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The Electron Transfer System of Syntrophically Grown Desulfovibrio vulgaris

Description: Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic couplings between hydrogen producers and consumers are a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent upon growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, D. vulgaris up-regulated numerous genes involved in electron transfer and energy generation when compared with sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn) and the well-characterized high-molecular weight cytochrome (Hmc) were among the most highly expressed and up-regulated. Additionally, a predicted operon coding for genes involved in lactate transport and oxidation exhibited up-regulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd and Hyn impaired or severely limited syntrophic growth but had little affect on growth via sulfate-respiration. These results demonstrate that syntrophic growth and sulfate-respiration use largely independent energy generation pathways and imply that understanding of microbial processes sustaining nutrient cycling must consider lifestyles not captured in pure culture.
Date: June 22, 2009
Creator: PBD; ENIGMA; GTL; VIMSS; Walker, Christopher B.; He, Zhili et al.
Partner: UNT Libraries Government Documents Department

Simulations of highly reactive fluids

Description: We report density functional molecular dynamics simulations to determine the early chemical events of hot (T = 3000 K) and dense (1.97 g/cm{sup 3}, V/V{sub 0} = 0.68) nitromethane (CH{sub 3}NO{sub 2}). The first step in the decomposition process is an intermolecular proton abstraction mechanism that leads to the formation of CH{sub 3}NO{sub 2}H and the aci ion H{sub 2}CNO{sub 2}{sup -}, in support of evidence from static high-pressure and shock experiments. An intramolecular hydrogen transfer that transforms nitromethane into the aci acid form, CH{sub 2}NO{sub 2}H, accompanies this event. This is the first confirmation of chemical reactivity with bond selectivity for an energetic material near the condition of fully reacted specimen. We also report the decomposition mechanism followed up to the formation of H{sub 2}O as the first stable product.
Date: July 21, 2005
Creator: Fried, L E; Manaa, M R & Reed, E J
Partner: UNT Libraries Government Documents Department

Spectroscopic signatures of proton transfer dynamics in the water dimer cation

Description: Using full dimensional EOM-IP-CCSD/aug-cc-pVTZ potential energy surfaces, the photoelectron spectrum, vibrational structure, and ionization dynamics of the water dimer radical cation, (H2O)+2, were computed. We also report an experimental photoelectron spectrum which is derived from photoionization efficiency measurements and compares favorably with the theoretical spectrum. The vibrational structure is also compared with the recent experimental work of Gardenier et al. [J. Phys. Chem. A 113, 4772 (2009)] and the recent theoretical calculations by Cheng et al. [J. Phys. Chem. A 113 13779 (2009)]. A reduced dimensionality nuclear Hamiltonian was used to compute the ionization dynamics for both the ground state and first excited state of the cation. The dynamics show markedly different behavior and spectroscopic signatures depending on which state of the cation is accessed by the ionization. Ionization to the ground-state cation surface induces a hydrogen transfer which is complete within 50 femtoseconds, whereas ionization to the first excited state results in a much slower process.
Date: December 21, 2009
Creator: Kamarchik, Eugene; Kostko, Oleg; Bowman, Joel M.; Ahmed, Musahid & Krylov, Anna I.
Partner: UNT Libraries Government Documents Department

The electron transfer system of syntrophically grown Desulfovibrio vulgaris

Description: Interspecies hydrogen transfer between organisms producing and consuming hydrogen promotes the decomposition of organic matter in most anoxic environments. Although syntrophic couplings between hydrogen producers and consumers are a major feature of the carbon cycle, mechanisms for energy recovery at the extremely low free energies of reactions typical of these anaerobic communities have not been established. In this study, comparative transcriptional analysis of a model sulfate-reducing microbe, Desulfovibrio vulgaris Hildenborough, suggested the use of alternative electron transfer systems dependent upon growth modality. During syntrophic growth on lactate with a hydrogenotrophic methanogen, D. vulgaris up-regulated numerous genes involved in electron transfer and energy generation when compared with sulfate-limited monocultures. In particular, genes coding for the putative membrane-bound Coo hydrogenase, two periplasmic hydrogenases (Hyd and Hyn) and the well-characterized high-molecular weight cytochrome (Hmc) were among the most highly expressed and up-regulated. Additionally, a predicted operon coding for genes involved in lactate transport and oxidation exhibited up-regulation, further suggesting an alternative pathway for electrons derived from lactate oxidation during syntrophic growth. Mutations in a subset of genes coding for Coo, Hmc, Hyd and Hyn impaired or severely limited syntrophic growth but had little affect on growth via sulfate-respiration. These results demonstrate that syntrophic growth and sulfate-respiration use largely independent energy generation pathways and imply that understanding of microbial processes sustaining nutrient cycling must consider lifestyles not captured in pure culture.
Date: May 1, 2009
Creator: Walker, C. B.; He, Z.; Yang, Z.K.; Ringbauer, J. A., Jr.; He, Q.; Zhou, J. et al.
Partner: UNT Libraries Government Documents Department

