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The role of redox reactions in the formation of aroma volatiles, colour and other non-volatiles in the Maillard Reaction was investigated. The electrochemical properties of individual reactants and Maillard model mixtures were monitored via ORP (oxidation-reduction potential) and oxygen electrodes. All models exhibited unique electrochemical activities represented by their corresponding ORP profiles. Investigation into the redox potentials of several model systems demonstrated that the increased negative value of a redox potential is not necessarily correlated with its browning potential. An optimal redox potential range, where browning is favoured, was found to represent a balance between carbonyl and hydroxyl moieties in the structure. Adjustment of this redox potential by introducing reducing or oxidizing species can shift this balance resulting in modifications in browning capacities. However, it was concluded that there is a clear relationship between browning ability and reducing capacity of the model systems. Furthermore, a novel oxidative pyrolysis technique was developed to study the role of oxidative environment on the product distribution during pyrolysis and to investigate the mechanism of formation of non-volatiles through 13C and 15N-label incorporation. Application of this technique to glucoselglycine model system have indicated that most non-volatile Maillard reaction products can arise from glucose oxidation intermediates such as glucosone, gluconic acid and deoxyglucosones. To study the specific role of redox reactions in the formation of non-volatiles, a post-pyrolytic derivatization technique was developed and optimized. Several non-volatile end products were identified and mechanistically confirmed to involve oxidation and reduction reactions for their formation, such as lactones, hydroxylated benzenes and hydroxylated pyrazine. The latter was identified and confirmed to be generated via the dimerization of glycine and subsequent oxidation. In addition, the formation of different volatiles such as pyrazole, imidazole and oxazole was mechanistically confirmed to depend on redox reactions.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1992. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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The product state specific stereodynamics of the photon–initiated reaction of O(¹D₂) with H₂ has been investigated by polarised Doppler–resolved laser induced fluorescence, under room temperature bulb conditions. Product state resolved differential cross sections, excitation functions and rotational angular momentum alignments are reported for the following product channels, O(¹D₂) + H₂(¹Σ⁺_g ; v = 0) -> OH(X²Pi; v' = 0;N' = 14; f) + H(²S). at a mean collision energy of 12 kJ mol^-1. The data are compared with extensive state resolved quasi–classical trajectory (QCT) calculations of the linear and angular momentum distributions and excitation functions conducted on the Schinke–Lester (SL1) and K ab initio ground state (1¹A') potential energy surfaces. Overall, good agreement is obtained between the QCT calculated and experimentally determined stereodynamical features. The results are discussed in light of other recent work on this prototypical insertion reaction, and on the related systems of O(¹D₂) + HD and CH₄.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18092号 / 理博第3970号 / 新制||理||1572(附属図書館) / 30950 / 京都大学大学院理学研究科化学専攻 / (主査)教授 丸岡 啓二, 教授 大須賀 篤弘, 教授 時任 宣博 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

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Three simultaneous isotope effects have been determined for the Tschugaeff reaction, i.e., the thermal decomposition of an xanthate ester. The ratio of the rate constants, k32/k34, was found to be 1.009 for the thion sulphur atom and 1.002 for the thio-ether sulphur atom. The rate ratio k12/k13, for the xanthate carbon atom was found to have a value of 1.000. The findings of previous investigators of the Tschugaeff reaction had been consistent with either of two mechanisms. However, with the determination of the isotope effects referred to above, it has been possible to distinguish between the two possible mechanisms and to gain considerable information about the transition state of the Tschugaeff reaction. The measured isotope effects have been discussed in terms of Bigeleisen's isotopic rate equation. In addition, it has been possible to carry out theoretical calculations for each reaction mechanism by assuming an appropriate potential function and in this manner make the interpretation of the measured isotope effects more quantitative. / Thesis / Master of Science (MS)

