1,758 Matching Results

Search Results

Advanced search parameters have been applied.

Pathway and kinetic analysis on the iso-propyl radical + O{sub 2} reaction system

Description: We analyze the isopropyl + 02 reaction system using thermochemical Transition State Theory (TST), molecular thermodynamic properties, analysis (quantum RRK) for k(E) and modified strong collision analyze Cyclic transition states for both hydrogen transfer and concerted propylene from isopropylperoxy are calculated using semi-empirical theory in addition to transition states for H02 elimination from hydroperoxy-isopropyl. Computed rate constants are compared to constant measurements of for isopropyl + H02.
Date: April 7, 1997
Creator: Bozzelli, J. W. & Pitz, W. J.
Partner: UNT Libraries Government Documents Department

Compression ratio effect on methane HCCI combustion

Description: We have used the HCT (Hydrodynamics, Chemistry and Transport) chemical kinetics code to simulate HCCI (homogeneous charge compression ignition) combustion of methane-air mixtures. HCT is applied to explore the ignition timing, bum duration, NO<sub>x</sub> production, gross indicated efficiency and gross IMEP of a supercharged engine (3 atm. Intake pressure) with 14:1, 16:l and 18:1 compression ratios at 1200 rpm. HCT has been modified to incorporate the effect of heat transfer and to calculate the temperature that results from mixing the recycled exhaust with the fresh mixture. This study uses a single control volume reaction zone that varies as a function of crank angle. The ignition process is controlled by adjusting the intake equivalence ratio and the residual gas trapping (RGT). RGT is internal exhaust gas recirculation which recycles both thermal energy and combustion product species. Adjustment of equivalence ratio and RGT is accomplished by varying the timing of the exhaust valve closure in either 2-stroke or 4-stroke engines. Inlet manifold temperature is held constant at 300 K. Results show that, for each compression ratio, there is a range of operational conditions that show promise of achieving the control necessary to vary power output while keeping indicated efficiency above 50% and NO<sub>x</sub> levels below 100 ppm. HCT results are also compared with a set of recent experimental data for natural gas.
Date: September 29, 1998
Creator: Aceves, S. M.; Pitz, W.; Smith, J. R. & Westbrook, C.
Partner: UNT Libraries Government Documents Department

Ultrafast studies of solution dynamics

Description: This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). Fast chemical dynamics generally must be initiated photochemically. This limits the applicability of modern laser methods for following the structural changes that occur during chemical and biological reactions to those systems that have an electronic chromophore that has a significant yield of photoproduct when excited. This project has developed a new and entirely general approach to ultrafast initiation of reactions in solution: laser-induced temperature jump (T-jump). The results open entire new fields of study of ultrafast molecular dynamics in solution. The authors have demonstrated the T-jump technique on time scales of 50 ps and longer, and have applied it to study of the fast events in protein folding. They find that a general lifetime of alpha-helix formation is ca 100 ns, and that tertiary folds (in apomyoglobin) form in ca 100 {mu}s.
Date: October 1, 1997
Creator: Woodruff, W.H.; Dyer, R.B. & Callender, R.H.
Partner: UNT Libraries Government Documents Department

Free-radical kinetics of coal liquefaction

Description: A rate expression with first- and second-order terms in the concentration of extractable compounds in solid coal particles is derived from a fundamental free-radical mechanism. The expression was suggested empirically by prior experiments for coal liquefaction in the presence of a hydrogen-donor solvent. Radical reactions are considered to occur in both coal and in solvent. The long-chain approximation justifies the neglect of initiation, hydrogen abstraction, and termination rates as quantitatively insignificant relative to propagation reaction rates.
Date: July 16, 1994
Creator: Wang, M.; Smith, J.M. & McCoy, B.J.
Partner: UNT Libraries Government Documents Department

Dynamics of ultrafast internal conversion processes studied by femtosecond time-delayed photoelectron spectroscopy

Description: The authors have studied the dynamics of ultrafast internal conversion processes using femtosecond time-resolved photoionization and photoelectron spectroscopy. In hexatriene, following femtosecond pulse excitation at 250 nm, they use time-delayed photoionization to observe the formation and decay of an intermediate species on the subpicosecond time scale. With time-resolved photoelectron spectroscopy, the rapid evolution of vibrational excitation in this intermediate is observed, as electronic energy is converted to vibrational energy in the molecule. The photodynamics of cis and trans isomers of hexatriene are compared and found to be surprisingly different on the 2-3 psec time scale. These results are important for understanding the fundamental photochemical processes in linear polyenes, which have served as models for the active chromophores of many biological photosystems.
Date: August 1, 1995
Creator: Cyr, D.R. & Hayden, C.C.
Partner: UNT Libraries Government Documents Department

