Shin-ichi Furukawa

Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character

The origin of the diradical character dependence of the second hyperpolarizability (gamma) of neutral singlet diradical systems is clarified based on the perturbation formula of gamma using the simplest diradical molecular model with different diradical characters, i.e., H2 under bond dissociation. The enhancement of gamma in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, respectively, with the increase in diradical character. This feature confirms that open-shell singlet conjugated molecules with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose gamma values can be controlled by the diradical character in addition to the conjugation length.

Investigation of the dominant hydration structures among the ionic species in aqueous solution: Novel quantum mechanics/molecular mechanics simulations combined with the theory of energy representation

In the present work, we have performed quantum chemical calculations to determine preferable species among the ionic complexes that are present in ambient water due to the autodissociation of water molecule. First, we have formulated the relative population of the hydrated complexes with respect to the bare ion (H(3)O(+) or OH(-)) in terms of the solvation free energies of the relevant molecules. The solvation free energies for various ionic species (H(3)O(+), H(5)O(2) (+), H(7)O(3) (+), H(9)O(4) (+) or OH(-), H(3)O(2) (-), H(5)O(3) (-), H(7)O(4) (-), H(9)O(5) (-)), categorized as proton or hydroxide ion in solution, have been computed by employing the QM/MM-ER method recently developed by combining the quantum mechanical/molecular mechanical (QM/MM) approach with the theory of energy representation (ER). Then, the computed solvation free energies have been used to evaluate the ratio of the populations of the ionic complexes to that of the bare ion (H(3)O(+) or OH(-)). Our results suggest that the Zundel form, i.e., H(5)O(2) (+), is the most preferable in the solution among the cationic species listed above though the Eigen form (H(9)O(4) (+)) is very close to the Zundel complex in the free energy, while the anionic fragment from water molecules mostly takes the form of OH(-). It has also been found that the loss of the translational entropy of water molecules associated with the formation of the complex plays a role in determining the preferable size of the cluster.

Novel quantum mechanical/molecular mechanical method combined with the theory of energy representation: free energy calculation for the Beckmann rearrangement promoted by proton transfers in the supercritical water.

The Beckmann rearrangement of acetone oxime promoted by proton transfers in the supercritical water has been investigated by means of the hybrid quantum mechanical/molecular mechanical approach combined with the theory of energy representation (QM/MM-ER) recently developed. The transition state (TS) structures have been explored by ab initio calculations for the reaction of hydrated acetone oxime on the assumption that the reaction is catalyzed by proton transfers along the hydrogen bonds connecting the solute and the solvent water molecules. Up to two water molecules have been considered as reactants that take part in the proton transfers. As a result of the density functional theory calculations with B3LYP functional and aug-cc-pVDZ basis set, it has been found that participation of two water molecules in the reaction reduces the activation free energy by -12.3 kcal/mol. Furthermore, the QM/MM-ER simulations have revealed that the TS is more stabilized than the reactant state in the supercritical water by 2.7 kcal/mol when two water molecules are involved in the reaction. Solvation free energies of the reactant and the TS have been decomposed into terms due to the electronic polarization of the solute, electron density fluctuation, and others to elucidate the origin of the stabilization of the TS as compared with the reactant. It has been revealed that the promotion of the chemical reaction due to the hydration mainly originates from the interaction between the nonpolarized solute and the solvent water molecules at the supercritical state.

Computer simulation studies on gas permeation of propane and propylene across ZSM-5 membranes by a non-equilibrium molecular dynamics technique

A non-equilibrium molecular dynamics (NEMD) technique has been utilized to simulate permeation of pure and mixed gases across ZSM-5 membranes. Propane and propylene are chosen as permeating gases. The ideal separation factor and the permselectivity of propylene to propane calculated from simulations are in good agreement with the experimental data though the permeabilities from simulations are about one order higher than those of experiments. For pure gas permeation, permeability of propane is smaller than that of propylene, while for mixed-gas permeation, it becomes larger than that of propylene due to the competitive adsorption of propane. The present simulations have suggested that the permeation mechanism of propane and propylene across ZSM-5 membranes is selective adsorption followed by diffusion in zeolite pores.

Simulation Performance of a Non-Equilibrium Molecular Dynamics Method Using Density Difference as Driving Force

Abstract The simulation performance of two NEMD algorithms, the constant density difference (DD) and the constant chemical potential difference (CPD) methods, has been compared in fluctuations of molar flux for He and CH4 permeation across the ZSM-5 membrane. The CPD method and the DD method are found to give almost the same performance; however, the former seems slightly superior to the latter in terms of low fluctuations of molar flux though the former needs more CPU time than the latter. An advantage of the DD method is that it can simulate the mixed-gas permeation through a membrane under the specification of high and low pressures and the composition of feed gas. It is shown that the density profile of permeating gas could provide important information about the relative resistance at the entrance, inside, and exit regions for permeation.

