Competitive Reaction Rates of Hydrogen Atoms with HCl and Cl2. Entropy Considerations of the HCl2 Transition State

1961 ◽  
Vol 34 (5) ◽  
pp. 1494-1498 ◽  
Author(s):  
Fritz S. Klein ◽  
Max Wolfsberg
Keyword(s):  
Transition State ◽  
Reaction Rates ◽  
Competitive Reaction ◽  
Hydrogen Atoms

scholarly journals Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage

2020 ◽  
Vol 635 ◽  
pp. A9 ◽  
Author(s):  
Sandra D. Wiersma ◽  
Alessandra Candian ◽  
Joost M. Bakker ◽  
Jonathan Martens ◽  
Giel Berden ◽  
...  
Keyword(s):  
Transition State ◽  
Ir Spectra ◽  
Density Functional ◽  
Zero Point ◽  
Reaction Rates ◽  
Ion Cyclotron Resonance ◽  
Point Energy ◽  
Polycyclic Aromatic ◽  
Atom Loss ◽  
Ground State Structures

Aims. We investigate the possible role of polycyclic aromatic hydrocarbons (PAHs) as a sink for deuterium in the interstellar medium (ISM) and study UV photolysis as a potential underlying chemical process in the variations of the deuterium fractionation in the ISM. Methods. The UV photo-induced fragmentation of various isotopologs of deuterium-enriched, protonated anthracene and phenanthrene ions (both C14H10 isomers) was recorded in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Infrared multiple photon dissociation spectroscopy using the Free-Electron Laser for Infrared eXperiments was applied to provide IR spectra. Infrared spectra calculated using density functional theory were compared to the experimental data to identify the isomers present in the experiment. Transition-state energies and reaction rates were also calculated and related to the experimentally observed fragmentation product abundances. Results. The photofragmentation mass spectra for both UV and IRMPD photolysis only show the loss of atomic hydrogen from [D − C14H10]+, whereas [H − C14D10]+ shows a strong preference for the elimination of deuterium. Transition state calculations reveal facile 1,2-H and -D shift reactions, with associated energy barriers lower than the energy supplied by the photo-excitation process. Together with confirmation of the ground-state structures via the IR spectra, we determined that the photolytic processes of the two different PAHs are largely governed by scrambling where the H and the D atoms relocate between different peripheral C atoms. The ∼0.1 eV difference in zero-point energy between C–H and C–D bonds ultimately leads to faster H scrambling than D scrambling, and increased H atom loss compared to D atom loss. Conclusions. We conclude that scrambling is common in PAH cations under UV radiation. Upon photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher probability for the aliphatic D atom to migrate to a strongly bound aromatic site, protecting it from elimination. We speculate that this could lead to increased deuteration as a PAH moves towards more exposed interstellar environments. Also, large, compact PAHs with an aliphatic C–HD group on solo sites might be responsible for the majority of aliphatic C–D stretching bands seen in astronomical spectra. An accurate photochemical model of PAHs that considers deuterium scrambling is needed to study this further.

Dynamics of biochemical and biophysical reactions: insight from computer simulations

2001 ◽  
Vol 34 (4) ◽  
pp. 563-679 ◽  
Author(s):  
Arieh Warshel ◽  
William W. Parson
Keyword(s):  
Electron Transfer ◽  
Transition State ◽  
Transmission Coefficient ◽  
Enzyme Catalysis ◽  
Experimental Studies ◽  
Reaction Rates ◽  
State Theory ◽  
Enzyme Reactions ◽  
Formidable Challenge ◽  
Nuclear Tunneling