Diesel combustion: an integrated view combining laser diagnostics, chemical kinetics, and empirical validation

Description: This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the two-stage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors. A framework is also proposed for understanding the heat release and emission formation processes.
Date: February 1, 1999
Creator: Akinyami, O C; Dec, J E; Durrett, R P; Flynn, P F; Hunter, G L; Loye, A O et al.
Partner: UNT Libraries Government Documents Department

Reactivity of hydrogen with uranium in the presence of Pt

Description: The surface-reaction of di-hydrogen with uranium in the presence of Pt clusters has been studied using scanning tunneling microscopy (STM). Uranium was deposited on highly oriented pyrolytic graphite (HOPG) and annealed at temperatures up to 1200{degrees}C to obtain atomically pyrolytic flat surfaces. Pt clusters were then formed using evaporation from a Pt source onto the surface and subsequent annealing. Hydrogen mainly attacked uranium in the vicinity of Pt clusters and formed hydride. The hydride formation probability is almost constant at 2.3x10{sup -4} over the range of exposures studied.
Date: July 1, 1997
Creator: Balooch, M. & Siekhaus, W.J.
Partner: UNT Libraries Government Documents Department

Wide range modeling study of dimethyl ether oxidation

Description: A detailed chemical kinetic model has been used to study dimethyl ether (DME) oxidation over a wide range of conditions. Experimental results obtained in a jet-stirred reactor (JSR) at I and 10 atm, 0.2 < 0 < 2.5, and 800 < T < 1300 K were modeled, in addition to those generated in a shock tube at 13 and 40 bar, 0 = 1.0 and 650 :5 T :5 1300 K. The JSR results are particularly valuable as they include concentration profiles of reactants, intermediates and products pertinent to the oxidation of DME. These data test the Idnetic model severely, as it must be able to predict the correct distribution and concentrations of intermediate and final products formed in the oxidation process. Additionally, the shock tube results are very useful, as they were taken at low temperatures and at high pressures, and thus undergo negative temperature dependence (NTC) behavior. This behavior is characteristic of the oxidation of saturated hydrocarbon fuels, (e.g. the primary reference fuels, n-heptane and iso- octane) under similar conditions. The numerical model consists of 78 chemical species and 336 chemical reactions. The thermodynamic properties of unknown species pertaining to DME oxidation were calculated using THERM.
Date: April 1, 1997
Creator: Pitz, W.J.; Marinov, N.M.; Westbrook, C.K.; Dagaut, P.; Boettner, J-C & Cathonnet, M.
Partner: UNT Libraries Government Documents Department

Effect of Palladium Form on Tetraphenylborate Decomposition Rate

Description: Palladium catalyzes the decomposition of tetraphenylborate in alkaline solutions. Researchers postulate several decomposition mechanisms that differ in the form of the palladium catalyst. Potential forms include solid and soluble, different soluble species (such as aqueous or organic soluble), and different oxidation states (i.e., 0, II, and IV). Initial tests measured the reactivity and distribution of four Pd forms in tetraphenylborate slurries.
Date: April 28, 1998
Creator: Walker, D. D.
Partner: UNT Libraries Government Documents Department

Recent advances in the measurement of high temperature bimolecular rate constants

Description: Recent advances in the measurement of high temperature reaction rate constants are discussed. The studies carried out by shock tube methods are particularly considered because these results are important not only in theoretical chemical kinetics but also in practical applications. The work on 5 chemical reactions are reviewed in detail. These are: D + H{sub 2}, Cl + H{sub 2}, H + O{sub 2}, CH{sub 3} + CH{sub 3}, and H + NO{sub 2}.
Date: July 1, 1995
Creator: Michael, J.V.
Partner: UNT Libraries Government Documents Department

The Reaction Kinetics of Amino Radicals with Sulfur Dioxide

Description: Abstract: Application of the laser photolysis–laser-induced fluorescence method to the reaction NH2 + SO2 in argon bath gas yields pressure-dependent, third-order kinetics which may be summarized as 𝑘 = (1.49 ± 0.15) × 10^−31 (𝑇/298 K) − 0.83 cm^6 molecule^−2 s^−1 over 292 – 555 K, where the uncertainty is the 95% confidence interval and includes possible systematic errors.The quenching of vibrationally excited NH2 is consistent with a high-pressure limit for NH2 + SO2 of (1.62 ± 0.25) × 10^−11 cm^3 molecule^−1 s^−1 over the temperature range 295–505 K, where again the 95% confidence interval is shown. Abinitio analysis yields a H2N – SO2 dissociation enthalpy of 73.5 kJmol−1, and comparison with RRKM theory and the exponential down model for energy transfer yields ⟨Δ𝐸⟩down = 350 cm^−1 for Ar at room temperature.
Date: April 29, 2015
Creator: Gao, Yide; Glarborg, Peter & Marshall, Paul
Partner: UNT College of Arts and Sciences

