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P J Hore - One of the best experts on this subject based on the ideXlab platform.
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Model calculations of magnetic field effects on the Recombination Reactions of radicals with anisotropic hyperfine interactions
Chemical Physics Letters, 2020Co-Authors: Christiane R Timmel, Brian Brocklehurst, F Cintolesi, P J HoreAbstract:The effects of anisotropic hyperfine interactions on the Recombination Reactions of spin correlated radical pairs in a weak applied magnetic field are discussed in the context of the radical pair mechanism (RPM). Model calculations are presented for radical pairs containing a single spin-1/2 nucleus with an axial or rhombic coupling to one of the unpaired electrons. The so-called low field effect (LFE) and various resonances in the magnetic field effect (MFE) are calculated. Approximate analytical expressions are given for the field positions of the resonances which are shown to arise from energy level crossings. © 2001 Elsevier Science B.V
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spin selective Recombination Reactions of radical pairs experimental test of validity of reaction operators
Journal of Chemical Physics, 2013Co-Authors: Kiminori Maeda, Paul A Liddell, Devens Gust, P J HoreAbstract:Spin-selective Reactions of radical pairs are conventionally modelled using an approach that dates back to the 1970s [R. Haberkorn, Mol. Phys. 32, 1491 (1976)]. An alternative approach based on the theory of quantum measurements has recently been suggested [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010)]. We present here the first experimental attempt to discriminate between the two models. Pulsed electron paramagnetic resonance spectroscopy has been used to investigate intramolecular electron transfer in the radical pair form of a carotenoid-porphyrin-fullerene molecular triad. The rate of spin-spin relaxation of the fullerene radical in the triad was found to be inconsistent with the quantum measurement description of the spin-selective kinetics, and in accord with the conventional model when combined with spin-dephasing caused by rotational modulation of the anisotropic g-tensor of the fullerene radical.
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an improved semiclassical theory of radical pair Recombination Reactions
Journal of Chemical Physics, 2013Co-Authors: David E Manolopoulos, P J HoreAbstract:We present a practical semiclassical method for computing the electron spin dynamics of a radical in which the electron spin is hyperfine coupled to a large number of nuclear spins. This can be used to calculate the singlet and triplet survival probabilities and quantum yields of radical Recombination Reactions in the presence of magnetic fields. Our method differs from the early semiclassical theory of Schulten and Wolynes [J. Chem. Phys. 68, 3292 (1978)] in allowing each individual nuclear spin to precess around the electron spin, rather than assuming that the hyperfine coupling-weighted sum of nuclear spin vectors is fixed in space. The downside of removing this assumption is that one can no longer obtain a simple closed-form expression for the electron spin correlation tensor: our method requires a numerical calculation. However, the computational effort increases only linearly with the number of nuclear spins, rather than exponentially as in an exact quantum mechanical calculation. The method is therefore applicable to arbitrarily large radicals. Moreover, it approaches quantitative agreement with quantum mechanics as the number of nuclear spins increases and the environment of the electron spin becomes more complex, owing to the rapid quantum decoherence in complex systems. Unlike the Schulten-Wolynes theory, the present semiclassical theory predicts the correct long-time behaviour of the electron spin correlation tensor, and it therefore correctly captures the low magnetic field effect in the singlet yield of a radical Recombination reaction with a slow Recombination rate.
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measurement of magnetic field effects on radical Recombination Reactions using triplet triplet energy transfer
Chemical Physics Letters, 2006Co-Authors: Kevin B Henbest, Kiminori Maeda, P J Hore, E Athanassiades, Christiane R TimmelAbstract:Abstract Previously, the effect of applied magnetic fields on the yield of the reaction of the radical ions of pyrene and 1,3-dicyanobenzene (DCB) has been measured by monitoring the emission of the pyrene/1,3-DCB exciplex formed from the singlet state of the spin-correlated radical pair (SCRP). This Letter describes an alternative approach that relies on energy transfer from the excited triplet state of pyrene to the tris(2,2′-bipyridyl)ruthenium(II) ion, Ru ( bpy ) 3 2 + , allowing the Recombination of the triplet SCRP to be monitored by emission spectroscopy. Measurements of magnetic field effects in combination with time-resolved flash photolysis and luminescence experiments confirm triplet–triplet energy transfer as the reaction mechanism.
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influence of dipolar interactions on radical pair Recombination Reactions subject to weak magnetic fields
Journal of Physical Chemistry A, 2005Co-Authors: Anthony R Odea, Christiane R Timmel, Ailsa F Curtis, Nicholas J B Green, P J HoreAbstract:Monte Carlo simulations of the effects of weak magnetic fields on the Recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet−triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase...
