Kinetics and Reaction Mechanisms for Methylidyne Radical Reactions with Small Hydrocarbons

The chemical evolution with respect to time of complex macroscopic mixtures such as interstellar clouds and Titan’s atmosphere is governed via a mutual competition between thousands of simultaneous processes, including thousands of chemical reactions. Chemical kinetic modeling, which attempts to understand their macroscopic observables as well as their overall reaction mechanism through a detailed understanding of their microscopic reactions and processes, thus require thousands of rate coefficients and product distributions. At present, however, just a small fraction of these have been well-studied and measured; in addition, at the relevant low temperatures, such information becomes even more scarce. Due to the recent developments in both theoretical kinetics as well as in ab initio electronic structure calculations, it is now possible to predict accurate reaction rate coefficients and product distributions from first-principles at various temperatures, often in less time, than through the running of an experiment. Here, the results of a first principles theoretical investigation into both the reaction rate coefficients as well as the final product distributions for the reactions between the ground state CH radical (X2Π) and various C1-C3 hydrocarbons is presented; together, these constitute a set of reactions important to modeling efforts relevant to mixtures such as interstellar clouds and Titan’s atmosphere

Crossed Beam Imaging Of The Reaction Dynamics Of Halogen Atoms With Selected Hydrocarbons

This dissertation presents results of applying dc slice imaging in crossed molecular beams to probe the dynamics of the reactions of halogen atoms (chlorine and fluorine) with polyatomic hydrocarbons and alcohols such as deuterated propanes, butane isomers, pentane, alkenes and propanol. The full velocity-flux contour maps of the radical products were measured with 157nm single photon ionization at various collision eneriges. Secondary and tertiary abstractions were found in Cl with normal and deuterated propanes and butane isomers and show distinct differences. The differences were explained in terms of the nature of abstraction sites, energy disposal of the radical product, and kinetic isotope effects. For Cl reaction with butene isomers, the coupling of translational energy and center of mass angular distributions reflect the energetics of competition between direct abstraction and addition/elimination pathways in accordance with ab initio thermochemical data. A possible Cl atom roaming mediating the indirect mechanism is suggested and further addressed with investigations of Cl + isobutene reactions at various collision energies. For reaction of chlorine atoms with butenes, the combined experimental theoretical calculations result shows that Cl addition-HCl elimination occurs from an abstraction-like Cl-H-C geometry, rather than a conventional three-center or four-center transition state. This geometry is accessed exclusively by Cl atom roaming from the initial adduct. For fluorine atom reaction with linear alkanes, i.e. propane, n-butane and n-pentane, little effect of reaction exoergicity appears in the reduced translational energy distributions. The fraction of available energy in translation for pentane is smaller than the other two. Sharp forward scattering were found in the center of mass angular distributions of all targets and the backscattering decreases in with the size of the molecule increasing. The analyzed data were compared with corresponding theoretical studies. For fluorine atom reaction with 1-propanol, the translational energy distribution and center of mass angular distributions is quite similar to the results of F + n-butane; it is possible that the greater fraction of collision energy in translation comes from the existence of O-H group. The product scattering distributions of fluorine reaction with 1-butene and 1-hexene provide evidence of a long-lived complex mediated mechanism

Probing The Unimolecular Decay Of Atmospherically Important Criegee Intermediates

Ozonolysis of alkenes is an important source of hydroxyl (OH) radicals, key oxidants in the Earth’s troposphere. Alkene ozonolysis proceeds via carbonyl oxide species known as Criegee intermediates. Infrared (IR) action spectroscopy is used to study OH production from jet-cooled, stabilized Criegee intermediates. IR activation drives the rate-limiting 1,4 H-atom transfer from a syn-alkyl substituent to the terminal oxygen of the carbonyl oxide group, followed by rapid unimolecular decay to OH, which is detected by ultraviolet (UV) laser-induced fluorescence (LIF). IR action spectra provide spectral fingerprints of the Criegee intermediates. OH appearance rates are measured following IR activation by varying the IR-UV time delay. This technique is applied to two prototypical Criegee intermediates, syn-CH3CHOO and (CH3)2COO, in three different energy regimes, including at energies significantly below the calculated transition state (TS) barrier, indicating the importance of quantum mechanical tunneling in the H-atom transfer reaction. The role of tunneling is further confirmed by a significant observed kinetic isotope effect for the D-atom transfer reaction of syn-CD3CHOO. The IR action spectroscopy technique is also extended to more complex Criegee intermediates. Methyl vinyl ketone oxide (MVK-oxide) is an unsaturated four-carbon Criegee intermediate formed from the ozonolysis of isoprene, the most abundant non-methane volatile organic compound in the atmosphere. MVK-oxide was generated via a novel synthetic method and identified by its IR action spectrum. OH appearance rate measurements validate the calculated TS barrier for the H-atom shift reaction, and provide insight into an alternative unimolecular decay mechanism. The methyl-ethyl substituted Criegee intermediate (MECI) is a saturated four-carbon Criegee intermediate and is unique among Criegee intermediates studied by IR action spectroscopy because multiple conformational forms can undergo H-atom transfer to OH. Comparisons among the Criegee intermediates studied provides insight into substituent effects on unimolecular decay. The experimental OH appearance rates across many systems are in good agreement with statistical RRKM rate calculations incorporating tunneling, validating the unimolecular decay mechanism. Finally, UV LIF is used to detect vinoxy radicals, a coproduct in the H-atom transfer reaction. LIF detection of vinoxy radicals may be a probe for alternative unimolecular chemistry of vinyl-substituted Criegee intermediates from isoprene ozonolysis