ENERGY RECEPTION AND TRANSFER IN PHOTOSYNTHESIS

Description: The basic information about the path of carbon in photosynthesis is reviewed together with the methods that were used to discover it. This has led to the knowledge of what is required of the photochemical reaction in the form of chemical species. Attention is then directed to the structure of the photochemical apparatus itself insofar as it is viewable by electron microscopy, and some principoles of ordered structure are devised for the types of molecules to be found in the chloroplasts. From the combination of these, a structure for the grana lamella is suggested and a mode of function proposed. Experimental test for this mode of function is underway; one method is to examine photoproduced unpaired electrons. This is discussed.
Date: September 23, 1958
Creator: Calvin, Melvin
Partner: UNT Libraries Government Documents Department

A Survey of the Rates and Products of Short-Term Photosynthesis inPlants of 9 Phyla

Description: The conclusions of this paper are: (1) Short-term photosynthetic experiments using C{sup 14}O{sub 2} and paper chromatography were performed with 27 different plants representing nine phyla: Schizophyta (Schizophyceae), Euglenophyta, Chlorophyta, Charophyta, Chrysophyta, Rhodophyta, Bryophyta, Pteridophyta, and Spermatophyta. (2) There is a remarkable uniformity in the types of ethanol-soluble compounds which became radioactive in the entire group of plants used. The amounts of the different compounds varied considerably percentage-wise among the various plants as would be expected because of their inherent metabolic differences and the variations in their physiological states induced by experimental conditions. (3) Sucrose became radioactive in very different amounts in two major groupings of plants: (a) those containing only photosynthetic tissue and (b) those containing non-photosynthetic tissue as well. The amount of radioactive sucrose in the former group was much lower than that in the latter. (4) An unidentified compound became radioactive in appreciable amounts in two of the blue-green algae, but was radioactive in very small amounts or not visible at all on the chromatograms of all other plants.
Date: May 1, 1954
Creator: Calvin, M.; Norris, R.E. & Norris, Louisa
Partner: UNT Libraries Government Documents Department

The Path of Carbon in Photosynthesis VI.

Description: This paper is a compilation of the essential results of our experimental work in the determination of the path of carbon in photosynthesis. There are discussions of the dark fixation of photosynthesis and methods of separation and identification including paper chromatography and radioautography. The definition of the path of carbon in photosynthesis by the distribution of radioactivity Within the compounds is described.
Date: June 30, 1949
Creator: Calvin, M.
Partner: UNT Libraries Government Documents Department

Beyond transition state theory: Rigorous quantum approaches for determining chemical reaction rates

Description: Transition state theory (TST) has historically been the most important and widely used theoretical approach for describing the rates of chemical reactions, and for qualitative pictures and order-of-magnitude estimates one does not expect this situation to change. However a rigorous, quantitative treatment of chemical reaction rates must go beyond TST. A rigorous description, for example, must be based on a quantum mechanical description of the molecular system, but the fundamental assumption on which TST is based - namely that the molecular dynamics is {open_quotes}direct,{close_quotes} i.e., that no trajectories re-cross a dividing surface which separates reactants and products (vide infra) - is couched inherently in the language of classical mechanics. There is no unambiguous way to quantize TST, for the various ways of trying to do so invariably require one to introduce additional assumptions about the reaction dynamics. As one tries to eliminate these {open_quotes}additional assumptions{close_quotes} one is driven ultimately to an exact quantum treatment of the reaction dynamics which is then no longer a transition state theory (i.e., approximation) but simply an exact formulation. It is such exact approaches, those without inherent approximations, that are the subject of this chapter.
Date: January 1, 1995
Creator: Miller, W.H.
Partner: UNT Libraries Government Documents Department

