Reactive Resonance Research Articles

New Analytical Potential Energy Surface for the F(2P) + CH4 Hydrogen Abstraction Reaction: Kinetics and Dynamics

A new potential energy surface for the gas-phase F(2P)+CH4 reaction and its deuterated analogues is reported, and its kinetics and dynamics are studied exhaustively. This semiempirical surface is completely symmetric with respect to the permutation of the four methane hydrogen atoms, and it is calibrated to reproduce the topology of the reaction and the experimental thermal rate constants. For the kinetics, the thermal rate constants were calculated using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 180-500 K. The theoretical results reproduce the experimental variation with temperature. The influence of the tunneling factor is negligible, due to the flattening of the surface in the entrance valley, and we found a direct dependence on temperature, and therefore positive and small activation energies, in agreement with experiment. Two sets of kinetic isotope effects were calculated, and they show good agreement with the sparse experimental data. The coupling between the reaction coordinate and the vibrational modes shows qualitatively that the FH stretching and the CH3 umbrella bending modes in the products appear vibrationally excited. The dynamics study was performed using quasi-classical trajectory calculations, including corrections to avoid zero-point energy leakage along the trajectories. First, we found that the FH(nu',j') rovibrational distributions agree with experiment. Second, the excitation function presents an oscillatory pattern, reminiscent of a reactive resonance. Third, the state specific scattering distributions present reasonable agreement with experiment, and as the FH(nu') vibrational state increases the scattering angle becomes more forward. These kinetics and dynamics results seem to indicate that a single, adiabatic potential energy surface is adequate to describe this reaction, and the reasonable agreement with experiment (always qualitative and sometimes quantitative) lends confidence to the new surface.

Spectroscopic and Computational Studies of Co1+Cobalamin: Spectral and Electronic Properties of the “Superreduced” B12 Cofactor

The 4-coordinate, low-spin cob(I)alamin (Co1+Cbl) species, which can be obtained by heterolytic cleavage of the Co-C bond in methylcobalamin or the two-electron reduction of vitamin B12, is one of the most powerful nucleophiles known to date. The supernucleophilicity of Co1+Cbl has been harnessed by a number of cobalamin-dependent enzymes, such as the B12-dependent methionine synthase, and by enzymes involved in the biosynthesis of B12, including the human adenosyltransferase. The nontoxic nature of the Co1+Cbl supernucleophile also makes it an attractive target for the in situ bioremediation of halogenated waste. To gain insight into the geometric, electronic, and vibrational properties of this highly reactive species, electronic absorption, circular dichroism (CD), magnetic CD, and resonance Raman (rR) spectroscopies have been employed in conjunction with density functional theory (DFT), time-dependent DFT, and combined quantum mechanics/molecular mechanics computations. Collectively, our results indicate that the supernucleophilicity of Co1+Cbl can be attributed to the large destabilization of the Co 3dz2-based HOMO and its favorable orientation with respect to the corrin macrocycle, which minimizes steric repulsion during nucleophilic attack. An intense feature in the CD spectrum and a prominent peak in the rR spectra of Co1+Cbl have been identified that may serve as excellent probes of the nucleophilic character, and thus the reactivity, of Co1+Cbl in altered environments, including enzyme active sites. The implications of our results with respect to the enzymatic formation and reactivity of Co1+Cbl are discussed, and spectroscopic trends along the series from Co3+Cbls to Co2+Cbl and Co1+Cbl are explored.

Reactive resonance and formation mechanism of the F+CH 4→HF+CH 3 reaction

The partial potential energy surface is constructed using ab initio method [QCISD (T)/6-311++G (2df, 2dp)] for F+CH 4→HF+CH 3 reaction. It not only explains the reaction mechanism brought forward by D. Troya, but also successfully validates K.P. Liu's experimental phenomena about the existence of a reactive resonance. Finally, we estimate the lifetime of the scattering resonance state, which is about 0.07 ps. All these are in agreement with the experiments.

