Study Of Reaction Dynamics Research Articles

Quantum Chemical Molecular Dynamics Study of Chemical Reaction Dynamics on Ni-base Alloy Surfaces in Gas-cooled Reactors

Quantum Chemical Molecular Dynamics Study of Chemical Reaction Dynamics on Ni-base Alloy Surfaces in Gas-cooled Reactors

Structural role of polyethylene glycol in the formation of anatase nanocrystalline titania at low temperature

AbstractAnatase nanocrystalline TiO2 thin films were obtained by a sol–gel dip‐coating method, in which the nanocrystallization is effected by a simple hot water treatment of the deposited films at temperatures below 90 °C under atmospheric pressure for 1 h. The dip‐coating sol was prepared by reacting titanium tetra‐n‐butoxide [Ti(OnBu)4] with polyethylene glycol (PEG) in ethanol. Films obtained from a sol that do not contain PEG show no sign of crystallization, demonstrating the importance of PEG in the crystallization process. Raman studies of reaction dynamics show that PEG undergoes a nucleophilic substitution reaction replacing butoxy groups in Ti(OnBu)4. Stoichiometric reactions of Ti(OnBu)4 with PEG in polar and nonpolar solvents were performed, and they yielded different titanium–PEG hybrid polymers, which were isolated and characterized by various spectroscopic techniques such as IR, Raman, solid‐state NMR and MALDI‐TOF‐MS. NMR studies evidenced the location and the way in which PEG is bonded with titanium atoms in the titanium–PEG hybrid polymers. On the basis of these studies, we have proposed structures for these polymers. It is demonstrated that the structure of the obtained polymers plays an important role in the formation of anatase TiO2 nanoparticles in hot water at temperatures below 90 °C under atmospheric pressure. Copyright © 2009 John Wiley & Sons, Ltd.

Application of stimulated Raman pumping toward the first study of chemical reaction dynamics of the muonium atom with H2*

AbstractThe chemical reaction of interest, from the field of μSR spectroscopy, is that of the muonium atom (Mu) with molecular hydrogen in its first vibrational excited state. The stimulated Raman pumping (SRP) technique has been used to prepare H2 in its ν = 1 state in a series of off‐line experiments at the LASIR Lab at UBC. Three different SRP excitation schemes employing the second harmonic of a Nd:YAG laser have been analysed in connection with this application, for 5.5, 13, and 26 cm‐long reaction cells. The laser energy varied from ∼80 to ∼550 mJ/pulse (or from ∼0.07 GW/cm2 to ∼0.46 GW/cm2) at a 10 Hz repetition rate. The importance of injection seeding of the Nd:YAG laser for efficient molecular excitation is shown. The major challenge of this experiment was to ensure a homogeneous excitation over a large volume (few cm3) while achieving a high amount of vibrationally excited H2 (3‐15 Torr). This is demanded by the requirements of superposition of a pulsed muon beam with the laser beam and the expected rate constant of the Mu + H2*(ν=1) →MuH + H reaction. In this endeavour it was also of interest to explore the effect of added moderator gas to the hydrogen and so the effect of Ar/Xe moderator on the efficiency of hydrogen pumping for different SRP excitation schemes is also shown. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cross sections for single electron capture in collisions of N2+ with He at low energies

The absolute cross sections of single electron capture during the collision of N2+ with He have been measured over the energy range 0.1-300 eV/u. From the plot showing the dependence of cross sections on energy, the following three remarkable observations can be made: (1) the cross sections exhibit an oscillatory structure below 1eV/u, (2) a bend in the cross sections appears at approximately 1 eV/u and (3) there is a dip in the cross sections at approximately 70 eV/u. The formation of an oscillatory structure is attributed to the glory effect. The bend in the cross sections occurs owing to the attractive potential part of a l2n state of quasimolecule NHe2+. The dip in the cross sections appears as a result of opening a new reaction path above 70eV/u. The glory effect can be observed for the first time on a total cross section even for slow ion-neutral collisions. The objective of this study is to explore the possibilities of providing a detailed explanation of energy dependence measurements in order to study reaction dynamics for a charge changing process.

