Nuclear quantum effects, such as zero-point vibration and tunneling, are important aspect in molecular systems containing light atoms like hydrogen, and it is considered to have substantial influence on their chemical behavior and physical properties. In this report, I will describe the basic idea of path integral molecular dynamics simulations which enables one to compute nuclear quantum effects in complex many-body systems. When the method is combined with ab initio electron structure calculations, the whole molecular entity composed of electrons and nuclei is treated quantum mechanically based on first principles within the framework of Born-Oppenheimer approximation. Using this technique, the quantum nature of strong hydrogen bonds in protonated and deprotonated water clusters have been studied, with a focus on the geometrical isotope effect.
Intense (1012-1013 W/cm2) phase-controlled laser fields consisting of a fundamental light and a second-harmonic light induce the directionally asymmetric tunneling ionization and the resultant orientation-selective molecular ionization. It is demonstrated that orientation-selective molecular ionization induced by phase-controlled ω + 2ω laser fields reflects the geometric structure of the highest occupied molecular orbital. This method was robust, being free of both laser wavelength and pulse-duration constraints, and thus can be applied to a wide range of molecules.
Highly localized plasmons can be excited at atomically smooth metal surfaces by using a sphere-plane type plasmonic cavity. This plasmonic structure can be optimized for use in well-organized self-assembled monolayers formed on various single crystalline metal substrates including highly damping platinum-group catalytic metals, and is therefore useful in the fields of molecular science, catalytic science, and surface science. Surface enhanced Raman scattering (SERS) observation at well-defined molecule-metal interfaces revealed crystallographic orientation dependence not only in adsorption geometry of the molecules but also in the signal enhancement, suggesting a contribution of interfacial charge transfer resonances between metal states and molecular affinity levels. Moreover, this method enables us to increase efficiency of various photochemical processes such as photo-energy conversion when photo-sensitive molecular layers are formed on a substrate. The use of well-defined metal-organic system in plasmonic cavities opens up a new possibility of spectroscopy and photochemistry.
Ferroelectricity has been time-honored subject in terms of various electronic, electro-mechanical, and optical functions. Developing organic ferroelectrics with advantages of light-weight, flexible, low-cost, and environmentally benign characteristics is surely in demand, yet needs elaborate chemical designs of objective functions. This review describes modern chemical approach to improved dielectric, ferroelectric and/or thermal properties, based on the charge-transfer complexes and hydrogen-bonded compounds.
It has been a long standing problem to answer the following fundamental questions in chemistry; (1) what kinds of chemical compounds (isomers) can be produced from a set of atoms given by a chemical formula, such as H4C2O2, (2) how the isomers can be converted to one another, (3) how they are decomposed into smaller species, or conversely (4) how they are made of smaller species. Although this problem can be solved theoretically if all minima and pathways among them via saddles could be searched on the potential energy hypersurface, it has been believed to be impossible, when the number of atoms exceeds four in the target chemical formula. A very simple tool like a compass for voyage could be discovered for global reaction route mapping (GRRM) in the chemical world. That is the anharmonic downward distortion (ADD) which enables one to follow all reaction pathways from an equilibrium (EQ) point toward structures of transition states (TS) surrounding the EQ point. Subsequent downward followings from already found TSs can easily be made as conventional intrinsic reaction coordinate (IRC) followings to reach some EQ points and dissociation channels (DC). Further quests around newly found EQs will yield many more reaction pathways via many other TSs. Such one-after-another procedures will continue until no new EQ could be found, and finally one can obtain a global reaction route map of the chemical formula as well as the answers to the fundamental questions in chemistry. Now, one should step into the new era of chemical problems to perform a perfect microscopic control of chemistry involved in the stereo reaction dynamics of atoms and molecules, based on leading information on typical trajectories searched by the automated exploration of chemical reaction pathways.
Rhodopsins contain a retinal molecule, and convert light into chemical energy or signal. The chromophore of visual or microbial rhodopsins is a retinal Schiff base of the 11-cis or all-trans form, respectively, where specific chromophore-protein interaction determines their colors. Upon light absorption, ultrafast photoisomerization initiates protein structural changes, leading to each functional expression. By use of spectroscopic methods, we have been studying how rhodopsins respond to light. Ultrafast spectroscopy of visual rhodopsin revealed that cis-trans isomerization is the primary event in our vision, which is optimized in protein environment. Fourier-transform infrared (FTIR) spectroscopy of visual and microbial rhodopsins provides various important vibrational bands related to structural changes of these proteins. Detection of protein-bound water molecules is one of the research highlights, and the comprehensive FTIR study has shown that a strongly hydrogen-bonded water molecule is the functional determinant of light-driven proton pump proteins. Here I review our spectroscopic challenge for > 25 years, particularly focusing our recent findings.
Chemical reactions and pathways involving a metal cluster change dramatically with the cluster size and composition. These dependences originate essentially from the electron correlation of the cluster, and hence are closely related to the geometrical and electronic structures of the cluster. Along with our recent studies, the following subjects are reviewed: (i) reactivity, (ii) geometrical and electronic structures in relation to the reactivity, (iii) reaction dynamics, and (iv) catalytic properties.