Imaging a reactive resonance in the Cl+CH4 reaction

The title reaction has been under extensive experimental and theoretical investigations. Presented here is the experimental evidence suggesting a reactive resonance in this reaction-an intriguing possibility that has been hitherto unsuspected. The initial speculation was inferred from theoretical works in the literature, and subsequent confirmation came from the observed pattern of the angular distributions of the HCl(nu'=1)+CH(3)(upsilon=0) product pair, when plotted in the angle-collision energy plane. This characteristic pattern proves particularly incisive and universal in revealing the imprint of reactive resonance in experimental observable. The nature of the proposed resonance in this reaction is also elucidated.

An improved potential energy surface for the F+H2 reaction

A new ground state potential energy surface has been developed for the F+H2 reaction. Using the UCCSD(T) method, ab initio calculations were performed for 786 geometries located mainly in the exit channel of the reaction. The new data was used to correct exit channel errors that have become apparent in the potential energy surface of Stark and Werner [J. Chem. Phys. 104 (1996) 6515]. While the entrance channel and saddlepoint properties of the Stark–Werner surface are unchanged on the new potential, the exit channel behavior is more satisfactory. The exothermicity on the new surface is much closer to the experimental value. The new surface also greatly diminishes the exit channel van der Waals well that was too pronounced on the Stark–Werner surface. Several preliminary dynamical scattering calculations were carried out using the new surface for total angular momentum equal to zero for F+H2 and F+HD. It is found that gross features of the reaction dynamics are quite similar to those predicted by the Stark–Werner surface, in particular the reactive resonance for F+HD and F+H2 survive. However, the most of the exit channel van der Waals resonances disappear on the new surface. It is predicted that the differential cross-sections at low collision energy for the F+H2 reaction may be drastically modified from the predictions based on the Stark–Werner surface.

Observation of a reactive resonance in the integral cross section of a six-atom reaction: F+CHD3.

The title reaction was investigated under crossed-beam conditions at collisional energies ranging from about 0.4 to 7.5 kcal/mol. Product velocity distributions were measured by a time-sliced, velocity-map imaging technique to explicitly account for the density-to-flux transformation factors. Both the state-resolved, pair-correlated excitation functions and vibrational branching ratios are presented for the two isotopic product channels. An intriguing resonance tunneling mechanism occurring near the reaction threshold for the HF+CD3 product channel is surmized, which echoes the reactive resonances found previously for the F+HD-->HF+D reaction and more recently for the F+CH4 reaction.

Reactive resonance in a polyatomic reaction.

In the course of an extensive investigation aimed at understanding the detailed mechanisms of a prototypical polyatomic reaction, several remarkable observations were uncovered. To interpret these findings, we surmise the existence of a reactive resonance in this polyatomic reaction. The reaction of concern is F+CH4-->HF+CH3, and the abnormal attributes were revealed only near the reaction threshold. The discovery of reactive resonance in a polyatomic reaction is more than just an extension from a typical atom+diatom reaction. As shown here, it holds great promise to disentangle the elusive intramolecular vibrational dynamics of transient collision complex in the critical transition-state region.

Study of irradiated black pepper antioxidant activity changes

Some antioxidant activities of extracts of irradiated black pepper (Piper nigrum) were evaluated. The ground black pepper was exposed to gamma-irradiation at doses from 5 to 30 kGy. The effect of irradiation on antioxidant properties of black pepper extracts was investigated by these methods: radical scavenging effect on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals, determination of reducing power, thiobarbituric acid reactive substances assay and electron paramagnetic resonance spectroscopy (EPR). Irradiation resulted in a decrease of DPPH antiradical activity of black pepper extracts according to dose of irradiation. Reducing power was not changed by action of irradiation. Very important and sensitive differences in antioxidant activity of irradiated black pepper were investigated by EPR spectrometry and thiobarbituric acid method.

Time delay as a tool to identify the signatures of reactive resonance: F+HD and F+H2 reactions

The formalism of scattering time delay is investigated as a tool to identify the signatures of resonance in reactive molecular collisions. The concept of state and angle resolved time delay is reviewed and applied to model problems. Several numerical pathologies of the time delay formalism are discussed that potentially may obscure a resonance signature, or may lead to a false positive result. The time delay is computed explicitly for the F+HD→HF+D and F+H2→HF+H reactions using the results of full scattering calculations on the Stark–Werner potential energy surface. The reactive resonance known to exist for the F+HD reaction is clearly apparent both in the state resolved and angle resolved time delay functions. On the other hand, the analogous resonance in the F+H2 reaction is masked in the state resolved time delay function through the influence of a nearby energetic threshold. However, the angle resolved time delay does provide a clear signature of the resonance, thus settling a controversy on the existence of a resonance for the F+H2 system.