Impulsive orientation and alignment of quantum-state-selected NO molecules

A technique that produces significant alignment of molecules in a beam should aid a wide range of experiments geared towards understanding and controlling molecular processes in the gas phase. Manipulation of the molecular-axis distribution is an important ingredient in experiments aimed at understanding and controlling molecular processes1,2,3,4,5,6. Samples of aligned or oriented molecules can be obtained following the interaction with an intense laser field7,8,9, enabling experiments in the molecular rather than the laboratory frame10,11,12. However, the degree of impulsive molecular orientation and alignment that can be achieved using a single laser field is limited13 and crucially depends on the initial states, which are thermally populated. Here we report the successful demonstration of a new technique for laser-field-free orientation and alignment of molecules that combines an electrostatic field, non-resonant femtosecond laser excitation14 and the preparation of state-selected molecules using a hexapole2. As a unique quantum-mechanical wavepacket is formed, a large degree of orientation and alignment is observed both during and after the femtosecond laser pulse, which is even further increased (to 〈cosθ〉=−0.74 and 〈cos2θ〉=0.82, respectively) by tailoring the shape of the femtosecond laser pulse. This work should enable new applications such as the study of reaction dynamics or collision experiments in the molecular frame, and orbital tomography11 of heteronuclear molecules.

Toward the first study of chemical reaction dynamics of Mu with vibrational-state-selected reactants in the gas phase: The reaction by stimulated Raman pumping

Abstract Stimulated Raman pumping (SRP) is used to produce H 2 in its first vibrational state, in order to measure, for the first time, the Mu + H 2 * ( v = 1 ) → MuH + H reaction rate at room temperature, as a prototypical example of new directions in gas-phase muonium chemistry, utilizing the pulsed muon beam and a new dedicated laser system at the RIKEN/RAL Laboratory. Reported here is a preliminary result but the final results are expected to provide definitive new tests of reaction rate theory on the highly accurate H 3 potential energy surface. The major difficulty in this experiment, compared to the standard SRP process, is to ensure a homogeneous excitation over a volume of several cm 3 and of sufficient intensity to ensure a measurable Mu relaxation rate. The techniques used to accomplish this are described. The experiment utilizes the 2nd harmonic output of a Nd:YAG laser (532 nm) with pulse energies up to 500 mJ at a repetition rate of 25 Hz. Different optical setups have been constructed and tested in order to optimize the number of laser-pumped H 2 molecules and their overlap with the stopping profile of the muon beam in the reaction cell (total volume ∼ 100 × 40 × 4 mm 3 ). The first result of this experiment gives a measured relaxation rate due to laser excitation of λ * = 0.085 ± 0.051 μ s - 1 , consistent with theory but limited by both low statistics and particularly a high background relaxation rate.

Infrared Spectra of Metallacyclopropane, Insertion, and Dihydrido Complex Products in Reactions of Laser-Ablated Group 6 Metal Atoms with Ethylene Molecules

Reactions of laser-ablated group 6 metal atoms with ethylene have been investigated. The insertion and dihydrido products (MH-CHCH(2) and MH(2)-C(2)H(2)) are identified from reactions of W and Mo with ethylene isotopomers, whereas products in the Cr spectra are assigned to the insertion and metallacyclopropane (M-C(2)H(4)) complexes. Our experiments with CH(2)CD(2) show that the dihydrido complex is formed by beta-hydrogen transfer in the insertion complex because the MHD-CHCD isotopic product is favored. The present matrix infrared spectra and DFT computational results support the general trend that the higher oxidation-state complexes become more stable on going down the group 6 column. Unlike the cases of group 4 and 5 metals, binary metal hydride (MH(x)) absorptions are not observed in the infrared spectra, suggesting that the H(2)-elimination reactions of ethylene by group 6 metals are relatively slow, consistent with previous gas-phase reaction dynamics studies.

Formation of stable sessile interstitial complexes in reactions between glissile dislocation loops in bcc Fe

Clusters of self-interstitial atoms (loops) are commonly observed in the microstructure of irradiated metals. These clusters can be formed directly in high-energy displacement cascades or by growth as a result of interaction between individual self interstitials. The majority of these clusters have features of glissile dislocation loops and migrate by fast one-dimensional glide. In this paper, we present results of a systematic molecular dynamics (MD) study of reactions involving glissile interstitial loops. By the example of bcc iron we demonstrate that the reactions can produce a number of specific, stable microstructural features, with different properties compared to the reactants. Namely, the reactions between the most common glissile clusters of 〈1 1 1〉 crowdions can result in coarsening or formation of immobile self interstitial complexes. The coarsening leads to a decrease of the total dislocation line length and therefore is favourable. The structure and stability of the junction formed in the reactions has been studied using many-body potentials and density functional theory (DFT) techniques. No evidence of the formation of a 〈1 0 0〉 loop from two glissile 〈1 1 1〉 clusters was found among the studied reactions. The immobile self interstitial complexes that form as a result of these reaction have, however, high binding energies, of the order of tens of eV, implying that a relatively long life time should be assigned to the resulting configurations and therefore that such objects are expected to contribute to the evolution of the microstructure under irradiation.