Christiane R Timmel - One of the best experts on this subject based on the ideXlab platform.
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Model calculations of magnetic field effects on the Recombination Reactions of radicals with anisotropic hyperfine interactions
Chemical Physics Letters, 2020Co-Authors: Christiane R Timmel, Brian Brocklehurst, F Cintolesi, P J HoreAbstract:The effects of anisotropic hyperfine interactions on the Recombination Reactions of spin correlated radical pairs in a weak applied magnetic field are discussed in the context of the radical pair mechanism (RPM). Model calculations are presented for radical pairs containing a single spin-1/2 nucleus with an axial or rhombic coupling to one of the unpaired electrons. The so-called low field effect (LFE) and various resonances in the magnetic field effect (MFE) are calculated. Approximate analytical expressions are given for the field positions of the resonances which are shown to arise from energy level crossings. © 2001 Elsevier Science B.V
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measurement of magnetic field effects on radical Recombination Reactions using triplet triplet energy transfer
Chemical Physics Letters, 2006Co-Authors: Kevin B Henbest, Kiminori Maeda, P J Hore, E Athanassiades, Christiane R TimmelAbstract:Abstract Previously, the effect of applied magnetic fields on the yield of the reaction of the radical ions of pyrene and 1,3-dicyanobenzene (DCB) has been measured by monitoring the emission of the pyrene/1,3-DCB exciplex formed from the singlet state of the spin-correlated radical pair (SCRP). This Letter describes an alternative approach that relies on energy transfer from the excited triplet state of pyrene to the tris(2,2′-bipyridyl)ruthenium(II) ion, Ru ( bpy ) 3 2 + , allowing the Recombination of the triplet SCRP to be monitored by emission spectroscopy. Measurements of magnetic field effects in combination with time-resolved flash photolysis and luminescence experiments confirm triplet–triplet energy transfer as the reaction mechanism.
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influence of dipolar interactions on radical pair Recombination Reactions subject to weak magnetic fields
Journal of Physical Chemistry A, 2005Co-Authors: Anthony R Odea, Christiane R Timmel, Ailsa F Curtis, Nicholas J B Green, P J HoreAbstract:Monte Carlo simulations of the effects of weak magnetic fields on the Recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet−triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase...
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model calculations of magnetic field effects on the Recombination Reactions of radicals with anisotropic hyperfine interactions
Chemical Physics Letters, 2001Co-Authors: Christiane R Timmel, Brian Brocklehurst, F Cintolesi, P J HoreAbstract:The effects of anisotropic hyperfine interactions on the Recombination Reactions of spin correlated radical pairs in a weak applied magnetic field are discussed in the context of the radical pair mechanism (RPM). Model calculations are presented for radical pairs containing a single spin-1/2 nucleus with an axial or rhombic coupling to one of the unpaired electrons. The so-called low field effect (LFE) and various resonances in the magnetic field effect (MFE) are calculated. Approximate analytical expressions are given for the field positions of the resonances which are shown to arise from energy level crossings.
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the effects of weak magnetic fields on radical Recombination Reactions in micelles
International Journal of Radiation Biology, 2000Co-Authors: R W Eveson, Brian Brocklehurst, Christiane R Timmel, P J Hore, K A MclauchlanAbstract:Purpose : To demonstrate the effects of weak magnetic fields (> ˜ 1 mT) on chemical Reactions involving free radicals, in the context of possible effects of environmental electromagnetic radiation on biological systems. Materials and methods : Transient absorption, flash photolysis experiments have been performed to study the kinetics and yields of radical Reactions. The triplet state of benzophenone has been used as a convenient source of radical pairs, whose identity is largely immaterial to the investigation of the so-called Low Field Effect. Hydrogen abstraction from surfactant molecules in micelles yields a pair of neutral radicals, one large and one small, in a region of restricted translational and rotational motion. Results : In alkyl sulphate and sulphonate micelles a weak field increases the concentration of free radicals that escape from the micelle to an extent that depends on the structure, dynamics and volume of the space in which the radical pairs are confined. The effect (up to 10%) is typ...
Dmitri Babikov - One of the best experts on this subject based on the ideXlab platform.