Signatures of reactive resonance: three case studies

Recent advances in the study of short-lived reactive resonances are reviewed. Special emphasis is given to addressing the question of how reactive resonances might be observed in molecular beam scattering experiments. Three case studies are presented for simple triatomic systems that are believed to exhibit resonance phenomena: F + HD → D + HF, F + H2 → H + HF, and H + HD → D + H2. It is seen that reactive resonances do strongly influence collision observables, but in a different way for each case. At this stage, there does not appear to be a unique resonance signature that can be applied to all reactions.

Reaction dynamics of F+HD→HF+D at low energies: Resonant tunneling mechanism

The complete state-resolved differential cross section σ(v′,j′,θ;Ec), investigated in a crossed-beam scattering study, is presented for the title reaction at six initial collision energies (Ec) which are below or near the barrier energy. At low energies, all reactive flux is gated through a trapped resonance state via a tunneling process. Hence, it serves as a benchmark system for better understanding the reactive resonance phenomenon. In addition to highlighting various resonance fingerprints of experimental observable, the concept of resonant tunneling reaction mechanism is elucidated. Particular emphasis is placed on its distinction from the more conventional transition-state reaction mechanism.

Induction of Activator Protein-1 through Reactive Oxygen Species by Crystalline Silica in JB6 Cells

We reported previously that freshly fractured silica (FFSi) induces activator protein-1 (AP-1) activation through extracellular signal-regulated protein kinases (ERKs) and p38 kinase pathways. In the present study, the biologic activities of FFSi and aged silica (ASi) were compared by measuring their effects on the AP-1 activation and phosphorylation of ERKs and p38 kinase. The roles of reactive oxygen species (ROS) in this silica-induced AP-1 activation were also investigated. We found that FFSi-induced AP-1 activation was four times higher than that of ASi in JB6 cells. FFSi also caused greater phosphorylation of ERKs and p38 kinase than ASi. FFSi generated more ROS than ASi when incubated with the cells as measured by electron spin resonance (ESR). Studies using ROS-sensitive dyes and oxygen consumption support the conclusion that ROS are generated by silica-treated cells. N-Acetylcysteine (an antioxidant) and polyvinyl pyridine-N-oxide (an agent that binds to Si-OH groups on silica surfaces) decreased AP-1 activation and phosphorylation of ERKs and p38 kinase. Catalase inhibited phosphorylation of ERKs and p38 kinase, as well as AP-1 activation induced by FFSi, suggesting the involvement of H(2)O(2) in the mechanism of silica-induced AP-1 activation. Sodium formate (an ( small middle dot)OH scavenger) had no influence on silica-induced MAPKs or AP-1 activation. Superoxide dismutase enhanced both AP-1 and MAPKs activation, indicating that H(2)O(2), but not O(2), may play a critical role in silica-induced AP-1 activation. These studies indicate that freshly ground silica is more biologically active than aged silica and that ROS, in particular H(2)O(2), play a significant role in silica-induced AP-1 activation.

The search for resonance signatures in H+D 2 reaction dynamics

To investigate the recent report of the observation of a reactive resonance in the title reaction, we have calculated the ro-vibrationally state-resolved integral cross-sections (ICS) and differential cross-sections (DCS), as well as the vibrational branching ratios for the H+D 2( v= j=0)→HD( v ′, j ′)+D reaction. Oscillatory structure occurring in the state-to-state reaction probabilities does survive partial wave averaging to appear in the integral cross-sections for low j ′-values, although not significantly for the j ′=7 state recently reported. The resonance picture interpretation of the oscillations is shown to be problematic.

