 12 K and 6.6 ± 0.1 bar, respectively. Pyrolytic experiments with two aromatic species, toluene (T5 = 1362 ± 22 K, P5 = 6.6 ± 0.2 bar) and ethylbenzene (T5 = 1327 ± 18 K, P5 = 6.7 ± 0.2 bar), are analyzed to test the method. Time dependent concentrations for molecular and radical species were corrected with the new method. The resulting signals were compared with chemical kinetic simulations using a recent mechanism for pyrolytic formation of polycyclic aromatic hydrocarbons. Excellent agreement was obtained between the experimental data and simulations, without adjustment of the model, demonstrating the validity of the external calibration method.

The chemisorption of atomic (H, O, N, S, C), molecular (N2, CO, NO, NH3), and radical (CH3, OH, NOH) species on Ir(111) has been systematically studied. Self-consistent, periodic, density functional theory (DFT-GGA) calculations, using PW91 and RPBE functionals, have been used to determine preferred binding sites, chemisorbed structures, binding energies, vibrational frequencies, and the effect of surface relaxation for the above species at 0.25 ML surface coverage. The following order in binding energies from least to most strongly bound was determined: N2 < NH3 < NO < CH3 < CO < OH < H < NOH < O < N < S < C. A preference for 3-fold sites for the atomic adsorbates was observed, with the exception of atomic H, which prefers top sites. Molecular species showed a preference for top sites with the exception of NOH; this species preferred fcc sites. Surface relaxation had only a small effect on energetics in most cases. Calculated vibrational frequencies, in general, were in good agreement with experimental frequencies and support experimentally proposed site preferences for all adsorbates where data are available. Finally, the thermochemistry of CO, NO, NOH, N2, NH3, and CH3 decomposition on Ir(111) was examined, leading to predictions for each species of the preference for either desorption or decomposition.

The small dimensions of novel electronic and optoelectronic device structures and their abruptly and unconventionally changing material compositions require high levels of spatial and kinetic control of the surface chemical reactions which are involved in the materials' growth or modification. These requirements exclude high temperature processes based on thermally activated chemical reactions. Directed energetic beams (electrons, photons, ions), however, can induce reactions at low temperatures, as well as provide for spatial and kinetic control in microfabrication processes. Electrons and photons of sufficient energy can cause fragmentation and desorption of adsorbed molecules or fragments of them, by inducing electronic excitations to dissociative states1. The surface species after such excitations are mostly of highly reactive radical character, which readily react with the substrate and with other radical or molecular species in the adsorbed layer. The formation of such reactive surface species in a controlled way can provides a basis for non-thermal selective area film growth.

This paper describes the kinetic study of a number of gas-phase reactions involving neutral Mg-containing species, which are important for the chemistry of meteor-ablated magnesium in the upper mesosphere/lower thermosphere region. The study is motivated by the very recent observation of the global atomic Mg layer around 90 km, using satellite-born UV-visible spectroscopy. In the laboratory, Mg atoms were produced thermally in the upstream section of a fast flow tube and then converted to the molecular species MgO, MgO(2), OMgO(2), and MgCO(3) by the addition of appropriate reagents. Atomic O was added further downstream, and Mg was detected at the downstream end of the flow tube by laser-induced fluorescence. The following rate coefficients were determined at 300 K: k(MgO + O → Mg + O(2)) = (6.2 ± 1.1) × 10(-10); k(MgO(2) + O → MgO + O(2)) = (8.4 ± 2.8) × 10(-11); k(MgCO(3) + O → MgO(2) + CO(2)) ≥ 4.9 × 10(-12); and k(MgO + CO → Mg + CO(2)) = (1.1 ± 0.3) × 10(-11) cm(3) molecule(-1) s(-1). Electronic structure calculations of the relevant potential energy surfaces combined with RRKM theory were performed to interpret the experimental results and also to explore the likely reaction pathways that convert MgCO(3) and OMgO(2) into long-lived reservoir species such as Mg(OH)(2). Although no reaction was observed in the laboratory between OMgO(2) and O, this is most likely due to the rapid recombination of O(2) with the product MgO(2) to form the relatively stable O(2)MgO(2). Indeed, one significant finding is the role of O(2) in the mesosphere, where it initiates holding cycles by recombining with radical species such as MgO(2) and MgOH. A new atmospheric model was then constructed which combines these results together with recent work on magnesium ion-molecule chemistry. The model is able to reproduce satisfactorily some of the key features of the Mg and Mg(+) layers, including the heights of the layers, the seasonal variations of their column abundances, and the unusually large Mg(+)/Mg ratio.