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properties of feshbach and shape resonances in ozone and their role in Recombination Reactions and anomalous isotope effects
Faraday Discussions, 2018Co-Authors: Alexander Teplukhin, Dmitri BabikovAbstract:Computational modelling of Recombination Reactions that form ozone require the inclusion of several quantum mechanical effects such as symmetry, zero-point energy, scattering resonances and tunneling. Major elements of theory for rigorous description of this process are reviewed, with emphasis on interpreting the famous anomalous isotope effect due to substitutions of 18O. Three reaction pathways, for the formation of symmetric and asymmetric isotopologues of ozone, are introduced and a hierarchy of theory levels is outlined. Lower levels of theory are used to account for the effects of symmetry, isotope mass, rotational excitations and vibrational zero-point energy differences. They happen to be equivalent to statistical descriptions of the process and do not show anomalous isotope effects. Properties of scattering resonances should be included at the next level of theory, and may finally explain the isotope effect. Shape resonances, trapped behind the centrifugal barrier and populated by tunneling, can be studied by neglecting couplings between the diabatic ro-vibrational states of the system. Inclusion of these couplings enables the formation of Feshbach resonances. Accurate calculations using hyper-spherical coordinates are performed to obtain resonance energies, lifetimes and wavefunctions. Differences between the shape resonances and Feshbach resonances are emphasized.
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Recombination Reactions as a possible mechanism of mass independent fractionation of sulfur isotopes in the archean atmosphere of earth
Proceedings of the National Academy of Sciences of the United States of America, 2017Co-Authors: Dmitri BabikovAbstract:A hierarchy of isotopically substituted Recombination Reactions is formulated for production of sulfur allotropes in the anoxic atmosphere of Archean Earth. The corresponding system of kinetics equations is solved analytically to obtain concise expressions for isotopic enrichments, with focus on mass-independent isotope effects due to symmetry, ignoring smaller mass-dependent effects. Proper inclusion of atom-exchange processes is shown to be important. This model predicts significant and equal depletions driven by reaction stoichiometry for all rare isotopes: 33 S, 34 S, and 36 S. Interestingly, the ratio of capital Δ values obtained within this model for 33 S and 36 S is −1.16, very close to the mass-independent fractionation line of the Archean rock record. This model may finally offer a mechanistic explanation for the striking mass-independent fractionation of sulfur isotopes that took place in the Archean atmosphere of Earth.
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semiclassical wave packet treatment of scattering resonances application to the delta zero point energy effect in Recombination Reactions
Physical Review Letters, 2007Co-Authors: Evgeny Vetoshkin, Dmitri BabikovAbstract:For the first time Feshbach-type resonances important in Recombination Reactions are characterized using the semiclassical wave packet method. This approximation allows us to determine the energies, lifetimes, and wave functions of the resonances and also to observe a very interesting correlation between them. Most important is that this approach permits description of a quantum delta\char21{}zero-point energy effect in Recombination Reactions and reproduces the anomalous rates of ozone formation.
J Troe - One of the best experts on this subject based on the ideXlab platform.
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representation of broad falloff curves for dissociation and Recombination Reactions
Zeitschrift für Physikalische Chemie, 2014Co-Authors: J Troe, V G UshakovAbstract:Expressions for representing the pressure dependence of unimolecular dissociation and the reverse Recombination Reactions are compared. Situations are considered where the broadening of the corresponding falloff curves is partic- ularly pronounced, i.e. where broadening factors at the center of the falloff curves, �� cent, are very small and falloff curves correspondingly become very "broad". Such situations arise when unimolecular Reactions of molecules with large numbers of low-frequency modes and high-temperature situations are considered. Recom- bination Reactions of polyatomic species in atmospheric chemistry and dissocia- tion Reactions of large molecules in combustion are the fields of application of the present results. While previously proposed expressions for asymmetric broaden- ing factors showed artifacts when �� cent decreased to values below about 0.4, al- ternative functions are described which avoid these problems for situations where �� cent decreases to values far below 0.1.
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limitations of variational transition state theory for barrierless radical radical Recombination Reactions
Zeitschrift für Physikalische Chemie, 2004Co-Authors: J TroeAbstract:Variational transition state theory (VTST) is widely used for the modelling of barrierless radical-radical Recombination Reactions. In this application, VTST suffers from a number of limitations some of whichare of more technical, others of more fundamental nature. The former are caused by inappropriate averaging over individual adiabatic channel potentials or by the neglect of quantum effects, the latter are due to deviations from adiabatic dynamics. It is shown that most radical-radical Recombination Reactions are characterized by Massey parameters which are smaller than unity such that the dynamics is nonadiabatic. VTST treatments which generally assume adiabatic dynamics, therefore, have a fundamental problem. Calculations of rate constants by VTST often exceed classical trajectory results by about 10 to 20 percent. This is normally attributed to "recrossing trajectories". In the present work it is shown, however, that deviations of this magnitude also have to be expected for nonadiabatic dynamics in comparison to adiabatic dynamics. It is, therefore, suggested that "recrossing" at least in part has to be attributed to nonadiabatic dynamics. A way out of the dilemma is the use of a combination of statistical adiabatic channel and classical trajectory concepts.
