Important Elementary Processes Research Articles

A chiral hydrogen atom abstraction catalyst for the enantioselective epimerization of meso-diols.

Hydrogen atom abstraction is an important elementary chemical process but is very difficult to carry out enantioselectively. We have developed catalysts, readily derived from the Cinchona alkaloid family of natural products, which can achieve this by virtue of their chiral amine structure. The catalyst, following single-electron oxidation, desymmetrizes meso-diols by selectively abstracting a hydrogen atom from one carbon center, which then regains a hydrogen atom by abstraction from a thiol. This results in an enantioselective epimerization process, forming the chiral diastereomer with high enantiomeric excess. Cyclic and acyclic 1,2-diols are compatible, as are acyclic 1,3-diols. Additionally, we demonstrate the viability of combining our approach with carbon-carbon bond formation in Giese addition. Given the increasing number of synthetic methods involving hydrogen atom transfer steps, we anticipate that this work will have a broad impact in the field of enantioselective radical chemistry.

(Invited) Exciton Dynamics in Triplet Fusion of Diphenylanthracene in Fluid Solution and Crystalline Solid

Triplet fusion (TF) has recently attracted attention because of its great potential of various applications. Since high reaction efficiency of TF is demanded in any applications, it is necessary to clarify the mechanism of triplet exciton dynamics that is an important elementary process to enhance the TF efficiency. Therefore, in this paper, we study the triplet exciton dynamics of 9,10-diphenylanthracene (DPA), which shows the TF-based fluorescence, in a solvent of tetraethylene glycol dimethyl ether and in polycrystal of DPA, in combination with the triplet sensitization using platinum octaethylporphyrin. The observed emission kinetics and their magnetoluminescence effect indicate that the rotational and the multitrapping diffusions of the triplet exciton play an inherent role respectively in the fluid solution and in the polycrystalline solid.

Activation energy of kink incorporation of particles into colloidal crystals with attractive interactions

The kink incorporation and diffusion of particles on the growth interface of colloidal crystals with attractive interactions between particles are important elementary processes.

Solvent Effects on Ultrafast Charge Transfer Population: Insights from the Quantum Dynamics of Guanine-Cytosine in Chloroform.

We study the ultrafast photoactivated dynamics of the hydrogen bonded dimer Guanine-Cytosine in chloroform solution, focusing on the population of the Guanine→Cytosine charge transfer state (GC-CT), an important elementary process for the photophysics and photochemistry of nucleic acids. We integrate a quantum dynamics propagation scheme, based on a linear vibronic model parameterized through time dependent density functional theory calculations, with four different solvation models, either implicit or explicit. On average, after 50 fs, 30∼40 % of the bright excited state population has been transferred to GC-CT. This process is thus fast and effective, especially when transferring from the Guanine bright excited states, in line with the available experimental studies. Independent of the adopted solvation model, the population of GC-CT is however disfavoured in solution with respect to the gas phase. We show that dynamical solvation effects are responsible for this puzzling result and assess the different chemical-physical effects modulating the population of CT states on the ultrafast time-scale. We also propose some simple analyses to predict how solvent can affect the population transfer between bright and CT states, showing that the effect of the solute/solvent electrostatic interactions on the energy of the CT state can provide a rather reliable indication of its possible population.

Soliton–Breather Interaction: The Modified Korteweg–de Vries Equation Framework

Pairwise interactions of particle-like waves (such as solitons and breathers) are important elementary processes that play a key role in the formation of the rarefied soliton gas statistics. Such waves appear in different physical systems such as deep water, shallow water waves, internal waves in the stratified ocean, and optical fibers. We study the features of different regimes of collisions between a soliton and a breather in the framework of the focusing modified Korteweg–de Vries equation, where cubic nonlinearity is essential. The relative phase of these structures is an important parameter determining the dynamics of soliton–breather collisions. Two series of experiments with different values of the breather’s and soliton’s relative phases were conducted. The waves’ amplitudes resulting from the interaction of coherent structures depending on their relative phase at the moment of collision were analyzed. Wave field moments, which play a decisive role in the statistics of soliton gases, were determined.