The study of pyrolysis (thermolysis) reactions has a long tradition in Australia. In the 1950s, G. M. Badger and his students at the University of Adelaide carried out extensive studies on the gasphase pyrolysis of aromatic hydrocarbons under flow conditions at atmospheric pressure. Experiments with low-pressure gasphase pyrolysis, later to be known as flash vacuum thermolysis (FVT) or flash vacuum pyrolysis (FVP) -a term coined by Eddie Hedaya

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The FS2 radical has been detected in the glow discharge of the SF6 gas by microwave spectroscopy. Both a-type and b-type rotational transitions with the fine and hyperfine structure were observed in the millimeter-wave range of 120–185 GHz. The least-squares analysis for the 440 line frequencies led to the determination of the rotational constants, the centrifugal distortion constants, the spin-rotation interaction constants with the centrifugal distortion corrections, and the hyperfine interaction constants. Especially, the off-diagonal constants (εab+εba)/2 and Tab were determined in good precision because some nearly degenerate energy levels were mixed strongly by the off-diagonal spin-rotation interaction, which separated the energy levels up to 3 GHz and even induced the forbidden transitions to be observed strongly. The spin density on the fluorine atom and the chemical bond moments were derived for FS2, and the possible signs of Tab and (εab+εba)/2 were discussed. The behaviors of the FS2 radical and other related transient species in the SF6-discharge system were compared by using the corresponding rotational transitions observed in the microwave spectra.

Kinetic isotope effect in direct ethane dissociation on Pt(111)

The interaction dynamics of NO with the reactive Ru(0001) surface has been investigated with the use of supersonic molecular beam techniques. Helium scattering, King and Wells sticking measurements and time-of-flight (TOF) experiments are performed. The initial sticking coefficient is remarkably high with incident energy, varying from unity at thermal energies to ∽90% for 0.25⩽Ei⩽2.4 eV and does not seem to depend on surface temperature for 400⩽Ts⩽850 K. Counter intuitively, the non-sticking ∽10% of the incident flux has only about 12% translational energy transfer in the specular scattering direction. Molecular sticking is observed at low surface temperatures for above thermal incident energies, up to at least 0.45 eV.An attempt is made to distinguish between molecular precursor mediated pathways and direct dissociation. In order to account for the surprisingly constant initial sticking coefficients, the opening of a direct dissociative channel is proposed for beam energies exceeding the depth of the molecular chemisorption well. Several open problems are identified.

In recent years, interesting materials have emerged which are only available as μm-scale flakes, and whose novel physics might be better understood through broadband microwave spectroscopy; examples include twisted bilayer graphene [Y. Cao S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras and P. Jarillo-Herrero Nature 556, 43 (2018).], 2D materials in which many-body phases are observed [S. Chen R. Ribeiro-Palau, K. Yang, T. Taniguchi, J. Hone, M. O. Goerbig and C. R. Dean Physical Review Letters 122¸ 026802 (2019)], and artificial lattices for analog quantum simulations [J. Salfi J. A. Mol, R. Rahman, G. Klimeck, M. Y. Simmons, L. C. L. Hollenberg and S. Rogge Nature Communications 7, 1 (2016)]. Most previous techniques are unfortunately not sensitive for flakes below mm lateral sizes. We propose a simple technique which does not require sophisticated sample preparation nor Ohmic contact and show through theory and simulations that one will be able to qualitatively measure spectral features of interest, and quantitatively measure the frequency-dependent complex conductivity.