Exciton recurrence motion in aggregate systems in the presence of quantized optical fields

The exciton dynamics of model aggregate systems, dimer, trimer, and pentamer, composed of two-state monomers is computationally investigated in the presence of three types of quantized optical fields, i.e., coherent, amplitude-squeezed, and phase-squeezed fields, in comparison with the case of classical laser fields. The constituent monomers are assumed to interact with each other by the dipole-dipole interaction, and the two-exciton model, which takes into account both the one- and two-exciton generations, is employed. As shown in previous studies, near-degenerate exciton states in the presence of a (near) resonant classical laser field create quantum superposition states and thus cause the spatial exciton recurrence motion after cutting the applied field. In contrast, continuously applied quantized optical fields turn out to induce similar exciton recurrence motions in the quiescent region between the collapse and revival behaviors of Rabi oscillation. The spatial features of exciton recurrence motions are shown to depend on the architecture of aggregates. It is also found that the coherent and amplitude-squeezed fields tend to induce longer-term exciton recurrence behavior than the phase-squeezed field. These features have a possibility for opening up a novel creation and control scheme of exciton recurrence motions in aggregate systems under the quantized optical fields.

Influence of Gas–Solid Kinetic Energy Exchange Processes on Gas Effusion from Slitpores in Non-equilibrium Molecular Dynamics Simulations

The boundary-driven type non-equilibrium molecular dynamics (BD-NEMD) simulations have been carried out to clarify the influence of solid flexibility on gas effusion through a slitpore, by using a rigid solid model (R-model) and a flexible solid model (F-model). The LJ potential particles with parameters of argon are chosen for both the effusing gas molecules and the solid atoms. It is found that the R-model combined with a velocity scaling technique provides a reasonable effusion flux, probably a maximum in magnitude, compared with the fluxes calculated from the F-model without a fictitious thermostat (a velocity scaling) for effusing molecules. In the F-model, temperature lowering has been observed at the pore exit, being enhanced with an increase in the strength of springs that unite the solid atoms. This observation indicates that the molecules escaping from the pore exit take the excess kinetic energy for evaporation from solid atoms or surrounding molecules, which results in the low exit temperature, and that the rate of gas–solid kinetic energy exchange decreases with increasing spring strength (i.e. decreasing solid flexibility). It is suggested that the effusion flux is influenced by two factors: the solid flexibility and the molar potential energy in the pore.

Theoretical Study on the Polarizabilities of Molecules in Solution by the Quantum Mechanical/Molecular Mechanical Approach: Comparison with the Polarizable Continuum Model

We compute the static polarizabilities of formaldehyde and ethylene in water solution by means of the QM/MM simulations and the polarizable continuum model for the purpose to examine the dependence of the polarizability on the method to include solvent effects. It is found that the polarizability of formaldehyde as well as ethylene is hardly changed by the solvent modeled by QM/MM approach. The results are reasonably consistent with suggestions given by other theoretical investigations which revealed that polarizabilities of a monomer in the linear hydrogen bonded chain is not increased by the hydrogen bondings. In contrast to the QM/MM method, it turns out that the polarizabilities are clearly enhanced for the solutes embedded in the dielectric continuums though both methods indicate that the σ electrons dominantly contribute to the polarizability.

Effects of peptide antigenic determinant properties on adsorption equilibrium of anti-peptide antibodies

Abstract The antibodies to hydrophilic and hydrophobic peptides representing residues 85–94 of cytochrome c (P1) and 148–158 of tobacco mosaic virus protein (P2), respectively, were produced in rabbits. Peptide-keyhole limpets hemocyanin (KLH) conjugates and peptide-polyvinylpyrrolidone (PVP) mixtures were used for immunization. From the binding experiments of analogous synthetic peptides in immunoadsorbents, the major fractions of all the polyclonal anti-peptide antibodies were found to bind to the antigenic determinants localized in the four to six C-terminal amino acids of the peptides. In both the anti-P1 and anti-P2 antibodies produced by immunizing peptide-KLH conjugates, the C-terminal carboxylic acid group was essential for antibody binding. On the other hand, the effect of pH and ionic strength on the adsorbed amount of the antibodies depended on the peptide. That is, the adsorbed amount of P1 to both the anti-P1 antibodies produced by the different immunization methods decreased significantly with increasing ionic strength, while the adsorbed amount of P2 to the antibodies exhibited a minimum at around ionic strength 1.0. These results indicate that the dependence of antibody affinities on pH and ionic strength is related to the properties of antigenic determinants.

Computation of the Reduction Free Energy of Coenzyme in Water: A Novel Approach within the Framework of the QM/MM‐ER Method

We have computed free energy for one‐electron reduction of the active site of the coenzyme (flavin adenine dinucleotide (FAD)) in aqueous solution by means of the QM/MM method combined with the theory of energy representation (QM/MM‐ER). In the present work, we have proposed a novel approach that the excess electron to be attached on the FAD has been regarded as a solute, while the remaining molecules including the active site of FAD have been considered as solvent in the implementation of the method of energy representation. The efficiency and the accuracy of the method have been examined by performing the conventional simulations where the oxidized and the reduced active sites of FAD are regarded as solutes and the surrounding water molecules are treated as solvent. The present approach is computationally more advantageous and is amenable to the extension to the reaction in the protein systems as compared with the conventional one. It has been found that the reduction free energy obtained by the present...