Real-space observation of far- and near-field-induced photolysis of molecular oxygen on an Ag(110) surface by visible light.

Dissociation of molecular oxygen is an important elementary process in heterogeneous catalysis. Here, we report on a real-space observation of oxygen photolysis on the Ag(110) surface at 78 K by far- and near-field excitation in the ultraviolet-near-infrared range using a low-temperature scanning tunneling microscope (STM) combined with wavelength-tunable laser excitation. The photolysis of isolated oxygen molecules on the surface occurs even by visible light with the cross section of ∼10-19 cm2. Time-dependent density functional theory calculations reveal optical absorption of the hybridized O2-Ag(110) complex in the visible and the near-infrared range which is associated with the oxygen photolysis. We suggest that the photolysis mechanism involves a direct charge transfer process. We also demonstrate that the photolysis can be largely enhanced in plasmonic STM junctions, and the cross section is estimated to be ∼10-17 cm-2 in the visible and the near-infrared range, which appears to be an interesting feature of plasmon-induced reactions from the perspective of photochemical conversion with the aid of solar energy.

Light scattering by dielectric bodies in the Born approximation

Light scattering is one of the most important elementary processes in near-field optics. We build up the Born series for scattering by dielectric bodies with sharp boundaries. The Green's function for a two-dimensional homogeneous dielectric cylinder is obtained. As an example, the formulas are derived for a scattered field of two parallel cylinders. The polar diagram is shown to agree with the numerical calculation by the known methods of discrete dipoles and boundary elements.

Communication: Vibrational relaxation of CO(1Σ) in collision with Ar(1S) at temperatures relevant to the hypersonic flight regime.

Vibrational energy relaxation (VER) of diatomics following collisions with the surrounding medium is an important elementary process for modeling high-temperature gas flow. VER is characterized by two parameters: the vibrational relaxation time τvib and the state relaxation rates. Here the vibrational relaxation of CO(ν=0←ν=1) in Ar is considered for validating a computational approach to determine the vibrational relaxation time parameter (pτvib) using an accurate, fully dimensional potential energy surface. For lower temperatures, comparison with experimental data shows very good agreement whereas at higher temperatures (up to 25 000 K), comparisons with an empirically modified model due to Park confirm its validity for CO in Ar. Additionally, the calculations provide insight into the importance of Δν>1 transitions that are ignored in typical applications of the Landau-Teller framework.

The influence of negative ions in helium–oxygen barrier discharges: II. 1D fluid simulation and adaption to the experiment

A 1D fluid simulation was developed to investigate the influence of negative ions in a helium–oxygen barrier discharge between two glass plates at a distance of . The paper describes setting up the simulation for a pressure of and an admixture of oxygen to helium. In order to enable the comparison with laser photodetachment experiments, the simulation is adapted to the experimentally observed discharge current and gap voltage by varying gas temperature, flux of thermally desorpted electrons and secondary electron emission coefficients. The discharge is characterized by evaluation of the most important elementary collision processes as well as the kinetics of the charged species. Besides, the influence of long-living species on the discharge behavior is taken into account by long-time simulations. The negative ions are characterized by their spatio-temporal distribution in the gap and their production and loss processes. The comparison between simulations without and with consideration of negative ions reveals the importance of negative ions on the discharge development.

Electron Transfer-Based Single Molecule Fluorescence as a Probe for Nano-Environment Dynamics

Electron transfer (ET) is one of the most important elementary processes that takes place in fundamental aspects of biology, chemistry, and physics. In this review, we discuss recent research on single molecule probes based on ET. We review some applications, including the dynamics of glass-forming systems, surface binding events, interfacial ET on semiconductors, and the external field-induced dynamics of polymers. All these examples show that the ET-induced changes of fluorescence trajectory and lifetime of single molecules can be used to sensitively probe the surrounding nano-environments.

