Double proton transfer and charge-transfer transitions in biological hydrogen-bonded systems guaninecytosine (GC) and GCMg2+ systems

Potential energy curves for intermolecular proton transfer have been calculated within a modified INDO scheme. Analysis of the nature of the excited states, with emphasis on charge-transfer transitions, has been performed. The proton-transfer probability was found to be markedly greater in some excited states than for the ground state. With the metal ions the double proton transfer should be most effective in charge-transfer states.     

Energy transfer of highly vibrationally excited 2-methylnaphthalene: Methylation effects

The methylation effects in the energy transfer between Kr atoms and highly vibrationally excited 2-methylnaphthalene in the triplet state were investigated using crossed-beam/time-sliced velocity-map ion imaging at a translational collision energy of approximately 520 cm(-1). Comparison of the energy transfer between naphthalene and 2-methylnaphthalene shows that the difference in total collisional cross section and the difference in energy transfer probability density functions are small. The ratio of the total cross sections is sigma(naphthalene): sigma(methylnaphthalene)=1.08+/-0.05:1. The energy transfer probability density function shows that naphthalene has a little larger probability at small T-->VR energy transfer, DeltaE(u)<300 cm(-1), and 2-methylnaphthalene has a little larger probability at large V-->T energy transfer, -800 cm(-1)相似文献   

On the theory of tunnelling in electron and proton transfer reactions

The concept of tunnelling in the theory of electron and proton transfer reactions has recently been questioned on the ground that the situation is a non-stationary one. It has been suggested that time-dependent perturbation theory should be applied to obtain the quantum mechanical transition probability. We have done this for a square barrier. The result for most reactions is the same as obtained by the WKB approximation.     

Transport of excitation energy in a three-dimensional doped molecular crystal. IV. Fourth-order propagation,exciton clothing,and exciton diffusion

The problem of excitons in interaction with phonons in a molecular crystal has been reinvestigated as a continuation of our earlier work. The exciton-phonon interaction has been taken to be linear in lattice displacements. The external medium, the phonon assembly, has been considered to be in thermal equilibrium. Following Simons, we have incorporated the effects of the medium on the exciton dynamics into a time-dependent effective potential that contains the equilibrium average exciton-phonon interaction as well as terms arising from the fluctuations in the medium's coordinates about their equilibrium values. A correlation function that represents the probability of exciton transfer has been given in the interaction picture. The time evolution of this correlation function has been determined by following Kubo's technique of cumulant expansion. The zeroth-, second-, and fourth-order contributions to the correlation function have been calculated in this way. The second- and fourth-order contributions have been diagrammatically represented. The second-order contribution has been explicitly calculated in different physical limits, namely, the slow exciton and the slow phonon limits at high and low temperatures and for very large and very small time. A few simple formulas for the transfer probability of a bare exciton in a molecular crystal of cubic symmetry have been derived from the Debye approximation for the dispersion of phonons. It has been specifically shown that the sum over phonon modes in the large time dynamics leads to a fully destructive interference in second order at a very low temperature and gives rise to a diffusive transport at a high enough temperature. A natural way of clothing the excitons has been considered and the clothed exciton has been represented diagrammatically. The dressing requires the correlation function to be redefined in terms of the clothed states and the clothed operators. The clothed exciton correlation function that represents the probability of transfer of excitons fully clothed by the phonons in thermal equilibrium turns out to be identical with the bare exciton correlation function. This attaches a novel interpretation to the correlation function which was originally defined by Simons. Transfer probabilities for a clothed exciton in a cubic crystal has been explicitly worked out for different physical limits under the Debye model of phonon dispersion. From these results a few expressions for the macroscopic diffusion coefficient of the clothed exciton have been obtained. A few critical comments have been incorporated. © 1996 John Wiley & Sons, Inc.     

Fluorescence resonance energy transfer-a spectroscopic probe for organized surfactant media

Dyes commonly used as biological labels have been used to probe resonance energy transfer in organized media. In neat water, energy transfer between the dye pairs fluorescein (donor):Nile red (acceptor) and acridine orange (donor):Nile red (acceptor) has a very low probability of occurrence. This study shows that the rate constant of energy transfer increases by more than an order of magnitude in organized surfactant media, viz., micelles and reverse micelles of the surfactant Triton X-100. The reverse micelles provide a better medium for energy transfer than the micelles. The energy transfer studies also provide an idea about the location and proximity of donor and acceptor dyes within the various organized media. Assuming Poissonian statistics for dye distribution, the donor-acceptor distances within micelles and reverse micelles are determined from energy transfer parameters. Acridine orange has been found to function better as a donor than fluorescein. This may be due to steric and electrostatic factors.     

