Hiroshi Ushiyama

Redox-responsive molecular helices with highly condensed π -clouds

Helices have long attracted the attention of chemists, both for their inherent chiral structure and their potential for applications such as the separation of chiral compounds or the construction of molecular machines. As a result of steric forces, polymeric o-phenylenes adopt a tight helical conformation in which the densely packed phenylene units create a highly condensed π-cloud. Here, we show an oligomeric o-phenylene that undergoes a redox-responsive dynamic motion. In solution, the helices undergo a rapid inversion. During crystallization, however, a chiral symmetry-breaking phenomenon is observed in which each crystal contains only one enantiomeric form. Crystals of both handedness are obtained, but in a non-racemic mixture. Furthermore, in solution, the dynamic motion of the helical oligomer is dramatically suppressed by one-electron oxidation. X-ray crystallography of both the neutral and oxidized forms indicated that a hole, generated upon oxidation, is shared by the repeating o-phenylene units. This enables conformational locking of the helix, and represents a long-lasting chiroptical memory.

Successive mechanism of double-proton transfer in formic acid dimer: A classical study

The dynamics of double-proton transfer reaction in formic acid dimer is investigated by performing ab initio molecular dynamics simulations. From the viewpoint of optimized energetics alone, the synchronous (simultaneous) proton transfer is more favorable than the successive one. However, a full-dimensional classical dynamics shows that there is a certain time lag, about 8 fs in average, between two proton transfers. When a proton undergoes the first transfer, it moves from an oxygen with higher electron density to the counterpart having the lower one. The proton thus needs an energy sufficient enough to break the chemical bond, resulting in a clime of a potential barrier. On the other hand, the second proton moves from the lower electron-density oxygen atom to the higher one. Hence, the second proton is shifted predominantly by the thus-formed electronic field. Not only due to the time lag observed but mainly because of the difference in the mechanism of transfer, therefore, the present double-proton tra...

Tunneling paths in multi-dimensional semiclassical dynamics

Abstract In light of the fundamental importance and renewed interest of the tunnel phenomena, we review the recent development of semiclassical tunneling theory, particularly from the view point of “tunneling path”, beginning from a simple one-dimensional formula to semiclassical theories making use of the analytic continuation, in time, coordinates, or momentum, which are the stationary solutions of semiclassical approximations to the Feynman path integrals. We also pay special attention to the instanton path and introduce various conventional and/or intuitive ideas to generate tunneling paths, to which one-dimensional tunneling theory is applied. Then, we review the recent progress in generalized classical mechanics based on the Hamilton–Jacobi equation, in which both the ordinary Newtonian solutions and the instanton paths are regarded as just special cases. Those new complex-valued solutions are generated along real-valued paths in configuration space. Such non-Newtonian mechanics is introduced in terms of a quantity called “parity of motion”. As many-body effects in tunneling, illustrative numerical examples are presented mainly in the context of the Hamilton chaos and chemical reaction dynamics, showing how the multidimensional tunneling is affected by the system parameters such as mass combination and anisotropy of potential functions.

Theoretical study of the mechanism of electron transfer at photosynthetic reaction centers. I. Singlet excited states of free base porphin

Free base porphin is a key unit in the electron transfer reaction at photosynthetic reaction centers. For the electron transfer reaction, the transfer integral of the rate constant depends strongly on the quality of the wave functions of porphin-based chromophores. Therefore, we need a stable method for calculating the wave functions of optically allowed excited states of the porphin. We developed such a method and verified its stability by calculating the wave functions for an ethylene molecule. We confirmed that the optically allowed excited states required the entire amount of valence molecular orbitals for the active space to adequately describe the wave function with molecular in–out polarization. We applied our calculation method to investigate the wave function of free base porphin. Our ab initio calculation used 4-31 G plus d polarization functions for the carbon and nitrogen atoms, and Rydberg 2d basis functions on the center of each pyrrol ring simultaneously. We also proposed an assignment for ...

Time-resolved photoelectron spectroscopy of proton transfer in the ground state of chloromalonaldehyde: wave-packet dynamics on effective potential surfaces of reduced dimensionality.

