Electronic mechanism of STM-induced diffusion of hydrogen on Si(100)

We have observed a scanning tunneling microscopy (STM) induced lateral transfer of a single hydrogen atom on the Si(100) surface. The transfer rate of the hydrogen atom is proportional to the electron dose, indicating an electron-assisted transfer mechanism. Measurements of the relations between the transfer rate and the sample bias and temperature give further support for an electronic mechanism. The bias dependence of the transfer rate shows a peak, and from a first principles electronic structure calculation we show that the position of the peak is related to the energy of a localized surface resonance. We propose that the hydrogen transfer is related to inelastic hole scattering with this surface resonance. We develop a microscopic model for the hydrogen transfer, and using the experimental data we extract information on the resonance lifetime and the transfer yield per resonant electron. The transfer takes place by tunneling through a small excited state transfer barrier. The transfer rate is increased if the hydrogen atom before the resonant excitation is vibrationally excited, and this gives rise to an increasing transfer rate with increasing sample temperature.     

New perspective of electron transfer chemistry

A new perspective of electron transfer chemistry is described for fine control of electron transfer reactions including back electron transfer in the charge separated state of artificial photosynthetic compounds and its synthetic application. Fundamental electron transfer properties of suitable components of efficient electron transfer systems are described in light of the Marcus theory of electron transfer, in particular focusing on the Marcus inverted region, and they are applied to design multi-step electron transfer systems which can well mimic the function of a photosynthetic reaction center. Both intermolecular and intramolecular electron transfer processes are finely controlled by complexation of radical anions, produced in the electron transfer, with metal ions which act as Lewis acids. Quantitative measures to determine the Lewis acidity of a variety of metal ions are given in relation to the promoting effects of metal ions on the electron transfer reactions. The mechanistic viability of metal ion catalysis in electron transfer reactions is demonstrated by a variety of examples of chemical transformations involving metal ion-promoted electron transfer processes as the rate-determining steps, which are made possible by complexation of radical anions with metal ions.     

Electron transfer versus proton transfer in gas-phase ion/ion reactions of polyprotonated peptides

The ion/ion reactions of several dozen reagent anions with triply protonated cations of the model peptide KGAILKGAILR have been examined to evaluate predictions of a Landau-Zener-based model for the likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragment ions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer are competitive processes for any combination of anionic and cationic reactants. For reagent anions in reactions with protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. If charge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kinetic grounds because the reactant and product channels cross at greater distances, provided conditions are favorable for electron transfer at the crossing point. The results are consistent with a model based on Landau-Zener theory that indicates both thermodynamic and geometric criteria apply for electron transfer involving polyatomic anions. Both the model and the data suggest that electron affinities associated with the anionic reagents greater than about 60-70 kcal/mol minimize the likelihood that electron transfer will be observed. Provided the electron affinity is not too high, the Franck-Condon factors associated with the anion and its corresponding neutral must not be too low. When one or the other of these criteria is not met, proton transfer tends to occur essentially exclusively. Experiments involving ion/ion attachment products also suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed, lead to either proton transfer or electron transfer.     

Shining a light on catalytic chain transfer

The catalytic chain transfer polymerization of styrene is only truly effective when the reaction mixture is exposed to (UV-)light. The apparent chain transfer constant depends inversely on radical concentration and can be increased up to 8000. These results can be explained by combining aspects of both catalytic chain transfer and the formation of cobalt-carbon bonds. For the catalytic chain transfer polymerization of n-butyl acrylate a chain transfer constant of 650 was found. The resulting transfer coefficient has the same order of magnitude as the one for n-butyl methacrylate. This means that the absence of an α-methyl group hardly influences the transfer step itself. Furthermore, the effect of possible impurities on the catalytic chain transfer polymerization of methyl methacrylate is investigated.     

