An extended dynamics method for classical and cumulant variables is formulated to take fluctuation effects into account directly. In particular, we have derived the coupled equations of motion (EOM) for the position, momentum, and second-order cumulants of the product of the momentum and position fluctuation operators for both quantum and classical regimes. The second-order quantal and classical cumulant dynamics have almost the same structure with proper initial conditions and more terms to describe friction for latter equations. We demonstrated that the present methods give the exact answer for the harmonic oscillator and are applied to analyze the dynamical quantum isotope effects on a model proton transfer reaction in a Guannine-Cytosine base pair and to obtain a thermal equilibrium state of 7-particles classical Morse cluster.

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Single molecule junctions, in which a single molecule bridges between metal electrodes, have attracted wide attention. It is because novel properties can appear in the single molecule junction due to its peculiar geometrical and electronic characters. The single molecule junction has also attracted attention due to its potential application in ultrasmall electronic devices, where single molecules are utilized as active electronic components. Thus, fabrication of single molecule junctions, as well as understanding and finding their unique properties (e.g. conductance, optical and magnetic properties), have become long-standing goals of scientists and engineers. Along with our studies on single molecular junction, the following subjects are reviewed (i) fabrication of single molecular junction (ii) characterization (iii) finding novel properties appeared in the single molecular junction.

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X-ray and electron scattering experiments are powerful tools for investigating the electronic structures of molecules. The momentum transfer dependence of the scattering cross sections provides valuable information about the shape of molecular orbitals, correlated motion of electrons, vibronic effects on electronic excitations and so on. In this article, recent developments in x-ray and electron scattering studies on electronic structures and electronic excitation dynamics of molecules are reviewed. In particular, we focus on the following three topics. The first topic is an x-ray scattering study on electron correlation. Total (elastic+inelastic) x-ray scattering intensity as well as inelastic scattering intensity are related to electron pair distribution function through Fourier transformation, and hence comparisons between observed and calculated x-ray scattering intensities allow us to make a critical assessment of whether or not electron correlation is properly taken into account in the theoretical wave function. The second topic is interference effects on electron momentum profiles studied using (e, 2e) electron momentum spectroscopy. An analysis of the measured electron momentum densities for the F 2p nonbonding orbitals of CF₄ has clearly shown the presence of oscillatory structure having information about the orientation in space of the constituent F 2p atomic orbitals. The third topic is electron energy loss spectroscopy (EELS) studies of vibronic effects on electronic excitations in molecules. Examining the momentum transfer dependence of the EELS cross section makes it possible to see coupling between electronic states. By using this method we have revealed the roles of vibronic coupling in some valence-shell electronic excitations in CF₄ and CO₂.

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Interface-selective vibrational sum frequency generation (VSFG) spectroscopy is a unique and powerful tool for the research in interface science. In particular, structures of water at various interfaces have been discussed based on VSFG spectroscopy. However, in conventional VSFG spectroscopy, only intensity of the sum frequency light is measured and therefore it provides only the modulus square of a second-order nonlinear susceptibility (|χ⁽²⁾|²). This results in a few serious drawbacks. First, sign of χ⁽²⁾ is lost, which means we lose information about polar orientation at the interface. Second, interpretation of |χ⁽²⁾|² spectrum is very complicated due to the spectral deformation caused by interference between neighboring resonances. To solve these problems, it is essential to measure complex χ⁽²⁾ directly. Recently, we have developed multiplex heterodyne-detection of VSFG (HD-VSFG), which enables us to measure complex χ⁽²⁾. Furthermore, because our HD-VSFG spectroscopy employs femtosecond infrared light, it can be extended to ultrafast time-resolved measurements by combining with pump-probe technique. Using this novel technique, we have been studying the structure and dynamics of water at various interfaces. This review article overviews our current understanding of the structure and dynamics of water at interfaces and the recent debate on the “ice-like” model of interfacial water.

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A new type of rechargeable battery, the molecular cluster battery (MCB), has been developed by utilizing molecular clusters such as Mn12 and polyoxometalate (POM) clusters, as cathode active materials and a lithium metal as an anode. It was found that MCBs can exhibit higher battery capacities than conventional lithium ion batteries. In operando X-ray absorption fine structure analyses on MCBs of Mn12 and POMs revealed that they exhibit 8- and 24-electrons reduction during discharge, respectively. Such electron sponge behavior realized only in solid-state electrochemistry resulted in a high battery capacity. To improve the battery performances furthermore, nanohybrid materials of single-walled carbon nanotubes and POMs were prepared, and both charging/discharging rate and battery capacity of their MCBs were significantly better than those of the non-nanohybrid MCBs. Finally, in situ magnetic measurements under solid-state electrochemistry on a mixed-valent chromium Prussian blue analogue ferrimagnet revealed continuous changes in magnetization, transition temperature and coercive force. It is demonstrated that solid-state electrochemistry is a useful technique to explore various physical properties from fundamental sciences to applications.

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Nanomaterials which exhibit both stability and functionality are currently considered to hold the most promise as components of nanotechnology devices. Thiolate (RS)-protected gold nanoclusters (Au_n(SR)_m) have attracted significant attention in this regard and, among these, the magic clusters are believed to be the best candidates since they are the most stable. We have investigated the effects of heteroatom doping, protection by selenolate ligands and protection by photoresponsive thiolates on the stability and physical/chemical properties of these clusters. Through such studies, we have attempted to establish methods of modifying magic Au_n(SR)_m clusters as a means of creating metal clusters that are both robust and functional. This paper summarizes our studies towards this goal and the obtained results.

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To understand molecular mechanism of biological functions, chemical reactions of biological molecules should be elucidated. For that purpose, we have developed new techniques to reveal the spectrally silent dynamics of the reactions and to unify the thermodynamics and kinetics of the reactions. In particular, we focus our attention on the “fluctuation” in this review. For example, proteins are flexible systems and the structures should be fluctuating at room temperature in aqueous solution due to the thermal energy. We found that the “fluctuations” of biological molecules are key factors for understanding reactions, and essentially important for understanding the functions. For proving the importance of the fluctuation, we showed that the fluctuation is indeed enhanced during the chemical reactions of biomolecules. We mainly used the transient grating (TG) method to detect the time dependence of the thermodynamical properties and diffusion coefficients. In particular, we succeeded in measurements of the thermal expansion coefficient, the heat capacity, and the isothermal compressibility, which are directly linked to the fluctuation, in time domain. These techniques have quite wide applicability to many reaction systems. We found that the fluctuation is closely related with the high efficiency and selectivity of the reactions of proteins.

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This paper reviews the spectroscopic research on the microscopic solvation of electron, alkali metal atoms, alkali-earth metal ions, alkali-like hypervalent radicals, and biological molecules in relation to widely-spread chemical and biological phenomena in solution. Motivation for this research is to understand how the properties of molecular clusters vary as a function of size, particularly the stepwise development of condensed phase attributes, thus allowing us to bridge the gap between isolated molecules and solution. These studies revealed the elemental processes of solvation phenomena. Alkali atom systems show how the solvated electron forms in small clusters and the role of solvents. Alkali-earth atom systems provide us the answer to the ultimate question how the solvent controls the oxidation modes of metal ions. In particular, the hypervalent radicals, which are the phantom species both in the gas and solution phases, behave like alkali atom and play the key role in many chemical reactions.

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