Brain Perceived Intuitively Mental Impasses in Insight Problem Solving: An ERP Study

Although previous studies on insight have fully investigated brain activities and neural correlates of "flash of insight", little knowledge was known on cognitive process of active solution-seeking in problem solving before insight. Studies revealed that we would not obtain a full and deep understanding on insight problem solving and its neural basis, unless given an equal and all-sided study for active solution-seeking before insight and "flash of insight" during hint presentation period. Mental impasse is an important elementary process not only for subsequent incubation and presentation restructuring but also for human problem solving behavior. However, yet to date there were few studies that have examined this issue. Existing studies on FOK in insight tasks solving revealed that people’s metacognition can not accurately monitor sudden insight, but they did not elucidate whether people’s intuition can perceive possible mental impasse subsequently encountered or not. Thus, the present study focused on this problem. The current study adopted normal three-word Chinese riddles and firstly employed high-density event related potentials (ERPs) to investigate the neural markers of insight during active solutions-seeking period. 13 paid participants were recruited to fulfill a three-character guessing task, and brain electrical activity was recorded. The results showed that, the process of active solutions-seeking of riddles with impasses compared to those without impasses elicited a more positive potential in the time windows of 120~210 ms (P170) and 620~800 ms (terminal LPC). P170 may be associated with the processing that people perceive intuitively mental impasses at the perceptual stage, whereas the terminal LPC may be associated with a conscious reappraisal and reflection of mental impasses. These results imply that mental impasse not only appears at the terminal phase, but also be intuition-sensitive at the perceptual stage. Human brain can perceive whether they would meet subsequent mental impasses or not during problem solving. The mental ruts hypothesis claims that mental impasses are formed because the repeated exploration of an unsuccessful search path or the search for the same knowledge element adds more and more activation to this path. However, our findings suggest that mental impasses in insight riddle solving task do not result from iterative repetitions of non-effective exploration. If necessary, mental impasses can be formed initially at an early stage, and then be reappraised at a late stage. The finding suggests that human brain can perceive intuitively subsequent mental impasses underlying our complex insight problem solving behaviors at an early stage.

Proton transfer events in GFP

Proton transfer is one of the most important elementary processes in biology. Green fluorescent protein (GFP) serves as an important model system to elucidate the mechanistic details of this reaction, because in GFP proton transfer can be induced by light absorption. Illumination initiates proton transfer through a 'proton-wire', formed by the chromophore (the proton donor), water molecule W22, Ser205 and Glu222 (the acceptor), on a picosecond time scale. To obtain a more refined view of this process, we have used a combined approach of time resolved mid-infrared spectroscopy and visible pump-dump-probe spectroscopy to resolve with atomic resolution how and how fast protons move through this wire. Our results indicate that absorption of light by GFP induces in 3 ps (10 ps in D(2)O) a shift of the equilibrium positions of all protons in the H-bonded network, leading to a partial protonation of Glu222 and to a so-called low barrier hydrogen bond (LBHB) for the chromophore's proton, giving rise to dual emission at 475 and 508 nm. This state is followed by a repositioning of the protons on the wire in 10 ps (80 ps in D(2)O), ultimately forming the fully deprotonated chromophore and protonated Glu222.

An ab initio chemical kinetic study on the reactions of H, OH, and Cl with HOClO3