Chemical dynamics of vibrationally excited molecules: Controlling reactions in gases and on surfaces.

Experimental studies of the chemical reaction dynamics of vibrationally excited molecules reveal the ability of different vibrations to control the course of a reaction. This Perspective describes those studies for the prototypical reaction of vibrationally excited methane and its isotopologues in gases and on surfaces and looks to the prospects of similar studies in liquids. The influences of vibrational excitation on the C-H bond cleavage in a single collision reaction with Cl and in dissociative adsorption on a Ni surface bear some striking similarities. Both reactions are bond-selective processes in which the initial preparation of a molecular eigenstate containing a large component of C-H stretching results in preferential cleavage of that bond. It is possible to cleave either the C-H bond or C-D bond in the reaction of Cl with CH3D, CH2D2, or CHD3 and, similarly, to use initial excitation of the C-H stretch to promote dissociation of CHD3 to CD3 and H on a Ni surface. Different vibrational modes, such as the symmetric and antisymmetric stretches in CH3D or CH4, lead to very different reactivities, and molecules with the symmetric stretching vibration excited can be as much as 10 times more reactive than ones with the antisymmetric stretch excited. The origin of this behavior lies in the change in the vibrational motion induced by the interaction with the atomic reaction partner or the surface.

A compact pulse shape discriminator module for large neutron detector arrays

A cost-effective high-performance pulse shape discriminator module has been developed to process signals from organic liquid scintillator-based neutron detectors. This module is especially designed for the large neutron detector array used for studies of nuclear reaction dynamics at the Inter University Accelerator Center (IUAC). It incorporates all the necessary pulse processing circuits required for neutron spectroscopy in a novel fashion by adopting the zero crossover technique for neutron-gamma (n- γ) pulse shape discrimination. The detailed layout of the circuit and different features of the module are described in the present paper. The quality of n- γ separation obtained with this electronics is much better than that of commercial modules especially in the low-energy region. The results obtained with our module are compared with similar setups available in other laboratories.

Development of a new molecular dynamics method for tribochemical reaction and its application to formation dynamics of MoS 2 tribofilm

Recently we have developed a novel molecular dynamics program NEW-RYUDO-CR, which can deal with chemical reactions. The developed method has been applied to the study of tribochemical reaction dynamics of MoS 2 tribofilm on iron surface. The initially amorphous MoS 2 layer self-organized its structure as result of the tribochemical reactions and formed layered MoS 2 tribofilm. The friction coefficient significantly decreased as the MoS 2 tribofilm was formed. Besides, sliding was observed between sulfur layers of MoS 2 tribofilms which occurred due to repulsive Coulombic interaction forces between sulfur atoms. This indicates that the formation of the layered MoS 2 tribofilm is important to achieve better lubrication properties.

First principles molecular dynamics study of catalytic reactions of biological macromolecularsystems: toward analyses with QM/MM hybrid molecular simulations

First principles molecular dynamics simulations performed on a fully solvated RNA modelstructure allowed us to investigate the mechanism for enzymatic cleavage reactions, invitro, of RNA enzymes (ribozymes). The concerted action of two metal catalysts turns outto be the most efficient way to promote, on the one hand, the proton abstraction from2′-OH that triggers the nucleophilic attack and, on the other hand, the cleavage of theP–O5′ bond. In fact, the elimination of one of the two metal cations leads to an increasein the activation energy of the reaction. The simulated pathway shows that anOH− in the coordinationshell of the Mg2+ close to O2′ promotes the initial proton abstraction and prevents its transfer to the ribozyme. Thissuggests that, in a real ribozyme, the double-metal-ion reaction mechanism is preferredwith respect to single-metal-ion mechanisms either in the presence or in absence of theOH− anion. Finally, an insight into the importance of hybrid quantum mechanics/molecularmechanics (QM/MM) schemes is discussed in view of the modelling of a realistic systemcarrying all the features of a true ribozyme.

On the mechanism of weak-field coherent control of retinal isomerization in bacteriorhodopsin

Experimental studies of the short time reaction dynamics controlling the chemical branching ratio provide direct evidence for the mechanism of coherent control of the retinal photoisomerization in bacteriorhodopsin in the weak-field limit with respect to the previous report [V. Prokhorenko, A. Nagy, S. Waschuk, L. Brown, R. Birge, R. Miller, Science 313 (2006) 1257]. The phase sensitivity of the reaction dynamics is directly revealed using time- and frequency-resolved pump–probe measurements. The high degree of control of the reaction branching ratio is theoretically explained through a combination of spectral amplitude shaping and phase-dependent coupling to selectively excite vibrations most strongly coupled to the reaction coordinate. Coherent control in this context must involve reaction dynamics that occur on time scales comparable to electronic and vibrational decoherence time scales.