Efficient Estimators for Quantum Instanton Evaluation of theKinetic Isotope Effects: Application to the Intramolecular HydrogenTransfer in Pentadiene

Description: The quantum instanton approximation is used to compute kinetic isotope effects for intramolecular hydrogen transfer in cis-1,3-pentadiene. Due to the importance of skeleton motions, this system with 13 atoms is a simple prototype for hydrogen transfer in enzymatic reactions. The calculation is carried out using thermodynamic integration with respect to the mass of the isotopes and a path integral Monte Carlo evaluation of relevant thermodynamic quantities. Efficient 'virial' estimators are derived for the logarithmic derivatives of the partition function and the delta-delta correlation functions. These estimators require significantly fewer Monte Carlo samples since their statistical error does not increase with the number of discrete time slices in the path integral. The calculation treats all 39 degrees of freedom quantum-mechanically and uses an empirical valence bond potential based on a modified general AMBER force field.
Date: June 13, 2007
Creator: Vanicek, Jiri & Miller, William H.
Partner: UNT Libraries Government Documents Department

DOE Laboratory Catalysis Research Symposium - Abstracts

Description: The conference consisted of two sessions with the following subtopics: (1) Heterogeneous Session: Novel Catalytic Materials; Photocatalysis; Novel Processing Conditions; Metals and Sulfides; Nuclear Magnetic Resonance; Metal Oxides and Partial Oxidation; Electrocatalysis; and Automotive Catalysis. (2) Homogeneous Catalysis: H-Transfer and Alkane Functionalization; Biocatalysis; Oxidation and Photocatalysis; and Novel Medical, Methods, and Catalyzed Reactions.
Date: February 1, 1999
Creator: Dunham, T.
Partner: UNT Libraries Government Documents Department

Shock-initiation chemistry of nitroarenes

Description: The authors present evidence that the shock-initiation chemistry of nitroarenes is dominated by the intermolecular hydrogen transfer mechanism discussed previously. The acceleration by pressure, kinetic isotope effect, and product distribution are consistent with the bimolecular transition state kinetic isotope effect, and product distribution are consistent with the bimolecular transition state rather than rate-determining C-N homolysis.GC-MS analysis of samples which were subjected to a shock wave generated by detonation of nitromethane shows that nitrobenzene produces aniline and biphenyl, and o-nitrotoluene forms aniline, toluene, o-toluidine and o-cresol, but not anthranil, benzoxazinone, or cyanocyclopentandiene. In isotopic labeling experiments o-nitrotoluene and TNT show extensive H-D exchange on their methyl groups, and C-N bond rupture is not consistent with the formation of aniline from nitrobenzene or nitrotoluene, nor the formation of o-toluidine from o-nitrotoluene. Recent work incorporating fast TOF mass spectroscopy of samples shocked and quenched by adiabatic expansion shows that the initial chemical reactions in shocked solid nitroaromatic explosives proceed along this path.
Date: November 1, 1997
Creator: Davis, L.L. & Brower, K.R.
Partner: UNT Libraries Government Documents Department

Quantum scattering studies of spin-orbit effects in the Cl({sup 2}P) + HCl {yields} ClH + Cl({sup 2}P) reaction

