Rotation Reaction Research Articles

On Relations Between Singlet and Triplet Recombination Yields for Singlet and Triplet Precursors

Abstract The relations have been analytically derived for recombination yields of radical pairs with different precursor states and by different reaction channels (singlet and/or triplet) in arbitrary magnetic and microwave fields. Recombination of two particles from a radical pair mostly with anisotropic reactivity is assumed to take place in a narrow reaction zone, and relative translation motion including rotation of reactants may be an arbitrary stochastic process. In addition, the approach may take into account the time-dependent spin Hamiltonian. The novel method for the calculation of recombination yields is suggested, and general expressions for recombination yields are derived for strong singlet-triplet dephasing. Some of the relations and expressions are the basis of the results given in the literature and their generalization to the case of non-model reacting systems that does not use simplification of reactants structure and reactants motion in solutions.

Effects of reactant rotation on the dynamics of the OH + CH4 → H2O + CH3 reaction: A six-dimensional study

The dynamics of the hydrogen abstraction reaction between methane and hydroxyl radical is investigated using an initial state selected time-dependent wave packet method within a six-dimensional model. The ab initio calibrated global potential energy surface of Espinosa-García and Corchado was used. Integral cross sections from several low-lying rotational states of both reactants have been obtained using the centrifugal sudden and J-shifting approximations. On the empirical potential energy surface, the rotational excitation of methane has little effect on the reaction cross section, but excited rotational states of OH inhibit the reactivity slightly. These results are rationalized with the newly proposed sudden vector projection model.

Kilogram-scale synthesis of Mg–Al–CO 3 LDHs nanosheets using a new two-stage reactor

A novel reactor with a two-stage reaction chamber was designed to kilogram-large-scale synthesized Mg–Al–CO 3 layered double hydroxide (Mg–Al–CO 3 LDHs) nanosheets. The effects of the reactant concentration and rotation speed of the dynamic structure on the crystal size and morphology were investigated. The results show that the size of Mg–Al–CO 3 LDH particles gained in typical condition is about 50 nm in length and 5.7 nm in thickness, and the size becomes larger as the reactant concentration or rotationl speed decreased. The possible reaction mechanism was explained by three critical factors of the reactor, N 2 gas and polyethylene glycol 2000.

Photoionization affected by chemical anisotropy

The kinetic constants of rhodamine 3B quenching by N,N-dimethyl aniline were extracted from the very beginning of the quenching kinetics, recently studied in a few solvents of different viscosities. They were well fitted with the conventional kinetic constant definition, provided the radial distribution function of simple liquids was ascribed to the reactant pair distribution and the contact electron transfer rate was different in all the cases. This difference was attributed to the chemical anisotropy averaging by the rotation of reactants, which is the faster in solvents of lower viscosity. With the proper choice of a space dependent encounter diffusion, the whole quenching kinetics was well fitted with an encounter theory, using the Marcus [J. Chem. Phys. 24, 966 (1956); 43, 679 (1965)] transfer rate instead of the contact Collins-Kimball [J. Colloid. Sci. 4, 425 (1949)] approximation. Not only the beginning and middle part of the quenching were equally well fitted, but the long time (Markovian) rate constant was also found to be the same as previously obtained. Moreover, the concentration dependencies of the fluorescence quantum yield and the Stern-Volmer constant were specified and await their experimental verification.

Resonances in Reaction Dynamics

Resonances--sharp changes in behavior when particles interact--in chemical reactions can reveal the vibration and rotation of reactants and products. This approach has been applied to the dissociation of formaldehyde and the reaction of fluorine with hydrogen.

Exact quantum dynamics of N(D2)+H2→NH+H reaction: Cross-sections, rate constants, and dependence on reactant rotation

Using an exact Chebyshev wave packet method, initial state-specified (upsilon(i)=0, j(i)=0,2) integral cross-sections and rate constants are obtained for the title reaction on the latest ab initio potential energy surface. Reaction probabilities up to J=29 are dependent on the reactant rotation and show mild oscillations superimposed on a broad background. Due to a barrier in the entrance channel, the cross sections increase with energy with clear thresholds and the rate constants vary with temperature in the Arrhenius form. The calculated canonical rate constant is in good agreement with the experimental measurements. Our results also indicate that the quasiclassical trajectory method underestimates the rate due to the neglect of tunneling, while the quantum statistical approach overestimates because of the short lifetime of the reaction intermediate.