Aqueous bimolecular proton transfer in acid–base neutralization

We report on a comparative femtosecond infrared study of the bimolecular proton transfer reaction dynamics of the photoacid pyranine (8-hydroxy-1,3,6-trisulfonate-pyrene, abbreviated as HPTS) with the family of carboxylate bases −OOCCH (3− x) Cl x ( x = 0–3) in aqueous solution. We use a generalized reaction model consisting of n proton transfer reaction pathways between photoacid and base, each pathway characterized by the number n of water molecules functioning as solvent switch facilitating an efficient transfer. The dynamics of the different proton transfer pathways have been followed by inspection of appropriate vibrational modes marking the progress of the reaction, and allow the direct observation of the transient relay mechanism of proton through one bridging water molecule in a loose photoacid–base configuration of the type HPTS −⋯H 3O +⋯ −OOCCH (3− x) Cl x ( x = 0–3). By applying a global fit procedure on the n-coupled reaction pathways, with full reversibility and detailed balance, we find for the reaction series that the encounter complex formed after mutual diffusion between photoacid and base, and subsequent desolvation, follows in general the loose complex pathway with one water molecule bridging acid and base. Only for the weakest base studied, trichloroacetate, larger solvent switches with n ⩾ 2 water molecules are important in the transfer dynamics. Whereas transfer in the tight complex with no water spacer ( n = 0) occurs within time resolution of 150 fs, the transfer in the loose ( n = 1) complex proceeds in a sequential fashion where the first transfer occurs within time resolution, irrespective of base strength, whereas the second transfer from the water to the base is activated and conforms to a correlation between free energy and reaction rate.

Matrix Infrared Spectroscopic Studies of the MH−C2H3 and MH2−C2H2 Intermediates in the Reactions of Ethylene with Laser-Ablated Group 5 Metal Atoms

Reactions of ethylene with laser-ablated group 5 metal atoms in excess argon have been carried out during codeposition at 8 K, and the matrix infrared spectra of intermediate products have been investigated. Oxidative C-H insertion of the transition metal into a C-H bond occurs and beta-hydrogen transfer follows to form the dihydrido complexes (MH2-C2H2). In the Ta spectra, the dihydrido complex is the primary product, whereas the Nb and V spectra reveal absorptions from both the insertion (MH-C2H3) and dihydrido complexes. The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies. Calculations also show that the higher oxidation-state complex becomes more stable relative to the insertion product going down the group 5 family.

Femtosecond Stimulated Raman Spectroscopy

Femtosecond stimulated Raman spectroscopy (FSRS) is a new ultrafast spectroscopic technique that provides vibrational structural information with high temporal (50-fs) and spectral (10-cm(1)) resolution. As a result of these unique capabilities, FSRS studies of chemical and biochemical reaction dynamics are expected to grow rapidly, giving previously unattainable insight into the structural dynamics of reactively evolving systems with atomic spatial and femtosecond temporal resolution. This review discusses the experimental and theoretical concepts behind FSRS, with an emphasis on the origins of its unique temporal and spectral capabilities. We illustrate these capabilities with vibrational studies of ultrafast electronic dynamics, as well as the direct structural observation of nonstationary vibrational wave-packet motion in small molecules and in complex biochemical reaction dynamics.

Internal-energy measurements of angle-resolved product CO2 in catalytic CO oxidation by means of infrared chemiluminescence

Measurements of both vibrational and rotational energies of product CO(2) in CO oxidation on palladium surfaces have been successfully performed as a function of the desorption angle by means of infrared chemiluminescence. The remarkable angle dependences of both energies indicate facile energy partitioning in repulsive desorption and provide new dimensions in the study of surface reaction dynamics as well as additional insights into the product formation site. Details of the apparatus for energy analysis of angle-resolved products are described, especially on how to pick up extremely weak infrared emission signals.

Modeling ionic unimolecular dissociations from a temperature controlled TPEPICO study on 1-C 4H 9I ions

We report on the development of a new temperature controlled inlet system for the study of gas phase unimolecular reaction dynamics using threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. Temperatures in the range of 220–400 K can be achieved, with a deviation of less than 5 K over the course of a 48 h experiment. Iodine loss from energy selected 1-butyl iodide (1-C 4H 9I) ions was studied at four temperatures, 220, 275, 298 and 400 K. The fractional ion abundances, in the form of breakdown diagrams are presented. Particular attention is paid to the slopes of the molecular and daughter ion abundances in crossover region of the breakdown curve since they are governed solely by the internal energy distribution, P( E), of the neutral precursor. P( E) is a function of both the temperature and vibrational frequencies of the neutral precursor. A detailed discussion regarding the transposition of the neutral thermal energy distribution to the ionic manifold is presented. From the four experimental measurements, the E 0 for the production of the 2-C 4H 9 + ion was determined to be 9.738 ± 0.015 eV.