1. Introduction 5632. Obtaining rate constants from molecular-dynamics simulations 5642.1 General relationships between quantum electronic structures and reaction rates 5642.2 The transition-state theory (TST) 5692.3 The transmission coefficient 5723. Simulating biological electron-transfer reactions 5753.1 Semi-classical surface-hopping and the Marcus equation 5753.2 Treating quantum mechanical nuclear tunneling by the dispersed-polaron/spin-boson method 5803.3 Density-matrix treatments 5833.4 Charge separation in photosynthetic bacterial reaction centers 5844. Light-induced photoisomerizations in rhodopsin and bacteriorhodopsin 5965. Energetics and dynamics of enzyme reactions 6145.1 The empirical-valence-bond treatment and free-energy perturbation methods 6145.2 Activation energies are decreased in enzymes relative to solution, often by electrostatic effects that stabilize the transition state 6205.3 Entropic effects in enzyme catalysis 6275.4 What is meant by dynamical contributions to catalysis? 6345.5 Transmission coefficients are similar for corresponding reactions in enzymes and water 6365.6 Non-equilibrium solvation effects contribute to catalysis mainly through Δg[Dagger], not the transmission coefficient 6415.7 Vibrationally assisted nuclear tunneling in enzyme catalysis 6485.8 Diffusive processes in enzyme reactions and transmembrane channels 6516. Concluding remarks 6587. Acknowledgements 6588. References 658Obtaining a detailed understanding of the dynamics of a biochemical reaction is a formidable challenge. Indeed, it might appear at first sight that reactions in proteins are too complex to analyze microscopically. At room temperature, even a relatively small protein can have as many as 1034 accessible conformational states (Dill, 1985). In many cases, however, we have detailed structural information about the active site of an enzyme, whereas such information is missing for corresponding chemical systems in solution. The atomic coordinates of the chromophore in bacteriorhodopsin, for example, are known to a resolution of 1–2 Å. In addition, experimental studies of biological processes such as photoisomerization and electron transfer have provided a wealth of detailed information that eventually may make some of these processes classical problems in chemical physics as well as biology.

THE REACTION OF DEUTERIUM ATOMS WITH ETHYLENE

1956 ◽  
Vol 34 (8) ◽  
pp. 1061-1073 ◽  
Author(s):  
S. Toby ◽  
H. I. Schiff
Keyword(s):  
Isotopic Composition ◽  
Isothermal Calorimetry ◽  
Reaction Rates ◽  
Recombination Coefficient ◽  
Tungsten Filament ◽  
Hydrogen Atoms ◽  
Atom Concentration ◽  
Metaphosphoric Acid ◽  
Detector Position ◽  
Reactant Flow

Deuterium was dissociated on a hot tungsten filament and the atom concentration measured by isothermal calorimetry. The recombination coefficient of deuterium atoms on a glass surface, coated with metaphosphoric acid, was found to be 3.8 × 10−5, and similar to that found for hydrogen atoms. The reactions of H-atoms and D-atoms with ethylene were found to be very rapid. The effects on the yields of the products and on their isotopic composition of variations of reactant flow rate, atom concentration, pressure, and atom-detector position were studied. The major products were methanes, ethanes, and ethylenes, with minor amounts of propanes and butanes. The methanes were always highly deuterated while the ethanes were slightly deuterated. A mechanism is proposed to explain the observations based on a flow pattern in the reaction zone. The possibility of differences in the reaction rates of variously deuterated intermediates is also discussed.

Reaction rates of BrH+Cl→Br+HCl using semiclassical transition state theory

1994 ◽  
Vol 223 (5-6) ◽  
pp. 459-464 ◽  
Author(s):  
Michael J. Cohen ◽  
Andrew Willetts ◽  
Nicholas C. Handy
Keyword(s):  
Transition State ◽  
Transition State Theory ◽  
Reaction Rates ◽  
State Theory

Exchange reactions of olefins with deuterium oxide on ion-exchanged X-type zeolites

1971 ◽  
Vol 321 (1545) ◽  
pp. 259-273 ◽  
Keyword(s):  
Active Sites ◽  
Deuterium Oxide ◽  
Reaction Rates ◽  
Main Reaction ◽  
Exchange Reactions ◽  
Hydrogen Atoms ◽  
Carbonium Ion ◽  
Mass Spectrometric Technique ◽  
Carbonium Ions ◽  
Cation Charge

Reactions of propylene, ethylene, but-1-ene , isobutene and isobutane with D 2 O on ion-exchanged X-type zeolites have been followed by a mass spectrometric technique. Exchange was usually the main reaction but polymerization of olefins also occurred with some catalysts. All the hydrogen atoms in isobutene were exchanged at similar rates by a stepwise process but with propylene only five atoms were replaced. Exchange was complicated by simultaneous isomerization with but-1-ene. Isobutane reacted only at high temperatures but gave multiply exchanged products. The order of activity for exchange was isobutene ⪢ but-1-ene > propylene ⪢ ethylene, isobutane, and this order appeared to reflect the relative ease of formation of carbonium ions from the hydrocarbons. The character of the exchange reactions as well as the rates were in accord with mechanisms involving carbonium ion intermediates . The order of activity of the zeolites for the exchange of propylene was CeX, LaX > NiX, CuX, CoX > CaX > NaX and a correlation was found to exist between the apparent activation energy for exchange and a function of the cation charge. Reaction rates on NiX and CeX increased with increasing degree of ion-exchange and decreased with increasing amounts of D 2 O. There was evidence that in some cases the active sites were associated with acidic OH(OD) groups rather than the cations themselves.

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