How Reactants Polarization Can Be Used to Change and Unravel Chemical Reactivity

This article presents theoretical methods for the description of the directional effect of reactant rotation on the reactivity of atom-diatom systems and suggests an experiment that could be used to test theoretical predictions. The theory can be used in conjunction with both quantum reactive scattering and quasiclassical trajectory calculations, and is stated in general terms, which allows it to deal with arbitrary reactant polarizations. The illustrative results obtained for the benchmark H + D2 reaction are also presented and show that under experimentally achievable conditions one can largely control reactive cross sections and product state distributions, while at the same time gaining valuable and at times surprising information on the reaction mechanism.

Reduced-dimensionality calculation of reaction cross sections and rate constant for the complex-forming gas-phase SN2 reaction Cl−+ CH3Cl′ → ClCH3+ Cl′−

Employing a 4D CCSD(T) potential energy surface, initial-state selected reaction cross sections for the complex-forming gas-phase identity S(N)2 reaction Cl(-) + CH3Cl' (v1, v2, v3) --> ClCH3 (v'1, v'2, v'3) + Cl'- havebeen calculated by means of time-independent quantum scattering theory in hyperspherical coordinates. The totally symmetric internal modes of the methyl group (C-H stretching vibration, quantum numbers v1 and v'1, and umbrella bending vibration, v2 and v'2) and the two C-Cl stretching modes (v3 and v'3) are included. The results for pure C-Cl stretching excitation in the reactants are similar to those obtained in earlier 2D calculations. The cooperative effect of C-Cl stretching and umbrella bending modes is even more pronounced for cross sections than for reaction probabilities. The same holds for excitations of the pure internal CH3 modes; in particular, the ratio of cross sections for reaction with the C-H stretch excited to reaction out of the vibrational ground state is five orders of magnitude larger than the ratio of the corresponding probabilities. This questions the concept of "spectator" modes in reaction dynamics which is valid only for thermal rate constants where the "spectator" modes play a negligible role due to their low population. Transition state theory rate constants fortuitously show good agreement with experiment while the reduced-dimensionality quantum calculations show larger deviations. Possible sources of this discrepancy are discussed in detail. Neglect of reactant CH3Cl rotation and the related modes in the transition state (doubly degenerate Cl-...CH3...Cl' bend and K rotation) yields very good agreement with experiment.

Quantum wave packet study of reactive and inelastic scattering between C(1D) and H2

Using a wave packet method, state-to-state inelastic transition probabilities and initial state specified total reaction probabilities are calculated for the title system (J=0) on a recent ab initio potential energy surface. Both the inelastic and reactive scattering probabilities are found to be strongly oscillatory, indicative of the involvement of long-lived resonances that are supported by a deep CH2 well. The oscillation becomes less pronounced at higher collision energies and with internal excitation of the reactant molecule. The reaction from the (νi=0, ji=0) initial state is clearly dominated by the insertion pathway, and this dominance is largely unaffected by the excitation of the reactant rotation or vibration. In addition, low-lying vibrational states of CH2 have been determined and compared with spectroscopic data.

A wave-packet calculation of the effect of reactant rotation and alignment on product branching in the O(1D)+HCl→ClO+H, OH+Cl reactions

We report results of wave-packet calculations of the reaction probabilities for the O(1D)+HCl(v=0,j,K)→ClO+H, OH+Cl, reactions, using a recent ab initio potential energy surface [K. A. Peterson, S. Skokov, and J. M. Bowman, J. Chem. Phys. 111, 2445 (1999)]. We find a striking effect of the initial rotation and alignment of HCl on the product branching ratio.