We report on a simple but widely useful method for obtaining time-independent potential surfaces of reduced dimensionality wherein the coupling between reaction and substrate modes is embedded by averaging over an ensemble of classical trajectories. While these classically averaged potentials with their reduced dimensionality should be useful whenever a separation between reaction and substrate modes is meaningful, their use brings about significant simplification in studies of time-resolved photoelectron spectra in polyatomic systems where full-dimensional studies of skeletal and photoelectron dynamics can be prohibitive. Here we report on the use of these effective potentials in the studies of dump-probe photoelectron spectra of intramolecular proton transfer in chloromalonaldehyde. In these applications the effective potentials should provide a more realistic description of proton-substrate couplings than the sudden or adiabatic approximations commonly employed in studies of proton transfer. The resulting time-dependent photoelectron signals, obtained here assuming a constant value of the photoelectron matrix element for ionization of the wave packet, are seen to track the proton transfer.

Tunneling splittings. A classical trajectory approach

Abstract We propose an extension of the well-known Makri–Miller model [J. Chem. Phys. 91 (1989) 4026] for the calculation of tunneling splittings of ground and excited states of symmetric double well potentials. Our approach is based on generalized trajectories which can penetrate into the classically forbidden region. The performance of the new method is demonstrated for a two-dimensional model potential. We compare our results with data from exact quantum mechanical calculations as well as from various semiclassical methods. The numerical effort is only slightly increased as compared to that needed for the Makri–Miller model.

Efficient calculations of classical trajectories and stability matrices for semiclassical theory with locally analytic integrator. The Hulme method revisited

We demonstrate that quantities such as classical paths, action integrals, stability matrix, caustics, and so on, which are all required in semiclassical chemical dynamics, can be integrated very efficiently by means of a locally analytic integrator (LAI). Hulmes collocation method is improved to carry out these integrations systematically. LAI solves ordinary differential equations (ODEs) by recasting the set of ODEs into a set of nonlinear equations. An individual solution in each dimension is represented in terms of an analytic function of time for a short interval. We explicitly show that the local analyticity brings about distinct advantages.

MOLECULAR MECHANICS WITH QEQ-CS (CHARGE EQUILIBRATION METHOD GENERALIZED FOR CHARGE SEPARATION SYSTEM)

Abstract In this article we put forward an idea to find a crossing point of a photoinduced electron transfer reaction of a large system such as a photosynthetic reaction center. The system requires describing the energy by Molecular Mechanics (MM) with the charge equilibration method generalized for a charge separation system (QEq-CS). The QEq-CS determines atomic charges on system circumstances, and reproduces ab initio results. A combination of MM with QEq-CS and the Koga–Morokuma scheme shows the useful applications to two test cases consisting of three benzene molecules.

Real-time observation of intramolecular proton transfer in the electronic ground state of chloromalonaldehyde: An ab initio study of time-resolved photoelectron spectra

The authors report on studies of time-resolved photoelectron spectra of intramolecular proton transfer in the ground state of chloromalonaldehyde, employing ab initio photoionization matrix elements and effective potential surfaces of reduced dimensionality, wherein the couplings of proton motion to the other molecular vibrational modes are embedded by averaging over classical trajectories. In the simulations, population is transferred from the vibrational ground state to vibrationally hot wave packets by pumping to an excited electronic state and dumping with a time-delayed pulse. These pump-dump-probe simulations demonstrate that the time-resolved photoelectron spectra track proton transfer in the electronic ground state well and, furthermore, that the geometry dependence of the matrix elements enhances the tracking compared with signals obtained with the Condon approximation. Photoelectron kinetic energy distributions arising from wave packets localized in different basins are also distinguishable and could be understood, as expected, on the basis of the strength of the optical couplings in different regions of the ground state potential surface and the Franck-Condon overlaps of the ground state wave packets with the vibrational eigenstates of the ion potential surface.

Dynamical hydrogen atom tunneling in dichlorotropolone: A combined quantum, semiclassical, and classical study

Based on the Cartesian Reaction Surface framework we construct a four-dimensional potential for the tropolone derivative 3,7-dichlorotropolone, a molecule with an intramolecular O-H...O hydrogen bond. The reduced configuration space involves the in-plane hydrogen atom coordinates, a symmetric O-O vibrational mode, and an antisymmetric mode related to deformations of the seven-membered ring. The system is characterized in terms of quantum mechanical computations of the low-lying eigenstates as well as a classical and semiclassical analysis of spectra obtained via Fourier transforming autocorrelation functions. For the semiclassical analysis we utilize the amplitude-free correlation function method [K. Hotta and K. Takatsuka, J. Phys. A 36, 4785 (2003)]. Our results demonstrate substantial anharmonic couplings leading to highly correlated wave functions even at moderate energies. Furthermore, the importance of dynamical tunneling in tropolone is suggested since many low-lying states--including the ground state--lie above the classical saddle point but nevertheless appear as split pairs.