PROTON TRANSFER IN THE PRIMARY PROCESS OF VISION

Abstract— Proton transfer was theoretically examined as a possible primary process of vision. The motion of protons in the adiabatic potential of the Schiff base hydrogen bond was investigated in terms of quantum mechanics. The probability of proton transfer from the Schiff base nitrogen (i.e. the unprotonation of Schiff base) was found to increase as the retinal rotated around 11–12. double bond by 90°. The results also suggested that the proton transfer can take place before or during the transition from the excited to ground state (excited state proton transfer). We proposed that such excited state proton transfer is one of the elementary processes in primary visual photochemistry, and this process leads to the unprotonated visual pigment, hyposorhodopsin, which has been experimentally verified as one of the primary photoproducts of rhodopsin. The probability of this process could be comparable to the conventional process leading to the protonated intermediate, bathorhodopsin. The relation of these results with the recent experimental data is discussed.     

Electron transfer compatible molecular design of benzothiophene-acetophenone bichromophores: a theoretical approach

Computational work has been done for a bichromophore (4MBA) comprising a donor 4-methoxy-benzo[b]thiophene (4MBT) and an acceptor molecule p-chloro-acetophenone (pclA) linked together by a HC=CH bond which shows large hyperpolarizability. The charge transfer in this bichromophoric system is computed by semiemperical theoretical calculation. Ground state and excited state dipole moment difference of the bichromophore 4MBA indicates a large electron transfer probability.     

Exciplexes in highly excited triplet states

A mechanism of energy transfer from highly excited triplet aromatic molecules has been developed, which involves a stage of formation of an exciplex between a highly excited energy-donor molecule and an unexcited energy-acceptor molecule. Interpretation of the experimental data on the shape and the intensity of triplet-triplet absorption bands and the energy transfer probability is presented. In this interpretation, the results of quantum-chemical calculations of the energies of highly excited triplet states of toluene and benzene molecules are used.     

Functional end-group polymers derived from free-radical chain transfer. I. Kinetic prediction of functionality

The kinetic scheme describing chain transfer in free-radical polymerizations, the Mayo equation, has been rearranged to yield the probability of generating a functional end group by chain transfer (e.g., to a disulfide). The prediction is that a mixture of functional and nonfunctional ends occurs. Examination of literature data shows a number of systems produce more functionality than predicted. Hence, the challenge to devise a new kinetic scheme to account for the increased functionality is made (e.g., by selective reaction of initiator with chain-transfer agent).     

Doubly charged ion mass spectra of organophosphorus compounds

Doubly charged ion mass spectra have been obtained for 11 organophosphorus compounds. Methane has been used as a target gas to increase the probability of single electron transfer collisions in the first field-free region of an Hitachi RMU-7L mass spectrometer. In general, the spectra of organophosphorus compounds do not exhibit molecular ions but are dominated by fragment ions, many of which must be formed by rearrangement processes. A geometry-optimized self-consistent field molecular orbital method has been employed to compute energies and structural parameters for prominent ions. In addition, a diabatic curve crossing model has been used to examine the single electron transfer reactions responsible for intense ions in the doubly charged ion mass spectra. Appearance energies measured for ions prominent in the 2E spectra of organophosphorus compounds have ranged from 23 to 38 eV.     

A model for energy transfer in inelastic molecular collisions applicable at steady state or non-steady state and for an arbitrary distribution of collision energies