Theoretical study of the effect of reagent rotation on the reaction of O + H2 (v, J)

Abstract Quasiclassical calculations have been carried out near threshold for the reaction of O + H2(v = 0, 1, 2; J) → OH + H for J = 0 to 10 gain an understanding of the effect of reactant rotation and vibration on this reaction. The calculations were done on the Johnson-Winter potential energy surface (PES); this surface has a barrier to reaction which is lowest in the linear configuration (Ξ = 180°). For values of r, the HH bond length, less than 1.0 A, the PES is fairly prolate in nature. (In the classification of Levine, a surface is prolate if for a given distance RCM from O to the center of mass of H2, the potential is higher for the collinear configuration than for the perpendicular configuration (Ξ = 90°). The surface is oblate if the reverse is true.) For the reaction of O + H2(v = 0) the reactive cross section QR increases monotonically with J. Our calculations demonstrate that the prolate PES “disorients” the OHH angle away from linear, and reduces QR substantially for J = 0. An increase in J reduces the effect of this disorientation and so increases QR. The reaction with O is much different for H2(v = 1,2). Now the average value of r is long enough that the PES is oblate during the last part of the reaction. Near threshold for both vibrational levels QR rises from J = 0 to 2, then declines to a minimum at J = 5, and finally rises monotonically to J = 10. The decline has been seen for other reactions which take place on oblate surfaces such as F + H2. The explanation of the decline is that for a rotating target the O atom approaches the reactant valley obliquely, and many trajectories bounce off the repulsive potential wall near Ξ = 90° rather than react. However, for J values above 5 the H2 molecule has enough rotational energy to rotate through the barrier at Ξ = 90° rather than bounce off it, so the collision continues on to reaction. This causes QR to rise. The results obtained here are compared with the quantal calculations by Chatfield et al. on this system. A number of interesting similarities are observed.

Vibrational and rotational effects in the Cl+HOD↔HCl+OD reaction

Quantum scattering calculations on the Cl+HODR⇌Cl+OD reaction have been performed at collisional energies up to 1.6 eV. The rotating bond approximation is used. In this method, the OD rotation and HCl vibration as well as the bending motion and OH local stretch of HOD are treated explicitly. Here, the theory is extended to account for thermal HOD reactant rotation. The potential energy surface used has accurate reactant and product rovibrational energy levels, correct bond dissociation energies, and a transition state geometry in accord with ab initio data. Mode selectivity is observed—HOD vibrational stretch energy enhances reaction more than vibrational bend energy. Translational energy enhances reaction more than vibrational stretch energy at low total energies, but not generally at higher total energies. Excitation of the local OH stretch in the reactant HOD produces vibrationally excited HCl product. The OD product rotation depends on the reactant HOD rovibrational state. The OD+HCl(v=0) reaction preferentially produces HOD in the vibrational ground state, while the OD+HCl(v=1) reaction preferentially produces HOD with one quantum of vibrational stretch energy. A calculated OH product rotational distribution for the Cl+H2O reaction agrees quite well with experiment.

Mechanical, Thermal and Flow Dynamics Issues in Compound Semiconductor MOCVD Reactor Design

ABSTRACTIn recent years Metalorganic Chemical Vapor Deposition (MOCVD) becomes a key epitaxial process for a variety of compound semiconductor devices such as: GaAs/AlGaAs lasers, HEMTs, LEDs, photocathodes, solar cells, and MESFETs; InP/InGaAsP long wavelength lasers and detectors; InP/InGaAs quantum wells and detectors, etc. Development of reliable, high throughput equipment is a major task in the implementation of MOCVD into cost-effective manufacturings. We have used Computational Fluid Dynamics (CFD) and Finite Element Analysis (FEA) software to model thermal, structural, and flow processes for the scaling of EMCORE vertical, high speed rotating disk reactor (RDR) to large dimensions (four 4″ wafers located on 12″ wafer carrier). Flow modeling was used to determine basic reactor geometry and the relation between process parameters such as total reactant flow, temperature, pressure, and rotation speed. Thermal and structural analysis was used to produce a uniform substrate temperature, avoid reactor overheating and decrease thermal stress. Flow and temperature distribution predicted by the modeling were found to be well correlated with experimental results.