The study of state-selected ion-molecule reactions using the vacuum ultraviolet pulsed field ionization-photoion technique

This paper presents the methodology to generate beams of ions in single quantum states for bimolecular ion-molecule reaction dynamics studies using pulsed field ionization (PFI) of atoms or molecules in high-n Rydberg states produced by vacuum ultraviolet (VUV) synchrotron or laser photoexcitation. Employing the pseudocontinuum high-resolution VUV synchrotron radiation at the Advanced Light Source as the photoionization source, PFI photoions (PFI-PIs) in selected rovibrational states have been generated for ion-molecule reaction studies using a fast-ion gate to pass the PFI-PIs at a fixed delay with respect to the detection of the PFI photoelectrons (PFI-PEs). The fast ion gate provided by a novel interleaved comb wire gate lens is the key for achieving the optimal signal-to-noise ratio in state-selected ion-molecule collision studies using the VUV synchrotron based PFI-PE secondary ion coincidence (PFI-PESICO) method. The most recent development of the VUV laser PFI-PI scheme for state-selected ion-molecule collision studies is also described. Absolute integral cross sections for state-selected H2+ ions ranging from v+ = 0 to 17 in collisions with Ar, Ne, and He at controlled translational energies have been obtained by employing the VUV synchrotron based PFI-PESICO scheme. The comparison between PFI-PESICO cross sections for the H2+(HD+)+Ne and H2+(HD+)+He proton-transfer reactions and theoretical cross sections based on quasiclassical trajectory (QCT) calculations and three-dimensional quantum scattering calculations performed on the most recently available ab initio potential energy surfaces is highlighted. In both reaction systems, quantum scattering resonances enhance the integral cross sections significantly above QCT predictions at low translational and vibrational energies. At higher energies, the agreement between experiment and quasiclassical theory is very good. The profile and magnitude of the kinetic energy dependence of the absolute integral cross sections for the H2+(v+ = 0-2,N+ = 1)+He proton-transfer reaction unambiguously show that the inclusion of Coriolis coupling is important in quantum dynamics scattering calculations of ion-molecule collisions.

Radical–radical reaction dynamics: A combined crossed-beam and theoretical study

We present an overview of our recent studies of the gas-phase reaction dynamics of O(3P) with a series of hydrocarbon radicals [allyl (C3H5), propargyl (C3H3), t-butyl (t-C4H9)] as prototypal radical–radical oxidation reactions. High-resolution laser spectroscopy in a crossed-beam configuration was applied to examine the nascent rovibrational state distributions and Doppler profiles of the reactive scattering products. The analyses of the product energy and population distributions demonstrated the existence of unusual dynamic characteristics and competition between the addition and abstraction reaction mechanisms at the molecular level. These features, which are in sharp contrast with those of the oxidation reactions of closed-shell hydrocarbon molecules, are discussed with the aid of ab initio and quantum statistical calculations. Contents PAGE 1. Introduction 614 2. Experiment 615 2.1. Characterization of the molecular beam in the supersonic flash pyrolysis source 616 2.2. Nascent product state distributions from reactive scattering processes 617 3. Ab initio calculations 619 4. Experimental and statistical analysis 620 4.1. Nascent internal state distributions of OH products 620 4.2. Translational energy release 621 4.3. Statistical analysis 621 5. Radical–radical reaction dynamics 623 5.1. Characteristics of the molecular beam in the supersonic flash pyrolysis source 623 5.2. Reaction dynamics of O(3P) with C3H5 626 5.2.1. Ab initio potential energy surface: addition vs. abstraction 626 5.2.2. O(3P) + C3H5 → C3H4 + OH 630 5.2.3. O(3P) + C3H5 → C3H4O + H 635 5.3. Reaction dynamics of O(3P) with C3H3 637 5.3.1. Ab initio potential energy surface 637 5.3.2. O(3P) + C3H3 → C3H2 + OH 640 5.3.3. O(3P) + C3H3 → C3H2O + H 643 5.4. Reaction dynamics of O(3P) with t-C4H9 644 5.4.1. Ab initio potential energy surface 644 5.4.2. O(3P) + t-C4H9 → iso-C4H8 + OH 646 6. Summary 646 Acknowledgments 648 References 649 Supplementary Materials 651