GENERALIZATION OF THE ACTIVATED COMPLEX THEORY OF REACTION RATES. I. QUANTUM MECHANICAL TREATMENT

Description: In its usual form activated complex theory assumes a quasi-equilibrium between reactants and activated complex, a separable reaction coordinate, a Cartesian reaction coordinate, and an absence of interaction of rotation with internal motion in the complex. In the present paper a rate expression is derived without introducing the Cartesian assumption. The expression bears a formal resemblance to the usual one and reduces to it when the added assumptions of the latter are introduced. The new equation for the transmission coefficient contains internal centrifugal terms. The derivation employs an extension of the Stackel-Robertson formalism for separation of variables in mechanics. The fourth assumption can also be weakened and a rotational interaction included in the formalism. In applications of the rate equation use is made of the recent findings that in the immediate vicinity of a saddle-point or a minimum a potential energy surface can be imitated in some major topographical respects by a surface permitting separation of variables. The separated wave equation for the reaction coordinate is then curvilinear because of the usual curvature of the path of steepest ascent to the saddle-point. Calculations of transmission coefficients and rates can be made and compared with those obtainable from the usual one-dimensional Cartesian-like calculations on the one hand and with some based on the numerical integration of the n-dimensional Schrodinger equation on the other. An application to a common three-center problem is discussed.
Date: November 1, 1964
Creator: Marcus, R.A.
Partner: UNT Libraries Government Documents Department

Activity Coefficient Derivatives of Ternary Systems Based on Scatchard's Neutral Electrolyte description

Description: Activity coefficient derivatives with respect to molality are presented for the Scatchard Neutral Electrolyte description of a ternary common-ion electrolyte system. These quantities are needed for the calculation of 'diffusion Onsager coefficients' and in turn for tests of the Onsager Reciprocal Relations in diffusion. The usually-omitted b{sub 23} term is included. The direct SNE binary approximations and a further approximation are discussed. Binary evaluation strategies other than constant ionic strength are considered.
Date: May 16, 2007
Creator: Miller, D G
Partner: UNT Libraries Government Documents Department

Fundamental Kinetics of Supercritical Coal Liquefaction: Effect of Catalysts and Hydrogen-Donor Solvents

Description: This is the quarterly report on our recent progress toward the overall objective to understand the supercritical fluid extraction of hydrocarbons from coal. Our strategy is to simulate coal as a high molecular-weight polymeric material by studying the degradation of polymers under various conditions. The hypothesis we are testing is that degradation of such macromolecules is applicable to the decomposition (depolymerization) of the coal network. Polymer degradation and coal liquefaction are influenced strongly by the solvent in the reaction. This motivated our investigation of the effect of hydrogen donor solvents on polymer degradation. In particular, we obtained new experimental data to show how a hydrogen donor, 6-hydroxy tetralin, influences the degradation rate of polystyrene. We also developed a detailed radical mechanism for hydrogen donation based on the Rice-Herzfeld chain reaction concept with the elementary steps of initiation, depropagation, hydrogen abstraction, and termination. Expressions for the degradation rate parameters were obtained by applying continuous distribution kinetics to the MWD of the reacting polymer. The theory explains the different influences of the hydrogen donor solvent on the degradation rate coefficients for different polymers. Though developed for the degradation of polymers, the mechanism and the theory are potentially applicable for chain scission and addition reactions among distributions of paraffins, olefins, and radicals of all chain lengths. The concepts can, in principle, be extended to examine the effect of hydrogen donors on coal liquefaction and on the complex mixture of liquefaction compounds. Based on this work, a research paper titled �Effect of Hydrogen Donors on Polymer Degradation,� has been submitted for publication. Our research paper entitled, �Molecular weight effect on the dynamics of polystyrene degradation,� has been accepted for publication by the journal, Industrial and Engineering Chemistry Research.
Date: April 16, 1997
Creator: McCoy, Ben J.; Madras, Girodhar; Smith, J. M. & Kodera, Yoichi
Partner: UNT Libraries Government Documents Department

Pathway and kinetic analysis on the propyl radical + 02 reaction system

Description: In this study of the reaction of alkyl radicals with molecular oxygen, we analyze the propyl + 02 reaction system using thermochemical kinetics, Transition State Theory (TST), molecular thermodynamic properties, quantum Kassel analysis (quantum RRK) for k(E) and modified strong collision analysis for fall off. Cyclic transition states for both hydrogen transfer and the H02 concerted elimination from propylperoxy are calculated using semi-empirical (MOPAC PM3) calculations [8] in addition to transition states for H02 elimination and epoxide formation from hydroperoxy-isopropyl. Computed rate constants for propyl + 02 are compared to the values of Gulati and Walker who measured the rate constants at 50 torr and over a temperature range of 653 to 773 K. Computed rate constants are also used in a detailed chemical kinetic mechanism and compared to the n- propyl + 02 data of Slagle. They measured the rate of disappearance of n-propyl by reaction with 02 over a temperature range of 297 to 635 K and a pressure range of 0.4 to 7 Torr, as well as the fall off data of the Kaiser and Wallington.
Date: May 1, 1997
Creator: Bozzelli, J.W. & Pitz, W.J.
Partner: UNT Libraries Government Documents Department