Quantum theory of planar four-atom reactions

An exact quantum mechanical theory is developed to treat four-atom reactions of the type AB+CD↔(BCD+A, ACD+B), where the atoms are constrained to move in a plane. The theory makes use of an unbiased set of hyperspherical coordinates. A method is proposed for implementing the theory that exploits the potential optimized discrete variable representation. Application is made to the calculation of rovibrational state-to-state reaction probabilities for the reaction H2+OH↔H2O+H, in which the length of the OH spectator bond is held fixed. The results show that a rotating bond approximation, in which the H2 molecule is not allowed to rotate, gives good results for vibrationally selected reaction probabilities. The effect of reactant rotation and vibration on the reactivity and product distributions is discussed for the reactions H2+OH→H2O+H and H2O+H→H2+OH.

The role of reactant rotation and rotational alignment in the dissociative chemisorption of hydrogen in Ni(100)

We have carried out classical trajectory calculations on the dissociative chemisorption of H 2(0, j) on Ni(100). Above threshold, the results are found to be in good agreement with a recent mixed quantum-classical treatment. We find that rotational effects in direct chemisorption reactions can be explained by the same concepts which have been employed in gas phase reactions.

Quasiclassical trajectory study of the effect of reactant rotation on the reaction B + + HD → BH + (BD +) + D(H)

Quasiclassical trajectories were calculated for the lowest triplet potential energy surface of the title reaction at different rotational states of the HD molecule and at two values of the relative translational energy of reactants. The opacity function, reaction cross sections, rotational and vibrational excitation of products, intramolecular isotopic effect, symmetry of backward-forward scattering and lifetime of the BHD+ collision complex are discussed in connection with the initial conditions defined. Results are compared with similar systems taken from the literature.

Effect of reactant rotation on hydrogen atom transfer

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTEffect of reactant rotation on hydrogen atom transferHoward R. MayneCite this: J. Am. Chem. Soc. 1990, 112, 22, 8165–8166Publication Date (Print):October 1, 1990Publication History Published online1 May 2002Published inissue 1 October 1990https://doi.org/10.1021/ja00178a050RIGHTS & PERMISSIONSArticle Views55Altmetric-Citations12LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (297 KB) Get e-Alerts Get e-Alerts

A rotationally resolved LIF study of the N+2 products of the thermal energy Penning ionization reaction: Ne*(3P2)+N2

Multiple temperature rotational distributions are observed in the N+2 products of the Penning ionization of N2 by Ne*(3P2). The rotational distributions for N=2–21 are well fit by a two-component Boltzmann distribution having a low temperature of 50 K±10 K and a high temperature ranging from 220 to 490 K±50 K. The relative contribution of each component varies smoothly with vibrational level from 62% Tlow:38% Thigh for v=0 to 43% Tlow:57% Thigh for v=4(±5%). We interpret the observed multiple rotational distributions as evidence for exit channel effects arising from details of the impulsive interaction in the electron transfer process. Since the overall degree of rotational excitation is small, observation of the multiple distributions requires suppression of the reactant initial rotation by using a supersonic molecular beam.

Effect of reactant rotation on reactivity: comparison of exact coplanar results and a model calculation for atomic hydrogen + molecular hydrogen

A simple classical model was proposed to study the effect of reactant rotation on reactivity for atom-diatom collisions. The model was able to reproduce trends recently reported in the reaction cross section for H + H/sub 2/ and other systems. Stretching of the target diatom bond during the collision was found to enhance the effectiveness of rotation in promoting reaction. This suggested that rotation may promote reaction more readily on surfaces with late barriers. The model results were found to be particularly sensitive to the long-range anisotropy of the potential, implying that rotational excitation of reactants may be a means of probing that part of the potential.

Effect of reactant rotation on reactivity: a comparison of classical and quantum effects in a model system

The effect of reactant rotation on reactivity has been investigated using a model system. Classical and exact quantum results are in excellent agreement above threshold. Trends are found which mirror those seen in recent trajectory studies on H + H 2.