SEARCH · Engineering Papers
Results for “ABSTRACT”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Hydrogen Abstraction Energies and Ammonia Binding to BEA, ZSM-5, and α-Quartz Doped with Al, Sc, B, or Ga
Not Available
Reaction of an Iron(IV) Nitrido Complex with Cyclohexadienes: Cycloaddition and Hydrogen-Atom Abstraction
Not Available
Reversible H Atom Abstraction Catalyzed by the Radical S -Adenosylmethionine Enzyme HydG
Not Available
Hydrogen Donation but not Abstraction by a Tyrosine (Y68) during Endoperoxide Installation by Verruculogen Synthase (FtmOx1)
Hydrogen-atom transfer (HAT) from a substrate carbon to an iron(IV)-oxo (ferryl) intermediate initiates a diverse array of enzymatic transformations. For outcomes other than hydroxylation, coupling of the resultant carbon radical and hydroxo ligand (oxygen rebound) must generally be averted. A recent study of FtmOx1, a fungal iron(II)- and 2-(oxo)glutarate-dependent oxygenase that installs the endoperoxide of verruculogen by adding O 2 between carbons 21 and 27 of fumitremorgin B, posited that tyrosine (Tyr or Y) 224 serves as HAT intermediary to separate the C21 radical (C21•) and Fe(III)–OH HAT products and prevent rebound. Our reinvestigation of the FtmOx1 mechanism revealed, instead, direct HAT from C21 to the ferryl complex and surprisingly competitive rebound. The C21-hydroxylated (rebound) product, which undergoes deprenylation, predominates when low [O 2 ] slows C21•–O 2 coupling in the next step of the endoperoxidation pathway. This pathway culminates with addition of the C21–O–O• peroxyl adduct to olefinic C27 followed by HAT to the C26• from a Tyr. The last step results in sequential accumulation of Tyr radicals, which are suppressed without detriment to turnover by inclusion of the reductant, ascorbate. Replacement of each of four candidates for the proximal C26 H• donor (including Y224) with phenylalanine (F) revealed that only the Y68F variant (i) fails to accumulate the first Tyr• and (ii) makes an altered major product, identifying Y68 as the donor. The implied proximities of C21 to the iron cofactor and C26 to Y68 support a new docking model of the enzyme–substrate complex that is consistent with all available data.
Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copper(III) Cores: Hydrogen-Atom Transfer or Concerted Proton-Coupled Electron Transfer?
In a possibly biomimetic fashion, formally copper(III)–oxygen complexes LCu(III)–OH (1) and LCu(III)–OOCm (2) (L 2– = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X–H bonds (X = C, O). Herein, we demonstrate similar X–H bond activation by a formally Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)–O 2 CAr 1 (3, Ar 1 = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O–H or C–H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied complexes and to a variation in X–H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have been employed to distinguish these two cases, both of which generally focus on whether the electron (e – ) and proton (H + ) travel “together” as a true H atom, (HAT), or whether the H + and e – are transferred in concert, but travel between different donor/acceptor centers (cPCET). In this work, we reveal that both mechanisms are active for X–H bond activation by 1–3, with interesting variations as a function of substrate and copper functionality.
Ab Initio Reaction Kinetics of Hydrogen Abstraction from Methyl Formate by Hydrogen, Methyl, Oxygen, Hydroxyl, and Hydroperoxy Radicals
Explore the source record for details and available documents.
Multistructural Variational Transition State Theory: Kinetics of the Hydrogen Abstraction from Carbon-2 of 2-Methyl-1-propanol by Hydroperoxyl Radical Including All Structures and Torsional Anharmonicity
Explore the source record for details and available documents.
Biofuel Combustion. Energetics and Kinetics of Hydrogen Abstraction from Carbon-1 in n -Butanol by the Hydroperoxyl Radical Calculated by Coupled Cluster and Density Functional Theories and Multistructural Variational Transition-State Theory with Multidimensional Tunneling
Explore the source record for details and available documents.
Ab initio kinetics studies of hydrogen atom abstraction from methyl propanoate
Explore the source record for details and available documents.
Multi-path variational transition state theory for chiral molecules: the site-dependent kinetics for abstraction of hydrogen from 2-butanol by hydroperoxyl radical, analysis of hydrogen bonding in the transition state, and dramatic temperature dependence of the activation energy
A hydrogen bond at the transition state can lower the enthalpy of activation, but raise the free energy of activation.
Kinetics and branching fractions of the hydrogen abstraction reaction from methyl butenoates by H atoms
Explore the source record for details and available documents.
Stereodynamical control of product branching in multi-channel barrierless hydrogen abstraction of CH 3 OH by F
Comprehensive dynamical simulations of a prototypical multi-channel reaction on a globally accurate potential energy surface show that the non-statistical product branching is dictated by unique stereodynamics in the entrance channels.
Redox chemistry and H-atom abstraction reactivity of a terminal zirconium( iv ) oxo compound mediated by an appended cobalt( i ) center
Bimetallic cooperativity is demonstrated with a Co/Zr complex featuring both nucleophilic Zr( iv ) oxo and redox active Co sites.
A full-dimensional quantum dynamics study of the mode specificity in the H + HOD abstraction reaction
We employ the initial state-selected time-dependent wave packet approach to an atom-triatom reaction to study the H + HOD → OH + HD/OD + H{sub 2} reaction without the centrifugal sudden approximation, based on an accurate potential energy surface which was recently developed by neural network fitting to high level ab initio energy points. The total reaction probabilities and integral cross sections, which are the exact coupled-channel results, are calculated for the HOD reactant initially in the ground and several vibrationally excited states, including the bending excited state, OD stretching excited states, OH stretching excited states, and combined excitations of them. The reactivity enhancements from different initial states of HOD are presented, which feature strong bond-selective effects of the reaction dynamics. The current results for the product branching ratios, reactivity enhancements, and relative cross sections are largely improved over the previous calculations, in quantitatively good agreement with experiment. The thermal rate constant for the title reaction and the contributions from individual vibrational states of HOD are also obtained.