磁性材料 有機可塑基板上に堆積させたＣｏ含有酸化鉄薄膜

Co-containing ferrite thin films were successfully deposited on organic flexible substrates by the reactive sputtering method using-an electron cyclotron resonance microwave plasma at a temperature lower than 150°C. The ferrite thin films had an amorphous-like structure, perpendicular magnetic anisotropy and high perpendicular coercivity of about 170kA/m. The perpendicular coercivity increased with increasing substrate thickness. Inserting SiO2 underlayer between organic flexible substrate and ferrite thin films enhanced the perpendicular coercivity.

In situ cleaning of a chamber wall divided in a reactive ECR plasma

Basic properties of in situ cleaning of a chamber wall divided in a reactive electron cyclotron resonance (ECR) plasma have been investigated by monitoring ion current flowing into a three-segmented side wall, to which radio-frequency (rf) surface voltage is applied externally. To keep the wall condition clean during the plasma processing, we have found that the positive sheath voltage toward the wall surface is an important factor as well as the wall temperature. The method of the chamber wall cleaning proposed here is quite useful for keeping the initial wall condition and plasma processing parameters without degrading the quality of the processing.

The case for a reactive resonance in F+H2

We have investigated the F+H2 reaction on the Stark–Werner potential energy surface using quantum scattering theory, adiabatic theory, and spectral quantization. Clear evidence is found for a transition state resonance on this potential surface. Comparison to experimental results suggests that a resonance is also present in the true reaction.

Resonance-Mediated Chemical Reaction:F+HD→HF+D

Conclusive evidence is presented for the existence of a reactive resonance in the F+HD reaction. In a molecular beam experiment, the resonance appears in the integral cross section as a distinct steplike feature, while in the differential cross section it is manifested as sharply varying forward-backward peaks in the product distribution. A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations. At collision energies below 1 kcal/mol, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism.

Observation of a transition state resonance in the integral cross section of the F+HD reaction

We have studied the reaction F+HD at low collision energies using a combination of experimental and theoretical methods. Clear evidence for a reactive resonance is found in the integral cross section for the reactive channel F+HD→HF+D. Using a crossed molecular beam apparatus, the total reactive cross sections for the HF+D and DF+H channels were obtained in the collision energy range of 0.2–5 kcal/mol. In addition, Doppler profiles were obtained over this range of energies, which provide information about the angularly resolved distribution of final vibrational states. The cross section shows a distinctive steplike feature near 0.5 kcal/mol. Furthermore, the Doppler profiles reveal a dramatic change in the angular distribution of products over a narrow energy range centered at 0.5 kcal/mol. This feature is shown to arise from a reactive resonance localized near the transition state. Theoretical scattering calculations have been carried out using the Stark–Werner potential energy surface, which accurately reproduce the shape of the resonance feature. A detailed analysis of quantum dynamics using the spectral quantization method reveals that a quantum resonance exists near 0.52 kcal/mol, which is localized about the collinear FHD geometry. At collision energies below 1 kcal/mol, the reaction was found to proceed almost exclusively through resonant tunneling with very little contribution from direct, over the barrier, reaction. The properties of the quantum resonance, such as the position, lifetime, and partial widths were found to correlate well with the experimental results and the quantum scattering calculations.

Time correlation function and its unifying role in molecular structure and dynamics

The utility of time correlation functions computed by time evolving suitably chosen wave packets to studies of bound states of molecules, quasibound states and transition state resonances in reactive systems, photoabsorption-dissociation and resonance Raman scattering in molecules is illustrated using examples from our own laboratory. The role of excited electronic states and wavefunction interference effects in influencing photoexcitation observables is examined using model systems.

Chamber wall cleaning in a reactive ECR plasma

The basic properties of the chamber wall cleaning in a reactive electron cyclotron resonance (ECR) plasma have been investigated using poly-Si wafers on the wall, to which a radio-frequency (RF) surface voltage was applied externally. The influence of the cleaning upon the processing plasma was also measured by a Langmuir probe. To keep the wall condition clean during the plasma processing, it was found that the ratio of cleaning on-time to off-time was an important factor as well as the potential difference between the plasma and the wall surface. The method of chamber wall cleaning proposed here was quite effective for keeping the initial wall condition constant without influencing the plasma parameters in the region of the processing too much.