A new model for energy exchange between translational and internal degrees of freedom in atom-molecule collisions has been developed. It is suitable for both steady state conditions (e.g., a large number of collisions with thermal kinetic energies) and non-steady state conditions with an arbitrary distribution of collision energies (e.g., single high-energy collisions). In particular, it does not require that the collision energies be characterized by a quasi-thermal distribution, but nevertheless it is capable of producing a Boltzmann distribution of internal energies with the correct internal temperature under quasi-thermal conditions. The energy exchange is described by a transfer probability density that depends on the initial relative kinetic energy, the internal energy of the molecule, and the amount of energy transferred. The probability density for collisions that lead to excitation is assumed to decrease exponentially with the amount of transferred energy. The probability density for de-excitation is obtained from microscopic reversibility. The model has been implemented in the ion trap simulation program ITSIM and coupled with an Rice-Rampsberger-Kassel-Marcus (RRKM) algorithm to describe the unimolecular dissociation of populations of ions. Monte Carlo simulations of collisional energy transfer are presented. The model is validated for non-steady state conditions and for steady state conditions, and the effect of the kinetic energy dependence of the collision cross-section on internal temperature is discussed. Applications of the model to the problem of chemical mass shifts in RF ion trap mass spectrometry are shown.     

银丝上环氧丙烷分解反应的研究

环氧丙烷均相热分解的反应机理在文献中存在着争论。Thompson和Meissner认为环氧丙烷先异构化为丙醛,生成的激活丙醛分子进一步反应得到一氧化碳、乙烯、乙烷和氢等产物。Lossing等根据在1100K高温下进行的均相热分解反应推断环氧丙烷在裂解过程中首先异构化为丙酮。Benson从热化学动力学的观点出发,认为环氧丙烷因活化形式的不同可以生成两种形式的双启由基,它们分别产生丙醛和丙酮。我们设计的热絲真空反应器曾应用于环氧乙烷在银絲表面的分解反应,发现同均相     

Nonlinearity and irreversibility in the theory of the solvent effect of proton transfer dynamics. II. Dissipative system

The quantum-statistical theory of the dynamics of proton transfer in solvated H-bond complexes is formulated. The theory takes into account a H-bond complex interaction with an environmental fast electronic polarisation as well as the coupling to the environment slow degrees of freedom that are connected with vibrational-rotational motion. The equation of motion for a reduced density matrix is derived in the form of a nonlinear generalised master equation. For the dynamics of the proton transfer in a symmetric double-well potential, the kinetic equation for the reactant state probability density has also been derived and solved. Results for different environments and temperatures are presented.     

Reaction dynamics simulations of the identity S(N)2 reaction H(2)O + HOOH(2)(+)--> H(2)OOH(+)+ H(2)O. Requirements for reaction and competition with proton transfer

Despite the fact that the transition structure of the gas phase S(N)2 reaction H(2)O + HOOH(2)(+)--> HOOH(2)(+)+ H(2)O is well below the reactants in potential energy, the reaction has not yet been observed by experiment. Variational transition state RRKM theory reveals a strong preference for the competing proton transfer reaction H(2)O + HOOH(2)(+)--> H(3)O(+)+ HOOH due to entropy factors. Born-Oppenheimer reaction dynamics simulations confirm these results. However, by increasing the collision energy to around 7.5 eV the probability for nucleophilic substitution increases relative to proton transfer. These observations are explained by the presence of the key common intermediate HOO(H)[dot dot dot]H-OH(2)(+) which leads to effective proton transfer, but can be avoided with increasing collision energy. However, the S(N)2 probability remains below 0.2 since successful passage through the TS requires optimum initial orientation of the reactants, excitation of the relative translational motion and good phase correlation between the O-O vibration and the motion of the incoming water.     

Evidence of the participation of electronic excited states in the mechanism of positronium formation in substitutional Tb(1-x)Eu(x)(dpm)3 solid solutions studied by optical and positron annihilation spectroscopies

Positronium formation in the bimary molecular solid solutions Tb(1-x)Eu(x) (dpm)(3) (dpm = dipivaloylmethanate) has been investigated. A strong linear correlation between the (5)D(4) Tb(iii) energy level excited state lifetime and the positronium formation probability has been observed. This correlation indicates that the ligand-to-metal charge transfer LMCT states act in both luminescence quenching and positronium formation inhibition, as previously proposed. A kinetic mechanism is proposed to explain this correlation and shows that excited electronic states have a very important role in the positronium formation mechanism.     