Chemical reactions can have a staggering amount of molecular complexity. Reaction mechanisms have been proposed with over one hundred elementary reaction steps that occur in the same system simultaneously. While several methods exist to simplify and make sense of the pathways and kinetics via which these reactions proceed, e.g., reaction graphs, sensitivity or flux analysis, microkinetic analysis, and comparison of energy landscapes, etc., these methods all have limitations and are often not able to capture a comprehensive picture of the kinetics of system. It has been found useful to view these mechanisms as a network, i.e., a reaction graph. These graphs enable the visualization of the pathways of the reaction and can provide an analytical tool for pathway and kinetic analysis. However, many of the specific graph-theoretic approaches in the literature are not the most suitable for kinetic analysis of complex mechanisms; as they are simply not based on rules that are rigorous enough to fully enumerate all the pathways or provide quantitative analysis of the reaction rates. Our Reaction Route (RR) Graph approach is different in that it depicts the mechanism by a graph that is consistent with all physical and chemical laws associated with reaction networks, particularly being consistent with mass and energy conservation, i.e., Kirchoff’s Flux Law (KFL) and Kirchoff’s Potential Law (KPL). Because of their adherence to these laws, RR Graphs are able to provide an accurate graph-theoretical tool not only for depicting all reactions routes as walks (hence the name RR Graph) but also for pruning mechanisms and allowing a simplified but accurate quantitative description of reaction rates. This adherence to KFL and KPL does mean that the construction and implementation of these graphs can be prohibitively difficult for large mechanisms. For large reaction systems,especially nonlinear mechanisms, it is not realistic to generate these graphs by hand. And although there exists an analytical solution to find a determinant matrix for the RR Graph of a mechanism, the process involves an exhaustive search for a solution which experiences a combinatorial explosion as the number of steps gets very large. This leads to the idea of developing an algorithm for a computer program that can determine how to generate these graphs automatically. Unfortunately, the same combinatorial explosion is present such that for a moderately sized twenty step mechanism, it could take an average computational processor over a decade to find a solution. We have determined, however, that this brute force combinatorial approach can be avoided if heuristics could be developed to bridge gaps in our knowledge of how these graphs are constructed. Thus, developing a better analytical approach and/or a tighter set of heuristics for a computer algorithm are the overarching goals of this work. To make progress toward developing such heuristics, a set of microkinetic mechanisms were analyzed with the notion that the realization of the RR Graphs would highlight a better approach to their construction and usage. In particular, a very large linear reaction system, a smaller linear system and two non-linear reaction systems were analyzed to develop insights into how each graph is manually constructed and analyzed. Furthermore, kinetic analysis was done for these mechanisms and compared to experimental data and other analytical tools to prove not only the validity of the RR Graphs, but also how they are a significant improvement over more commonly used approaches for mechanistic and kinetic analysis. Based on the lessons learned through a consideration of these examples, a set of heuristics are established and enumerated with the ultimate goal of developing an intuitive algorithm that can help automate drawing and kinetic analysis via RR Graphs of complex mechanisms.

The rate of destruction of 18F via the 18F + p reactions is of importance in both novae and X-ray burster explosive scenarios. The rate of the competing destructive reactions, 18F(p,γ)19Ne and 18F(p,α)15O, depend upon the level structure of the compound nucleus 19Ne. The first experiment in this thesis examines the level structure of 19Ne about the 18F + p threshold via elastic 15O(α,α)15O scattering. The experiment was performed at the CRC Louvain-la-Neuve. A radioactive 15O beam bombarded a thick 4He gaseous target with elastically scattered alpha particles detected using segmented silicon detectors. An R-matrix approach was used to analyse the data and extract the resonance parameters Er and Ѓα. Particular emphasis was placed on extracting information regarding the possible 3/2+ doublet, the parameters of which, to date, have been inferred from the mirror nucleus 19F. The nominal values for the doublet taken from the mirror are Er = 8 and 38 keV, with Ѓα = 0.27 and 1.3 keV respectively. Following this new analysis it has been found that the doublet straddles the threshold at Er = -22 and 3 keV, with Ѓα = 0.15 and 3.3 keV respectively. S-factor calculations and interference effects were also examined. The 18Ne(α,p)21Na reactions is believed to be a key process in X-ray bursters. It is thought to be a possible HCNO-breakout reaction; the mechanism responsible for producing energy to drive the X-ray burster. To date the reaction has been studied both directly and indirectly. The results from each previous experiment show su cient discrepancies to warrant a re-examination of the reaction rate. As such, the second experiment presented in this thesis revolves around a new direct measurement of 18Ne(α,p). The experiment was undertaken at the CRC Louvain-la-Neuve, where, a radioactive 18Ne beam impinged upon a thin gaseous 4He target. The beam energy was chosen to provide data points in common with previous experiments at Er = 1.7 and 2.5 MeV. Reaction protons were detected via a segmented silicon detector telescope system. The total cross section calculated at Er = 2.5 MeV is 1.22 0.151 mb. An upper limit for the cross-section of 0.0208 mb was evaluated at Er = 1.7 MeV.