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prediction of reduced falloff curves for Recombination Reactions at low temperatures
Zeitschrift für Physikalische Chemie, 2003Co-Authors: C J Cobos, J TroeAbstract:Strong collision falloff curves for barrierless Recombination Reactions at low temperatures are calculated taking into account transitional modes only. Specific rate constants k(E, J) from statistical adiabatic channel/classical trajectory (SACM/CT) calculations are used. Broadening factors of the falloff curves are found to depend only weakly on the temperature. A systematic analysis of the influence of the centrifugal barriers E 0 (J), which are governed by the potential energy surface of the bond energies E 0 , and of the number of transitional modes is made. Guidelines for estimating center broadening factors and shape functions for the broadening factors are given.
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experimental and theoretical study of the temperature and pressure dependences of the Recombination Reactions o no2 m no3 m and no2 no3 m n2o5 m
Physical Chemistry Chemical Physics, 2000Co-Authors: J Hahn, K Luther, J TroeAbstract:The Recombination Reactions O + NO2(+M) → NO3(+M) and NO2 + NO3(+M) → N2O5(+M) were studied at temperatures of 300 and 400 K, and at pressures of the bath gas M = N2 between 1 and 900 bar. Oxygen atoms were generated by laser flash photolysis of N2O at 193 nm, NO3 radicals were monitored by light absorption at 578 nm. The measured fall-off curve of the reaction O + NO2(+M) → NO3(+M) could be well represented by limiting low pressure rate constants k3,0 = (1.3 ± 0.3) × 10−31 (T/300 K)−1.5 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k3,∞ = (2.3 ± 0.2) × 10−11 (T/300 K)0.24 cm molecule−1 s−1, and fall-off broadening factors of Fc = 0.71 exp(−T/1700 K). The derived results are consistent with earlier relative rate measurements. Theoretical modeling of k3,0, Fc and k3,∞ led to consistency with the experimental data. There is strong evidence that, besides the electronic ground state NO3(2A′), the first excited electronic state NO3(2E′) also contributes to the observed Recombination reaction. The measured fall-off curve of the reaction NO2 + NO3(+M) → N2O5(+M) was represented by limiting low pressure rate constants k5,0 = 3.6 × 10−30(T/300 K)−5.0 [N2] cm6 molecule−2 s−1, limiting high pressure rate constants k5,∞ = (1.9 ± 0.3) × 10−12(T/300 K)0.2 cm3 molecule−1 s−1 and Fc = 0.38 exp(−T/4900 K). A theoretical analysis of these values is also presented.
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Pressure and temperature dependence of low temperature Recombination Reactions: O + O sub 2 yields O sub 3
1990Co-Authors: J Troe, H. Hippler, R. RahnAbstract:The pressure and the temperature dependences of the Recombination reaction O + O{sub 2} {yields} O{sub 3} show quite anomalous behavior. The reaction was studied by laser flash photolysis between 90 and 373 K in the pressure range 1-1,000 bar. With the third body He, the low pressure rate coefficient increases as T{sup {minus}1}, with Ar as T{sup {minus}2.5}, when the temperature is decreased. This behavior is in contrast to weak or strong collision unimolecular rate theory. The anomalous properties observed are interpreted in terms of radical-complex mechanisms. The possibility of similar results for other Recombination Reactions under stratospheric conditions is discussed.
M Rutigliano - One of the best experts on this subject based on the ideXlab platform.
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dynamics of plasma surface processes e r and l h atom Recombination Reactions
Plasma Sources Science and Technology, 2009Co-Authors: M Cacciatore, M RutiglianoAbstract:The dynamics and energetics of dissociative chemisorption and atom Recombination Reactions relevant to plasma–wall and plasma–surface processes are discussed in the light of recent progress in molecular dynamics and electronic structure studies on elementary heterogeneous systems. Emphasis is given to the Langmuir–Hinshelwood and Eley–Rideal Recombination processes involving H and its isotopes adsorbed on various substrates at different surface temperatures and collisional energy regimes.
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Dynamics of plasma–surface processes: E–R and L–H atom Recombination Reactions
Plasma Sources Science and Technology, 2009Co-Authors: M Cacciatore, M RutiglianoAbstract:The dynamics and energetics of dissociative chemisorption and atom Recombination Reactions relevant to plasma–wall and plasma–surface processes are discussed in the light of recent progress in molecular dynamics and electronic structure studies on elementary heterogeneous systems. Emphasis is given to the Langmuir–Hinshelwood and Eley–Rideal Recombination processes involving H and its isotopes adsorbed on various substrates at different surface temperatures and collisional energy regimes.