Description: The authors present quantum scattering calculations for the Cl + HCl {yields} ClH + Cl reaction in which they include the three electronic states that correlate asymptotically to the ground state of Cl({sup 2}P) + HCl(X{sup 1}{Sigma}{sup +}). The potential surfaces and couplings are taken from the recent work of C.S. Maierle, G.C. Schatz, M.S. Gordon, P. McCabe and J.N.L. Connor, J. Chem. Soc. Farad. Trans. (1997). They are based on extensive ab initio calculations for geometries in the vicinity of the lowest energy saddle point, and on an electrostatic expansion (plus empirical dispersion and repulsion) for long range geometries including the van der Waals wells. Spin-orbit coupling has been included using a spin-orbit coupling parameter {lambda} that is assumed to be independent of nuclear geometry, and Coriolis interactions are incorporated accurately. The scattering calculations use a hyperspherical coordinate coupled channel method in full dimensionality. AJ-shifting approximation is employed to convert cumulative reaction probabilities for total angular momentum quantum number J = 1/2 into state selected and thermal rate coefficients. Two issues have been studied: (a) the influence of the magnitude of {lambda} on the fine-structure resolved cumulative probabilities and rate coefficients (the authors consider {lambda}`s that vary from 0 to {+-}100% of the true Cl value), and (b) the transition state resonance spectrum, and its variation with {lambda} and with other parameters in the calculations. Cl + HCl is a simple hydrogen transfer reaction which serves as a canonical model both for heavy-light-heavy atom reactions, and for the reactions of halogen atoms with closed shell molecules.
Date: July 1, 1998
Creator: Schatz, G.C.; McCabe, P. & Connor, J.N.L.
Partner: UNT Libraries Government Documents Department

Molecular Mechanism of Biological Proton Transport

Description: Proton transport across lipid membranes is a fundamental aspect of biological energy transduction (metabolism). This function is mediated by a Grotthuss mechanism involving proton hopping along hydrogen-bonded networks embedded in membrane-spanning proteins. Using molecular simulations, the authors have explored the structural, dynamic, and thermodynamic properties giving rise to long-range proton translocation in hydrogen-bonded networks involving water molecules, or water wires, which are emerging as ubiquitous H{sup +}-transport devices in biological systems.
Date: September 1, 1998
Creator: Pomes, R.
Partner: UNT Libraries Government Documents Department

Role of aromatic structure in pathways of hydrogen transfer and bond cleavage in coal liquefaction: Theoretical studies

Description: The mechanisms by which strong carbon-carbon bonds between aromatic rings and side chains are cleaved under hydropyrolysis conditions remain a subject of wide interest to fuel science. Recently, the authors have studied in detail an alternate pathway for hydrogen atom transfer to {pi}-systems, radical hydrogen transfer (RHT). RHT is the direct, bimolecular transfer of hydrogen from the {beta}-position of an organic radical to the target {pi}-system. In the initial theoretical study, they examined the reaction ethyl radical + ethylene = ethylene + ethyl at the spin-projected UMP2/6-31G** level of theory. Recently, they have used a calibrated ROHF-MNDO-PM3 method to predict thermoneutral RHT barriers for hydrogen transfer between hydroaryl radicals and the corresponding arene. Because of the inherent limitations of semiempirical methods such as ROHF-MNDO-PM3, they have extended the initial work with the ethyl + ethylene study to examine this reaction at the ROHF-MBPT[2]-6-31G** and ROHF-CCSD[T]-6-31G** levels of ab initio theory. The primary objective was to determine how intrinsic RHT barriers change with conjugative stabilization of the radicals. The spin-restricted ROHF approach has been applied to study several RHT reactions, and they present completed ROHF results for the ethyl + ethylene system and preliminary results for the methallyl + butadiene system. The methallyl + butadiene system serves as a model for highly stabilized hydroaryl radicals: the methallyl radical exhibits a C-H bond strength of 46.5 kcal/mol compared to 9-hydroanthracenyl, 43.1 kcal/mol.
Date: September 1, 1995
Creator: Franz, J.A.; Autrey, T.; Camaioni, D.M.; Watts, J.D. & Bartlett, R.J.
Partner: UNT Libraries Government Documents Department