The Investigation and Characterization of the Reaction of 2-Methylfuran and 2-Methyl-3-Buten-2-Ol with O(3P) and the Photodissociation of Xylyl Bromide Isomers

This thesis has studied the oxidation behavior of different biofuels or additives, 2-methyl-3-buten-2-ol and 2-methylfuran, in combustion experiments at the Chemical Dynamics Beamline held at the Advanced Light Source of the Lawrence Berkley National Laboratory. The oxidation of these fuels were initiated through O(3P) and the combustion experiments were analyzed using a multiplexed chemical kinetics photoionization mass spectrometer with tunable synchrotron radiation. Products of the different reactions were identified using kinetic profiles and further characterized using the photoionization spectra. The amount of each species was calculated using branching fractions. Additionally, the unimolecular dissociation of the xylyl bromide isomers was studied using imaging and double imaging photoelectron photoion coincidence spectroscopy to obtain accurate thermochemical data. These experiments were conducted using the Swiss Light Source held at the Paul Scherrer Institute in Villigen, Switzerland. The importance of biofuels, fuel additives, and aromatic hydrocarbons is discussed in detail in Chapter 1 of this thesis. Further, the specific experimental components of the beamlines used at the ALS and the SLS are thoroughly explained in Chapter 2. The theory behind the experiments and the computational methods to analyze the substantial experimental findings from both experimental apparatuses are explained in Chapter 3. The two combustion systems, 2-methyl-3-buten-2-ol and 2-methylfuran with O(3P) are presented in Chapter 4 and 5. Lastly, the photodissociation dynamics of the xylyl bromide isomers is presented in Chapter 6, where a specific program, miniPEPICO, is used to determine the accurate appearance energy of the daughter ion and to calculate thermochemical data

Electronic and spatial characteristics of the retinylidene chromophore in rhodopsin

The G protein coupled receptor rhodopsin was characterised by physical chemical methods like solid-state NMR, FTIR and UV/Vis spectroscopy. Goal of the research was to determine the impact of steric and electronic properties of the retinal ligand on the rate and efficiency of the photochemical reaction of this light activated receptor. First the required 13C labelled and chemically modified retinal derivatives were obtained by chemical synthesis. Subsequently, solid-state 13C NMR was used as a tool to characterise the electronic structure of the native ligand bound to rhodopsin, while FTIR difference spectroscopy was applied to determine the effect retinal ligands that were modified in the isomerisation region. It transpires that the combined approach of synthesis and spectroscopic techniques can reveal fundamental aspects of the interplay of the electronic properties and the spatial arrangement of the ligand that may ultimately allow a more profound understanding of the activation of GPCRs, in addition to knowledge about the ultrafast and efficient isomerisation of the retinylidene chromophore in rhodopsinUBL - phd migration 201

Kinetics and decomposition mechanisms of selected Nitrogen-containing species

This thesis calculates the rate of hydrogen abstraction reactions and the mechanisms of nitrogen oxides (NOx and N2O) reduction, especially those relevant to the oxidation and pyrolysis of nitrogen-rich fuels such as biomass. The dissertation firstly focuses on the interaction of hydrocarbons with the amidogen radical (NH2) and nitrogen dioxide (NO2), before analysing in detail the decomposition of ammonium nitrate (AN) both in gas and liquid media. In addition to this, the moderation of nitrous oxide (N2O) and nitrogen oxides (NOx) via their reaction with a biomass surrogate of catechol was also studied. The underlying aims of the study were to report the mechanisms and kinetic factors controlling the interaction of NH2 and NO2 radicals with a wide array of hydrocarbons, then to map out the prominent reaction pathways prevailing in the decomposition of ammonium nitrate (AN) and conversion of N2O into N2 via dissociative adsorption onto a catechol moiety. Accurate quantum-mechanical calculations probed the hydrogen abstraction reactions from small aliphatic and aromatic hydrocarbons by NH2 and NO2 radicals. Reaction and activation energies for all plausible hydrogen abstraction channels were executed with the accurate chemistry model of CBS-QB3. Reaction rate parameters were obtained based on conventional transition-state theory, accounting for a plausible contribution from tunnelling effects and treating internal rotations as hindered rotors. We established that a linear correlation existed between the strength of the C-H bonds (i.e., primary, secondary, vinylic, and benzylic) and the activation energies for H abstraction channels operated by NH2 and NO2 radicals. Moreover, the meta-hybrid Density Functional Theory (DFT) of M05-2X/6-311+G(d,p) levels elucidated viable systematic conversion routes of N2O into N2 via interaction with a catechol molecule. Two theoretical methodologies were applied to study thermal decomposition of AN in gas and liquid phases. A continuum solvation model density-polarisable continuum model (SMD-PCM) expounds the catalysing effect of water on AN thermal cracking. The solvation model systematically predicts lower activation energies when contrasted with analogous gas phase values. An important part of the thesis investigates the potential of biomass constituents for the so-called selective non-catalytic reduction of NOx into nitrogen molecules. The laboratory-scale rig offers a continuous supply of carrier and reaction gases which run through a tubular reactor coupled with FTIR spectroscopy, micro-GC and a chemiluminescence NOx analyser. The consumption of the biomass surrogate (catechol) is analysed using a triple quadruple mass spectrometer (QQQ-MS) at temperatures starting from 400 °C. Fine-tuning of experimental conditions encompasses residence time and inlet reactant mixing ratios. Above 800 °C, we report more than 80 % NOx reduction efficiency. In summary, our findings throughout the thesis present previously unreported data and new insights pertinent to the combustion chemistry of several selected N-species