Weaker intermolecular forces expand the isomerization alternatives for molecular aggregation, as observed for the prototype models of the aniline trimer (An3) and the monohydrated aniline dimer (An2‐W) when compared to the phenol trimer. In this experiment the aniline clusters were generated in a jet‐cooled expansion and probed using broadband (chirped‐pulsed) microwave spectroscopy. Three isomers of the aniline trimer and two isomers of the hydrated dimer were detected and characterized in the rotational spectrum. In the homotrimer the weak N−H⋅⋅⋅N hydrogen bonds are assisted by subtle combinations of N−H⋅⋅⋅π and C−H⋅⋅⋅π interactions, producing several competing low‐lying ring species in the gas phase. One of the aniline trimers is a symmetric top, topologically equivalent to the only observed phenol trimer. Conversely, addition of a water molecule to the aniline dimer introduces a leading O−H⋅⋅⋅N interaction, making water to behave as dominant hydrogen‐bond pivot between the two aniline molecules. This combination of weak intermolecular interactions critically tests the performance of dispersion‐corrected or parametrized density‐functional methods. Evaluation of the B3LYP‐D3(BJ) and M06‐2X methods revealed deficiencies of the Truhlar functional to reproduce the experimental rotational data.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOptical systems unravel smog chemistryJames N. Pitts Jr., Barbara J. Finlayson-Pitts, and Arthur M. WinerCite this: Environ. Sci. Technol. 1977, 11, 6, 568–573Publication Date (Print):June 1, 1977Publication History Published online1 May 2002Published inissue 1 June 1977https://pubs.acs.org/doi/10.1021/es60129a014https://doi.org/10.1021/es60129a014research-articleACS PublicationsRequest reuse permissionsArticle Views90Altmetric-Citations32LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

The understanding of the chemical mechanisms occurring in the chemical vapor deposition of diamond, the development of reactor models, and process control and scaling all depend upon detailed knowledge of the composition, chemical kinetics, and transport in the gas phase and on the surface. The application of in situ optical probes to filament-assisted and combustion diamond growth environments will be presented. Atomic, radical, and stable molecular species have been detected using tunable infrared diode laser, resonance enhanced multiphoton ionization, third harmonic generation, and Fourier transform infrared spectroscopies. A simple model summarizing the current understanding of the growth of this metastable material will be presented.

The influence of solution conductivity on liquid phase pulsed electrical discharge was investigated by optical emission spectroscopy in a reactor with point-to-plane electrode geometry. The emission intensities of hydroxyl (OH) radical, hydrogen (H) radical, and oxygen (O) radical were measured using an emission intensity calibration method which used argon gas as a chemical actinometer. Control experiments showed that the addition of argon (Ar) bubbles did not disturb the electrical discharge. It was also found that the Ar emission peak at 750 nm is linearly related to the peaks of the other major species of interest (OH, H, and O) and that the slopes of these lines depend upon the solution conductivities. Therefore, the calibration method allows comparison of the radical generation rates in solutions of different conductivity. Hydrogen (H2), oxygen (O2), and hydrogen peroxide (H2O2) were measured using gas chromatography and spectrophotometric methods. The results showed that OH radicals and H2O2 decreased with increasing solution conductivity consistently with previous results. The intensities of O and H radicals and the generation rates of H2 and O2 were found to have extrema values at a conductivity of 150 μS/cm, and these results in combination with the electrical diagnostics (current waveform and overall power input) and images of the discharge channels imply changes in discharge properties at this conductivity due partly to power input changes from the spark gap. This work clearly demonstrates the close coupling of radical and molecular species formation in liquid phase electrical discharge.

A number of low-J rotational transitions have been measured for the radical or transient molecular species, SO, C 2 S, C 3 S, c-HC 3 , CH 2 CC, and c-C 3 H 2 . Measurements were made with a pulsed-nozzle, Fabry-Perot cavity, Fourier transform microwave spectrometer equipped with a DC voltage, electric discharge nozzle, Helmholtz coils around the outside of the vacuum chamber were used to maintain the magnetic field within the active volume element of the Fabry-Perot cavity to less than 10 mG