Intrinsic barriers for H-atom transfer reactions

Description: Hydrogen transfer reactions play a well-recognized role in coal liquefaction. While H-abstraction reactions between radicals and H-donors have been well-studied, understanding of structure-reactivity relationships remains surprisingly incomplete. Another form of hydrogen transfer known as radical hydrogen transfer (radical donation of H to an unsaturated compound) is currently the subject of much speculation. The barriers for identity reactions are key parameters in the Evans-Polanyi equation for estimating reaction barriers and are fundamentally significant for the insight they provide about bond reorganization energies for formation of transition state structures. Although knowable from experiment, relatively few H-abstraction identity barriers and no barriers for hydrocarbon radical hydrogen transfer reactions have been measured. This paper seeks to supplement and extend existing experimental data with results obtained by calculation. The authors have used ab initio and semiempirical molecular orbital methods (MNDO-PM3) to calculate barriers for a series of H-atom abstraction and radical-hydrogen-transfer identity reactions for alkyl, alkenyl, arylalkyl and hydroaryl systems. Details of this methodology and analyses of how barrier heights correlate with reactant and transition state properties will be presented and discussed.
Date: August 1, 1994
Creator: Camaioni, D.M.; Autrey, S.T. & Franz, J.A.
Partner: UNT Libraries Government Documents Department

Catalytic conversion of polycyclic aromatic hydrocarbons: Mechanistic investigations of hydrogen transfer from an iron-based catalyst to alkylarenes

Description: Results of our model compound studies suggest that free radical hydrogen transfer pathways from the catalyst to the alkylarene are responsible for the scission of strong carbon-carbon bonds. There are two requisites for the observed selective bond scission. First is the stability of the ipso adduct precursor leading to displacement, the more stable the adduct the more probable bond scission. This explains why benzyl radical displacement > phenoxy radical displacement in benzyldiphenyl ether and explains why PhCH{sub 2}CH{sub 2}PhCH{sub 2} radical > naphthylmethyl radical from NMBB. Second, given equal ipso adduct precursor stabilities, e.g. methyldiphenylmethane, the stability of the departing radical determines the selectivity. this explains benzyl radical > methyl radical in the methylated diphenylmethanes and explains why {alpha}-hydroxyphenethyl radical > methyl radical in 1,2-ditolylethanol. We have assumed little physical interaction between the molecules and the catalytic surface and have been able to satisfactorily explain most of the observed selectivity. However, for NMBB we expect a higher selectivity for -A- bond scission relative to -B- bond scission, given the ca. 6 kcal/mol difference between the radical adduct formed by the hydrogen atom addition to 1-methylnaphthalene and p-xylene. It is possible that physical properties play a role in lowering the selectivity in -B- bond scission. Also, catalysts prepared by other methods may contain different activity sites and operate by different mechanisms.
Date: August 1, 1995
Creator: Autrey, T.; Linehan, J.C.; Camaioni, D.M.; Powers, T.R.; McMillan, E.F. & Franz, J.A.
Partner: UNT Libraries Government Documents Department

Nonplanarity and the protonation behavior of porphyrins

Description: {sup 1}H NMR studies of the protonation of highly nonplanar porphyrins with strong acids reveal the presence of the previously elusive monocation, and show that its stability can be related to the amount of saddle distortion induced by protonation; the amount of saddle distortion for a porphyrin dication is also found to correlate well with the rate of intermolecular proton transfer.
Date: March 21, 2000
Creator: SOMMA,MARIA S.; MEDFORTH,CRAIG J.; TH,KEVIN M. & SHELNUTT,JOHN A.
Partner: UNT Libraries Government Documents Department

Hydrogen shutting pathways in thermal hydroliquefaction: Solvent-induced scission of coal model compound structures

Description: It has been demonstrated that donor solvents play a key role in the scission of thermal stable bonds in coal model compounds and therefore it has been speculated that they will improve liquefaction efficiencies. The authors have been studying the transfer of hydrogen from dihydroarene donor solvents to arene model compounds to quantify the barriers of competing hydrogen transfer mechanisms. Hydrogen can be transferred between arene rings by a variety of pathways. The specific hydrogen transfer pathway or pathways can be predicted given an understanding of the thermochemistry of the reactants intermediates and products. The individual pathways that contribute to strong bond scission have been shown to be dependent on the dihydroarene donor and the arene acceptor. In this paper they quantify the hydrogen pathways between the solvent components anthracene and phenanthrene. In addition, they describe reaction conditions requiring consideration of an additional hydrogen transfer pathway: a multi-step nonipso hydrogen transfer to coal model compounds to evaluate the hydrogen transfer steps to cleave strong diarylmethane bonds in coal structures.
Date: September 1, 1995
Creator: Autrey, S.T.; Powers, T.; Alborn, E.A.; Camaioni, D.M. & Franz, J.A.
Partner: UNT Libraries Government Documents Department