Optical energy transport and interactions between the excitations in a coumarin-perylene bisimide dendrimer

Energy transfer properties of novel coumarin-perylene bisimide dendrimer are studied by means of steady state and time-resolved UV/vis spectroscopy. At low donor excitation density fast (transfer rate approximately 10 ps(-1)) and efficient (quantum yield approximately 99.5%) donor-acceptor energy transfer is observed. The random distributions of donor-acceptor orientations and distances result in nonexponential energy transfer kinetics. The energy transfer remains independent of excitation density up to densities corresponding to one absorbed photon per 10 dendrimer molecules. At higher excitation densities the transfer rate is found to increase due to excitation of multiple donors per dendrimer. Control of the donor-acceptor energy transfer rate is achieved by pre-excitation of the acceptor and monitored by prepump-pump-probe experiments, which show that the energy transfer rate can be decreased by a factor of 2. The relative orientations of transition dipole moments in the donor and acceptor molecules are found to be one of the key factors determining the energy transfer dynamics at high excitation densities.     

Non-ideality in vinyl polymerization—VIII: Retarded polymerization of methyl methacrylate in presence of p-benzoquinone initiated by azobisisobutyronitrile

Detailed kinetic analysis of AIBN-initiated polymerization of methylmethacrylate in presence of p-benzoquinone has been reported. Primary radical transfer, whereby a primary radical transfers its radical reactivity to a transfer agent, has been considered along with macroradical transfer. It is found that the former process is quite appreciable in the system and must be allowed for to arrive at accurate values of transfer constants. Values of transfer constants for both primary radicals and macroradicals towards benzoquinone, and characteristic constants for degradative chain transfer and primary radical transfer have been evaluated applying the mathematical treatment developed previously. The mode of termination of macroradicals by fairly stable microradicals formed as a result of transfer has also been discussed.     

Theoretical Studies on the Hydrogen Bond Transfer and Proton Transfer between Anamorphoses of the Dihydrated Glycine Complex

The conversion between anamorphoses of the dihydrated glycine complex was studied by means of B3LYP/6-31++G**. It was found that proton transfer was accompanied by hydrogen bond transfer in the process of conversion between different kinds of anamorphoses. With proton transfer, the electrostatic action was notably increased and the hydrogen-bonding action was evidently strengthened when the dihydrated neutral glycine complex converts into dihydrated zwitterionic glycine complex. The activation energy required for hydrogen bond transfer between dihydrated neutral glycine complexes is very low (6.32 kJ·mol-1); however, the hydrogen bond transfer between dihydrated zwitterionic glycine complexes is rather difficult with the required activation energy of 13.52 kJ·mol-1 due to the relatively strong electrostatic action. The activation energy required by proton transfer is at least 27.33 kJ·mol-1, higher than that needed for hydrogen bond transfer. The activation energy for either hydrogen bond transfer or proton transfer is in the bond-energy scope of medium-strong hydrogen bond, so the four kinds of anamorphoses of the dihydrated glycine complex could convert mutually.     

Asymmetry of Hydrogen Transfer through a Composite Membrane

Hydrogen mass transfer through a composite membrane represented by a film of palladium (or its alloys) applied onto a porous substrate has been studied. The hydrogen flux through the composite membrane has been shown to be governed by the ratio between the diffusion permeabilities of the film and the porous substrate, the mechanism of the transfer through the film, and the external pressure. It has been found that the intensity of hydrogen transfer through the composite membrane may depend on the transfer direction. The transfer is most asymmetric when the diffusion permeabilities of both layers are close and the hydrogen transfer in the film is limited by diffusion. At the same time, the transfer asymmetry effect does not arise when the hydrogen transfer in the film is limited by adsorption processes on its surface.     

Mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin in CH3OH

[reaction: see text] The mechanism of NO transfer from NO-donors (SNAP and G-MNBS) to ferrous tetraphenylporphyrin (TPPFe(II)) in CH(3)OH is discovered for the first time by using a laser flash technique. The results show that the NO transfer is completed by NO(+) transfer followed by electron transfer rather than direct NO transfer in one step.     