AbstractThe mechanisms for reactions of H, HO, and Cl with HOClO3, important elementary processes in the early stages of the ammonium perchlorate (AP) combustion reaction, have been investigated at the CCSD(T)/6‐311+G(3df,2p)//PW91PW91/6‐311+G(3df) level of theory. The rate constants for the low‐energy channels have been calculated by statistical theory. For the reaction of H and HOClO3, the main channels are the production of H2 + ClO4 (k1a) and HO + HOClO2 (k1b); k1a and k1b can be represented as 1.07 × 10−17 T1.97 exp(−7484/T) and 6.08 × 10−17T1.96 exp(−7729/T) cm3 molecule−1 s−1, respectively. For the HO + HOClO3 reaction, the main pathway is the H2O + ClO4 (k2a) production process, with the predicted rate constant k2a = 1.24 × 10 −8 T−2.99 exp(1664/T) for 300–500 K and k2a = 1.27×10−19 T2.12 exp(−1474/T) for 500–3000 K. For the Cl + HOClO3 reaction, the formation of HOCl + ClO3 (k3a) and HCl + ClO4 (k3b) is dominant, with k3a = 1.33×10−12 T0.67 exp(−9658/T) and k3b = 1.75×1016 T1.63 exp(−11156/T) cm3 molecules−1 in the range of 300–3000 K. In addition, the heats of formation of ClO3 and HOClO3 have been predicted based on several isodesmic and/or isogyric reactions with ΔfHo0 (ClO3) = 47.0 ± 1.0 and ΔfHo0 (HOClO3) = 5.5 ± 1.5 kcal/mol, respectively. These data may be used for kinetic simulation of the AP decomposition and combustion reaction. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 253–261, 2010

Molecular mechanisms for disilane chemisorption on Si(100)-(2×1)

The dissociative chemisorption of disilane is an important elementary process in the growth of silicon films. Although factors governing the rate of film growth such as surface temperature and disilane flux have been extensively studied experimentally by a large number of groups, the molecular mechanism for disilane adsorption is not well established. In particular, although it is generally held that chemisorption occurs via silicon-silicon bond dissociation, there have been a number of suggestions that silicon-hydrogen bond dissociation also occurs. We consider this issue in detail hereby examining a number of different paths that disilane can take to chemisorb. In addition to silicon-silicon bond dissociation paths, we examine three different mechanisms for silicon-hydrogen bond dissociation, for each path considering both adsorption at interdimer and intradimer sites. The calculated barriers are critically compared to experimental data. We conclude that silicon-hydrogen bond dissociation is likely, finding two zero barrier paths for chemisorption at interdimer sites, and a precursor-mediated path with a low barrier. We also find two precursor states, and show that each can lead to chemisorption via either silicon-silicon or silicon-hydrogen bond dissociation. Finally, we calculated the barriers for reaction of coadsorbed disilyl and hydrogen to form gas phase silane. Our calculations are performed using density-functional theory within a planewave ultrasoft pseudopotential methodology. We traced the reaction paths with the nudged-elastic band technique.

基礎から学ぶマススペクトロメトリー／ 質量分析の源流第１回 衝突論

Recent development in mass spectrometry has far-reaching effects not only in physical and chemical studies of gaseous molecules and ions, but also in biological studies, including proteomics and metabolomics. In mass spectrometry, collisions are very important elementary processes in ionization, collision-induced dissociation, and mobility, whose methods are useful for the development of proteomics and metabolomics. In the present review, to understand the basis of collision, the concepts of center-of-mass system, laboratory system, cross section, kinetic energy release, and impact parameter are explained. The cross sections, which provide the magnitude of elementary reactions, depend on reaction species, collision energies, and reaction processes. I have attempted to present these dependences with the support of various examples.

Radiation Induced Spent Nuclear Fuel Dissolution under Deep Repository Conditions

The dynamics of spent nuclear fuel dissolution in groundwater is an important part of the safety assessment of a deep geological repository for high level nuclear waste. In this paperwe discussthe most important elementary processes and parameters involved in radiation induced oxidative dissolution of spent nuclear fuel. Based on these processes, we also present a new approach for simulation of spent nuclear fuel dissolution under deep repository conditions. This approach accounts for the effects of fuel age, burn up, noble metal nanoparticle contents, aqueous H2 and HCO3- concentration, water chemistry, and combinations thereof. The results clearly indicate that solutes consuming H202 and combined effects of noble metal nanoparticles and H2 have significant impact on the rate of spent nuclear fuel dissolution. Using data from the two possible repository sites in Sweden, we have employed the new approach to estimate the maximum rate of spent nuclear fuel dissolution. This estimate indicates that H2 produced from radiolysis of groundwater alone will be sufficient to inhibit the dissolution completely for spent nuclear fuel older than 100 years.

Measuring the rate of intramolecular contact formation in polypeptides.