Constraints in post-synthesis ligand exchange for hybrid organic (MEH-PPV)–inorganic (CdSe) nanocomposites

For an optimum charge/energy transfer performance of hybrid organic–inorganic colloidal nanocrystals for applications such as photonic devices and solar cells, the determining factors are the distance between the nanocrystal and polymer which greatly depends upon nanocrystal size/nanocrystal ligands. Short chain ligands are preferred to ensure a close contact between the donor and acceptor as a result of the tunnelling probability of the charges and the insulating nature of long alkyl chain molecules. Short distances increase the probability for tunnelling to occur as compared to long distances induced by long alkyl chains of bulky ligands which inhibit tunnelling altogether. The ligands on the as-synthesized nanocrystals can be exchanged for various other ligands to achieve desirable charge/energy transfer properties depending on the bond strength of the ligand on the nanocrystal compared to the replacement ligand. In this work, the constraints involved in post-synthesis ligand exchange process have been evaluated, and these factors have been tuned via wet chemistry to tailor the hybrid material properties via appropriate selection of the nanocrystal capping ligands. It has been found that both oleic acid and oleylamine (OLA)-capped cadmium selenide (CdSe) quantum dots (QDs) as compared with trioctylphosphine oxide (TOPO)-passivated CdSe QDs are of high quality, and they provide better steric stability against coagulation, homogeneity, and photostability to their respective polymer:CdSe nanocomposites. CdSe QDs particularly with OLA capping have relatively smaller surface energies, and thus, lesser quenching capabilities show dominance of photoinduced Forster energy transfer between donors (polymer) and acceptors (CdSe nanocrystals) as compared to charge transfer mechanism as observed in polymer:CdSe (TOPO) composites. It is conjectured that size quantization effects, stereochemical compatibility of ligands (TOPO, oleic acid, and oleyl amine), and polymer MEH-PPV stability greatly influence the photophysics and photochemistry of hybrid polymer–semiconductor nanocomposites.     

An application of the dynamical Lie algebraic method to energy transfer of the collinear scattering system AB+CD

The dynamical Lie algebraic method has been applied to treat the V–V and T–V energy transfers in the collinear scattering system AB+CD. The expression for the vibrational transition probability, which contains the main dynamical parameters, is given analytically. By using this expression we probe into the V–V resonance and T–V resonance phenomena appearing in the process of energy transfer. We find that the transition probability of V–V resonance is in good agreement with that obtained using the resonant exchange hypothesis. Then the reliability of the resonant exchange hypothesis is confirmed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two-color resonant multiphoton ionization study of CO. The rotational energy transfer behavior of CO A1 Πstate

Two-color resonant multiphoton ionization provides a novel technique, complementary to LIF, for studying state-to-state molecular relaxation dynamics. A set of formulae applicable to two-color pulsed laser ionization experiments has been derived to evaluate the rotational energy transfer probability for CO(A)-CO(X)collision pairs. The magnitude of this probability is on the order of 1/10 for rotational quantum jump ΔJ = ±1 and diminishes with increasing ΔJ. For the same > J(<±3), a transition with e/f symmetry conservation is always more probable than that with an e/f change.     

Electrochemical charge transfer mediated by metal nanoparticles and quantum dots

Electron transfer processes mediated by nanostructured materials assembled at electrode surfaces underpin fundamental processes in novel electrochemical sensors, light energy conversion systems and molecular electronics. Functionalisation of electrode surfaces with hierarchical architectures incorporating self-assembling molecular systems and materials, such as metal nanostructures, quantum dots, carbon nanotubes, graphene or biomolecules have been intensively studied over the last 20 years. Important steps have been made towards the rationalisation of the charge transfer dynamics from redox species in solution across molecular self-assembling systems to electrode surfaces. For instance, a unified picture has emerged describing the factors which determine the rate constant for electron transfer processes across rigid self-assembling molecular barriers. An increasing bulk of evidence has recently shown that the incorporation of nanomaterials into self-assembling monolayers leads to an entirely different electrochemical behaviour. This perspective rationalises some of the key observations associated with nanoparticle mediated charge transfer, such as the apparent distance independent charge transfer resistance observed for redox species in solution. This behaviour only manifests itself clearly in the case where the probability of direct charge transfer from the redox probe to the electrode is strongly attenuated by self-assembling molecular barriers. Here we will highlight specific issues concerning self-assembled monolayers as blocking barriers prior to discussing the effect of nanoparticles on the electrochemical response of the system. Selected examples will provide conclusive evidence that the extent of charge transfer mediation is determined by the overlap between the density of states of the nanostructures and the energy levels of redox species in solution. Only in the case where a strong overlap exists between the energy levels of the two components, the nanostructures behave as "electron launchers", allowing efficient charge transfer across insulating molecular layers.