卟啉-酞菁分子内能量传递和电子转移的溶剂效应

用稳态荧光光谱研究了以氧原子和哌嗪作为连接基的卟啉酞菁二元分子在不同溶剂中的分子内能量传递和电子转移过程结果表明;分子内的能量传递和电子转移是两个相互竞争的过程，在非极性溶剂中，激发单重态的能量传递是主要过程，而在极性溶剂中则以电子转移为主运用Rehm-Weller公式计算了两种二元化合物在不同溶剂中的电子转移反应的自由能变化△G0ET，表明溶剂的极性对电子转移反应的自由能变化△G0ET影响很大极性越大;体系中的电子转移反应的△G0ET、越负，电子转移反应越易进行由于电子转移过程较能量传递过程进行得快，所以表现为体系中能量传递效率降低而电子转移效率增大。两种二元化合物的能量传递效率（φEnT）利和电子转移效率（φET）随溶剂的极性的变化具有相同的变化趋势     

Applications of the study of end-groups. A new approach to the determination of transfer constants

A summary is given of established procedures depending on analyses of polymers for initiator fragments incorporated as end-groups; attention is paid to determinations of kinetic chain lengths and rates of initiation in polymerizations and also of the relative importances of combination and disproportionation during termination. Recent developments allow determination of transfer constants from information on the average number of initiator fragments in a molecule of a polymer, even in some cases where the transfer is degradative. Their applications are discussed for various transfer agents including iodoform and derivatives of propene from which allylic radicals could be produced. There is consideration of the possibilities of applying analyses for initiator fragments to study of transfer initiator, transfer to polymer and primary radical transfer.     

Multi-electron transfer processes for molecular conversions

Multi-electron transfer plays an important role in many chemical reactions. A collection of examples, both of metal complexes that exhibit one-step, multi-electron transfer process, and of chemical reactions based on a multi-electron transfer system, is presented. Emphasis is placed on the role of multi-electron transfer process in a sense that some molecular conversion systems such as reduction of O₂ and oxidation of H₂O become feasible through multi-electron transfer. As an important example of molecular conversion, oxidative polymerization of sulfur-containing compounds through two-electron transfer, is also reviewed.     

Chirality Transfer in Gold(I)‐Catalysed Direct Allylic Etherifications of Unactivated Alcohols: Experimental and Computational Study

Gold(I)‐catalysed direct allylic etherifications have been successfully carried out with chirality transfer to yield enantioenriched, γ‐substituted secondary allylic ethers. Our investigations include a full substrate‐scope screen to ascertain substituent effects on the regioselectivity, stereoselectivity and efficiency of chirality transfer, as well as control experiments to elucidate the mechanistic subtleties of the chirality‐transfer process. Crucially, addition of molecular sieves was found to be necessary to ensure efficient and general chirality transfer. Computational studies suggest that the efficiency of chirality transfer is linked to the aggregation of the alcohol nucleophile around the reactive π‐bound Au–allylic ether complex. With a single alcohol nucleophile, a high degree of chirality transfer is predicted. However, if three alcohols are present, alternative proton transfer chain mechanisms that erode the efficiency of chirality transfer become competitive.     

Multielectron transfer process for molecular conversions: Oxidative polymerization with vanadium catalysts

Multielectron transfer plays an important role in many chemical reactions. A collection of studies on metal complexes which exhibit one-step multielectron transfer processes and on chemical reactions based on multielectron transfer systems is presented. Emphasis is placed on the role of multielectron transfer process as essential prerequisites for some molecular conversion systems such as the reduction of O₂ and the oxidation of H₂O. As an important example of molecular conversion, oxidative polymerization of aromatic compounds through two-electron transfer is also reviewed.     

Enhanced energy transfer within a microparticle

Enhanced energy transfer between donor-acceptor pairs within a single aerosol particle is demonstrated for the first time. The overall transfer within a particle 10 μm in diameter is found to exceed conventional dipole-dipole transfer by more than a factor of 10². A simple model indicates that the energy transfer is mediated by morphological resonances of the particle.     