Formation of a specific contact between two residues of a polypeptide chain is an important elementary process in protein folding. Here we describe a method for studying contact formation between tryptophan and cysteine based on measurements of the lifetime of the tryptophan triplet state. With tryptophan at one end of a flexible peptide and cysteine at the other, the triplet decay rate is identical to the rate of quenching by cysteine. We show that this rate is also close to the diffusion-limited rate of contact formation. The length dependence of this end-to-end contact rate was studied in a series of Cys-(Ala-Gly-Gln)(k)-Trp peptides, with k varying from 1 to 6. The rate decreases from approximately 1/(40 ns) for k = 1 to approximately 1/(140 ns) for k = 6, approaching the length dependence expected for a random coil (n(-3/2)) for the longest peptides.

Dynamics of Atomic and Molecular Hydrogen Elimination from Hydrocarbons at VUV Excitation

AbstractElimination of atomic hydrogen (H) and molecular hydrogen (H2) are important elementary chemical processes in photochemistry and combustion chemistry. Recently, unique and sensitive detection techniques for atomic and molecular hydrogen detection were developed in our laboratory. Using the advanced molecular beam methods, we have studied the photodissociation of a few typical hydrocarbons at 157 nm excitation, especially their atomic and molecular hydrogen elimination processes. In this report, we will briefly describe the results from photodissociation of propane, ethylene, propyne and methanol at 157 nm excitation. These molecules represent different classes of hydrocarbons such as alkane, alkene, alkyne and alcohol. Through careful studies on differently deuterated compounds, clear pictures of selective atomic and molecular hydrogen elimination processes can be constructed for all of the above compounds. These results will help us to understand the dissociation dynamics of the small hydrocarbon molecules.

Kinetic Monte Carlo investigation of pit formation at the CaCO 3(101̄4) surface-water interface

On the basis of quantitative information derived from atomic-force microscopy (AFM) studies of shallow pit formation at the CaCO 3(101̄4) surface-water interface in the surface-reaction regime, we have developed a kinetic Monte Carlo (KMC) model which reproduces quantitatively the experimental behaviour of the time-evolution of the pits. This allows the rates of all the important elementary atomistic processes involved in the dissolution to be obtained, rates not readily obtainable directly from AFM data. The KMC model also provides important insight into the evolution of very small pits, which in principle can be resolved using AFM, but which in practice are very difficult to observe because of the low probability of their occurrence in the microscope scan area. The KMC simulations show that the growing pits exhibit two different growth regimes, in agreement with the predictions of a simple terrace-ledge-kink (TLK) model. For very small pits the linear pit size increases exponentially with time and the pit edges accelerate (double-kink self-annihilation regime). Having attained a certain pit size (∼25 nm for a temperature of 300 K), thereafter the pit sizes increase linearly with time, the pit edges maintaining a constant velocity (kink-kink annihilation regime). A comparison of the quantitative predictions of the TLK model with KMC simulations shows that, in spite of its simplicity, the TLK model provides a satisfactory semi-quantitative description of the pit evolution. The KMC model presented provides the starting point for the development of a more comprehensive model of the calcite-water interface which will include the effects of adsorbates and other variations in interface conditions. Published by Elsevier Science B.V.

Chemical Dynamics at the Gas−Surface Interface

We review recent progress in the understanding of the chemical dynamics of gas−surface reactions. Such reactions can involve many dynamically distinct interactions. Our review begins by considering a number of these important elementary processes, dealing in turn with (1) the initial gas−surface interaction including translational, rotational, and vibrational energy exchange, the discussion of vibrational interactions covering both direct vibrational excitation and vibrational relaxation at surfaces; (2) the trapping or adsorption of atoms and molecules on the surface; and (3) the important elementary processes of surface diffusion and desorption. We conclude with a detailed look at two examples of gas−surface reactions, first summarizing what is now known about the dissociative chemisorption of hydrogen at Cu surfaces (and the reverse process of recombinative desorption). Then we describe recent studies of direct gas−surface abstraction (or Eley−Rideal) reactions.