New Albany shale flash pyrolysis under hot-recycled-solid conditions: Chemistry and kinetics, II

Description: The authors are continuing a study of recycle retorting of eastern and western oil shales using burnt shale as the solid heat carrier. Stripping of adsorbed oil from solid surfaces rather than the primary pyrolysis of kerogen apparently controls the release rate of the last 10--20% of hydrocarbons. Thus, the desorption rate defines the time necessary for oil recovery from a retort and sets the minimum hold-time in the pyrolyzer. A fluidized-bed oil shale retort resembles a fluidized-bed cat cracker in this respect. Recycled burnt shale cokes oil and reduces yield. The kerogen H/C ratio sets an upper limit on yield improvements unless external hydrogen donors are introduced. Steam can react with iron compounds to add to the H-donor pool. Increased oil yield when New Albany Shale pyrolyzes under hot-recycled-solid, steam-fluidization conditions has been confirmed and compared with steam retorting of acid-leached Colorado oil shale. In addition, with retorted, but unburnt, Devonian shale present at a recycle ratio of 3, the authors obtain 50% more oil-plus-gas than with burnt shale present. Procedures to make burnt shale more like unburnt shale can realize some increase in oil yield at high recycle ratios. Reduction with H{sub 2} and carbon deposition are possibilities that the authors have tested in the laboratory and can test in the pilot retort. Also, eastern spent shale burned at a high temperature (775 C, for example) cokes less oil than does spent shale burned at a low temperature (475 C). Changes in surface area with burn temperature contribute to this effect. 15 refs., 8 figs., 4 tabs.
Date: November 1, 1990
Creator: Coburn, T.T. & Morris, C.J.
Partner: UNT Libraries Government Documents Department

Chemical structures and reactivities of coal as an organic natural product

Description: Some chemical reactions involved in coal liquefaction have been studied using carbon 14 labelled compounds and nuclear magnetic resonance. On the basis of these studies it is concluded that the role of tetralin during coal conversion is (1) to act as a dispersion vehicle; (2) to supply hydrogen radicals, when needed, to trap coal radicals, and (3) in a very minor way to undergo intermolecular reaction with the coal through making and breaking of C--C (and possibly other) bonds. As a result of other experiments it is concluded that to the methods previously employed for breaking bonds in coal molecules and thereby lowering their molecular weights, must now be added the use of solvated-electrons for breaking -CH/sub 2/--CH/sub 2/- linkages. A possible mechanism for the cleavage of bibenzyl (used as a model compound for coal) is given. (LTN)
Date: January 1, 1979
Creator: Collins, C J; Hombach, H P; Benjamin, B M; Roark, W H; Maxwell, B & Raaen, V F
Partner: UNT Libraries Government Documents Department

Studies of coal reactivity for direct liquefaction

Description: This paper presents a preliminary report on a continuing effort to quantify the reactivity of coal for direct liquefaction reactions in terms of the utilization of hydrogen, the selectivity to products and the properties of the products formed. Liquefaction processes are aimed at maximizing the yield of distillate that can serve as hydrocarbon fuels and chemical feedstocks. This study seeks to better describe: (1) how hydrogen consumed during liquefaction is distributed among product groups; (2) coal reactivity in terms of a relationship defining the selectivity to products formed during liquefaction; and (3) the relation of coal structure to reactivity for liquefaction reactions. Because of the limited space for reporting this effort in this special edition of Fuel, the experimental methods, calculations and data are presented in abbreviated form in order to focus on interpretation and discussion of the results in terms of coal structure and reactivity. 30 refs., 3 figs., 2 tabs.
Date: January 1, 1990
Creator: Stephens, H.P. & Kottenstette, R.J.
Partner: UNT Libraries Government Documents Department

Variational transition state theory calculations of tunneling effects on concerted hydrogen motion in water clusters and formaldehyde/water clusters