Pseudo cage effect in double energy transfer

In some systems, the donor of a triplet—triplet energy transfer can be sensitized in its singlet state through a singlet—singlet energy transfer (Dexter mechanism), where the donor is the acceptor of the triplet transfer itself. As a consequence an extra acceptor molecule in the triplet energy transfer is present in the vicinity of the donor, thus enhancing the efficiency of the transfer process. Experiments show clearly this effect and a diffusional model gives semi-quantitative agreement with the experimental data.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

Hydrogen transfer in the presence of amino acid radicals

Quantum chemical model studies of hydrogen transfer between amino acids in the presence of radicals have been performed using the density functional theory method B3LYP. These studies were made to investigate alternative mechanisms to the conventional electron transfer-proton transfer mechanisms. The model reactions studied are such that the net result of the reaction is a transfer of one neutral hydrogen atom. Simple models are used for the amino acids. Three different mechanisms for hydrogen transfer were found. In the first of these, a transition state with a protonated intermediate residue is found, in the second, the proton and electron take different paths and in the third, a neutral hydrogen atom can be identified along the reaction pathway. A key feature of these mechanisms is that charge separation is always kept small in contrast to the previous electron transfer-proton transfer mechanisms. It is therefore proposed that the processes normally considered as electron transfer in the biochemical literature could in fact be better explained as hydrogen atom transfer, at least in cases where a suitable hydrogen bonded chain pathway is present in the protein. The presence of such chains in principle allows the protein to define the path of net hydrogen transfer. Another important conclusion is that standard quantum chemical methods can be used to treat these mechanisms for hydrogen transfer, allowing for an accurate representation of the geometric changes during the reactions. Received: 10 February 1997 / Accepted: 11 February 1997     

Photo-Induced Intermolecular Electron Transfer-Effect of Acceptor Molecular Structures

Photo-induced electron transfer versus molecular structure of acceptors is investigated using ultrafast time-resolved transient grating spectroscopy. Typical laser dyes Rhodamine 101 (Rh101) and Rhodamine 6G (Rh6G) in electron donor solvent-aniline are adopted as the objects. The forward electron transfer time constant from aniline to the excited singlet state of two Rhodamine dyes and subsequent back electron transfer from two dyes to aniline are measured. The experimental results denote that Rh6G presents faster electron transfer rates with aniline in both forward electron transfer and back electron transfer processes. With chemical calculation and qualitative analysis, it is found that the flexible molecular geometry of Rh6G leads to stronger electron coupling with donor solvent and further gives rise to larger electron transfer rates.     

Improved calibration transfer between near-Infrared (NIR) spectrometers using canonical correlation analysis

The application of mobile near-infrared (NIR) spectrometers in field measurements is growing. Calibration transfer techniques offer simple solutions for enabling models constructed on benchtop instruments for use on mobile spectrometers. Since different types of spectrometers with different components, scanning ranges and resolutions cause great differences in the spectral response, calibration transfer is difficult to apply. In this paper, we focus on calibration transfer among benchtop, portable and handheld spectrometers by a method of calibration transfer based on canonical correlation analysis (CTCCA). Its capability was illustrated by the example of a group of NIR spectra dataset for predicting reducing sugars, total sugar, and nicotine contents in tobacco leaves. The experimental results showed that the transferability of CTCCA was superior to other conventional calibration transfer methods, including piecewise direct standardization, spectral space transformation, calibration transfer based on independent component analysis, and calibration transfer based on the weight matrix. Moreover, the best transfer results were obtained in the three cases by canonical correlation analysis method executing transfer while the spectra were not interpolated, which shows that this approach has the advantage of easy implementation for calibration transfer. Therefore, CTCCA without interpolation calculation offers a new and simple solution for transferring the spectra acquired by mobile spectrometers to the optimized spectral models built on benchtop devices to improve the accuracy of the results. Additionally, the results show that the benchtop spectrometer is more suitable as the master instrument for calibration transfer with more accurate prediction than using a portable device as the master.