A detailed chemical kinetic mechanism has been developed to describe the oxidation of small hydrocarbon and oxygenated hydrocarbon species. The reactivity of these small fuels and intermediates is of critical importance in understanding and accurately describing the combustion characteristics, such as ignition delay time, flame speed, and emissions of practical fuels. The chosen rate expressions have been assembled through critical evaluation of the literature, with minimum optimization performed. The mechanism has been validated over a wide range of initial conditions and experimental devices, including flow reactor, shock tube, jet-stirred reactor, and flame studies. The current mechanism contains accurate kinetic descriptions for saturated and unsaturated hydrocarbons, namely methane, ethane, ethylene, and acetylene, and oxygenated species; formaldehyde, methanol, acetaldehyde, and ethanol.

A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels

Kinetics of elementary reactions in low-temperature autoignition chemistry

A review of the theoretical and computational modeling of bimolecular reactions is given. The review is divided into several sections which are as follows: gas-phase thermal reactions; gas-phase state-selected reactions and product state distributions; and condensed-phase bimolecular reactions. The section on gas-phase thermal reactions covers the enthalpies and free energies of reaction, kinetics, saddle points and potential energy surfaces, rate theory for simple barrier reactions and bimolecular reactions over potential wells. The section on gas-phase state-selected reactions focuses on electronically adiabatic reactions and electronically nonadiabatic reactions. Finally, the section on condensed-phase bimolecular reactions covers reactions in liquids, reactions on surfaces and in solids and tunneling at low temperature.

Modeling the Kinetics of Bimolecular Reactions

Chemistry and enzymology of vitamin B12.

A photochemical model of Titan's atmosphere and ionosphere

The use of energy transfer data and models in describing nonequilibrium polyatomic reaction systems is discussed with particular emphasis on the information needed for modeling vibrational energy transfer. In the discussion, it is pointed out that key areas of energy transfer knowledge are still lacking and the available experimental data are limited in scope and are of uneven quality. Despite these limitations, it is still possible to carry out meaningful simulations of chemical systems in which vibrational energy transfer is important. The vibrational energy master equation, which is the basis for modeling, and various experiments and calculations that provide the basis for practical energy transfer models and parametrizations are described. Two examples of gas phase reaction systems are presented in which vibrational energy transfer is important. The decomposition of norbornene shows how energy transfer parameters can be obtained from measurements of shock-induced chemical reactions, but even the best ...

Vibrational Energy Transfer Modeling of Nonequilibrium Polyatomic Reaction Systems

An ultrafast transient absorption study of the primary photolysis of ethyl- and n-propylcobalamin in water is presented. Data have been obtained for two distinct excitation wavelengths, 400 nm at the edge of the UV γ-band absorption, and 520 nm in the strong visible αβ-band absorption. These data are compared with results reported earlier for the B12 coenzymes, methyl- and adenosylcobalamin. The data obtained for ethylcobalamin and n-propylcobalamin following excitation at 400 nm demonstrate the formation of one major photoproduct on a picosecond time scale. This photoproduct is spectroscopically identifiable as a cob(II)alamin species. Excitation of methyl-, ethyl-, and n-propylcobalamin at 520 nm in the low-lying αβ absorption band results in bond homolysis proceeding via a bound cob(III)alamin MLCT state. For all of the cobalamins studied here competition between geminate recombination of caged radical pairs and cage escape occurs on a time scale of 500 to 700 ps. The rate constants for geminate recomb...

Time-resolved spectroscopic studies of B12 coenzymes: A comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5′-deoxyadenosylcobalamin

The temperature and pressure dependence of the rate constant of the methyl−methyl recombination reaction with He bath gas has been studied using time-resolved time-of-flight mass spectrometry. Methyl radicals were produced by the 193 nm laser photolysis of acetone. In the observed temperature (300−700 K) and pressure (0.6−10 Torr) range, the rate constant exhibits a negative temperature dependence and falloff behavior typical for recombination reactions. The integrity of the measurements has been validated by determining the recombination rate constant with Ar (1 Torr) as the bath gas at room temperature and by analyzing the yield of the reaction product, ethane. In addition, rate constants were calculated theoretically using variable reaction coordinate transition state theory in a manner that improves upon the previous treatment of Wagner and Wardlaw by incorporating high-level ab initio results. The calculated high-pressure rate constant can be expressed as (T) = 7.42 × 10-11 (T/298 K)-0.69 e-88K/T cm3...

Experimental and Theoretical Investigations on the Methyl−Methyl Recombination Reaction

Femtosecond-to-nanosecond transient absorption spectroscopy is used to investigate the photolysis of coenzyme B12, 5‘-deoxyadenosylcobalamin, as a function of solvent environment comparing water, ethylene glycol, and mixtures of water and ethylene glycol. Photolysis in ethylene glycol is characterized by the clean formation of a cob(II)alamin species on a time scale ≤ 28 ps. Competition between cage escape and geminate recombination of the initial radical pair leads to a nanosecond photolysis quantum yield of ca. 8%. This is in contrast to the photolysis of adenosylcobalamin in water, where an additional intermediate state is identified, and the net quantum yield for photolysis is three times higher. The additional intermediate observed in aqueous solution may correspond to a base-off alkylcobalamin or to a cob(II)alamin-like state having an enhanced rate for ground-state recovery. The competition between cage escape and geminate recombination for adenosyl and cob(II)alamin radical pairs is investigated b...

Time-Resolved Spectroscopic Studies of B12 Coenzymes: Influence of Solvent on the Photolysis of Adenosylcobalamin

The dynamics of energy and charge transfer in the Photosystem II reaction center complex is an area of great interest today. These processes occur on a time scale ranging from femtoseconds to tens of picoseconds or longer. Steady-state and ultrafast spectroscopy techniques have provided a great deal of quantitative and qualitative data that have led to varied interpretations and phenomenological models. More recently, microscopic models that identify specific charge separated states have been introduced, and offer more insight into the charge transfer mechanism. The structure and energetics of PS II reaction centers are reviewed, emphasizing the effects on the dynamics of the initial charge transfer.

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Laurie M. Yoder

Papers

Vibrational Energy Transfer Modeling of Nonequilibrium Polyatomic Reaction Systems

Time-resolved spectroscopic studies of B12 coenzymes: A comparison of the primary photolysis mechanism in methyl-, ethyl-, n-propyl-, and 5′-deoxyadenosylcobalamin

Experimental and Theoretical Investigations on the Methyl−Methyl Recombination Reaction

Time-Resolved Spectroscopic Studies of B12 Coenzymes: Influence of Solvent on the Photolysis of Adenosylcobalamin

Structure and function in the isolated reaction center complex of Photosystem II: energy and charge transfer dynamics and mechanism.