Description: The direct participation of water molecules in aqueous phase reaction processes has been postulated to occur via both single-step mechanisms as well as concerted hydrogen atom or proton shifts. In the present work, simple prototypes of concerted hydrogen atom transfer processes are examined for small hydrogen-bonded water clusters -- cyclic trimers and tetramers -- and hydrogen-bonded clusters of formaldehyde with one and two water molecules. Rate constants for the rearrangement processes are computed using variational transition state theory, accounting for quantum mechanical tunneling effects by semiclassical ground-state adiabatic transmission coefficients. The variational transition state theory calculations directly utilize selected information about the potential energy surface along the minimum energy path as parameters of the reaction path Hamiltonian. The potential energy information is obtained from ab ignite electronic structure calculations with an empirical bond additivity correction (the BAC-MP4 method). Tunneling is found to be very important for these concerted rearrangement processes -- the semiclassical ground-state adiabatic transmission coefficients are estimated to be as high as four order of magnitude at room temperature. Effects of the size of the cluster (number of water molecules in the cyclic complex) are also dramatic -- addition of a water molecule is seen to change the calculated rates by orders of magnitude. 36 refs., 10 figs.
Date: August 1, 1990
Creator: Garrett, B.C. (Pacific Northwest Lab., Richland, WA (USA)) & Melius, C.F. (Sandia National Labs., Livermore, CA (USA))
Partner: UNT Libraries Government Documents Department

Analysis of the radical hydrogen transfer pathway for cleaving strong bonds in coal

Description: Hydrogen transfer processes involving radical intermediates are of key importance in the liquefaction of coal. While the primary function of donor solvents is to transfer H{lg_bullet} to coal-derived radicals that form when weak bonds are cleaved thermolytically, growing evidence suggests that the donor solvent can play a role in promoting cleavage of strong {alpha}-bonds. McMillen and Malhotra have explained the results in terms of a single-step mechanism referred to as radical H-transfer (RHT). Mechanistic kinetic models have been used to suggest the importance of RHT pathways in anthracene- and pyrene-based solvent systems. However, we question the reliability of these approaches because little experimental data exists to support the 16.5 kcal/mol intrinsic barriers they assume for RHT reactions. Unambiguous evidence for RHT is very difficult to obtain experimentally because at the temperatures required to activate the RHT reaction, a suite of multistep reactions can occur, which yield the same products, i.e. H-elimination from hydroaryl radicals followed by ipso addition. For this reason, we have sought to gain insight to barrier heights for RHT from theory. This paper reports our use of Marcus theory in combination with ab initio and semiempirical molecular orbital methods to show how the intrinsic barriers for RHT reactions depend on structural and thermodynamic properties of the reacting partners. In addition, reactions thought to be mediated by RHT are reexamined using mechanistic kinetic modeling (MKM) to determine the extent to which these reactions can be explained by conventional pathways.
Date: September 1, 1993
Creator: Autrey, S. T.; Camaioni, D. M.; Ferris, K. F. & Franz, J. A.
Partner: UNT Libraries Government Documents Department

Restricted mass transport effects on free radical reactions

Description: Coal possesses a complex chemical and physical structure. The cross-linked, network structure can lead to alterations in normal thermally-induced, free-radical decay pathways as a consequence of restrictions on mass transport. Moreover, in coal liquefaction, access of an external hydrogen donor to a reactive radical site can be hindered by the substantial domains of microporosity present in coals. However, previous work indicates that diffusion effects do not appear to be playing an important role in this coal conversion chemistry. Several possible explanations for this phenomenon were advanced including the potential involvement of a hydrogen hopping/radical relay mechanism recently discovered model systems in the authors laboratories. The authors have employed silica-anchored compounds to explore the effects of restricted mass transport on the pyrolysis mechanisms of coal model compounds. In studies of two-component systems, cases have been discovered where radical centers can be rapidly relocated in the diffusionally constrained environment as a consequence of rapid serial hydrogen atom transfers. This chemistry can have substantial effects on thermal decomposition rates and on product selectivities. In this study, the authors examine additional surfaces to systematically investigate the impact of molecular structure on the hydrogen atom transfer promoted radical relay mechanism. Silica-attached 1,3-diphenylpropane ({approx}Ph(CH{sub 2}){sub 3}Ph, or {approx}DPP) was chosen as the thermally reactive component, since it can be considered prototypical of linkages in coal that do not contain weak bonds easily cleaved at coal liquefaction temperatures (ca. 4000 {degrees}C), but which crack at reasonable rates if benzylic radicals can be generated by hydrogen abstraction. The rate of such hydrogen transfers under restricted diffusion will be highly dependent on the structure and proximity of neighboring molecules.
Date: September 1, 1994
Creator: Buchanan, A. C. III; Britt, P. F. & Thomas, K. B.
Partner: UNT Libraries Government Documents Department