Complex chemical systems consisting of transition metal element(s) are important and attractive research targets in both experimental chemistry and theoretical chemistry. Transition-metal complexes with carbon dioxide are discussed as one example, in which the coordination geometry, bonding nature, and reactivity are understood well and predicted with the HOMO and the number of d electrons. The spin-multiplicity of inverse sandwich-type dinuclear transition-metal complexes, which have been synthesized recently as a new type of compound, is discussed as another example based on CASPT2 calculations, in which the clear relationship between the spin multiplicity of its ground state and the number of d electrons is presented. Theoretical understanding of various organometallic reactions has been an important endeavor over the last two decades. Insertion reactions of olefin and carbon dioxide into M–H and M–alkyl bonds and σ-bond activation reactions are discussed with orbital interaction diagrams based on perturbation theory. In particular, detailed discussion of the characteristic features of a new type of oxidative addition to an M–L moiety (L = neutral ligand such as alkene and alkyne) and heterolytic σ-bond activation by an M–X moiety (X = anionic ligand). It is still a central challenge to elucidate the reaction mechanisms of catalytic reactions by transition metal complexes. The reaction mechanisms and electronic processes of Ru-catalyzed hydrogenation of carbon dioxide, Pt-, Rh-, and Zr-catalyzed hydrosilylations of alkene, Ir-catalyzed borylation of benzene, and the Hiyama cross-coupling reaction are analyzed based on computational results. We wish to present how to understand the mechanism based on the number of d electrons and the energy of d orbitals in discussion. Also, the importance of solvation and crystalline effects in the theoretical study of transition-metal complexes is discussed based on our recent theoretical studies of mixed-valence complex and a single crystal of a Pt(II) complex.
Complex systems consisting of transition metal element(s) are theoretically investigated with electronic structure theory. Well understanding of their geometries, chemical bonds, reactivities, and catalyses, as well as some theoretical predictions, is provided. Discussion is also presented on what are necessary in theoretical study of the complex systems in furture.
Time-resolved fluorescence spectra of 4′-N,N-diethylamino-3-methoxyflavon in ionic liquids have been measured by using a Kerr gate method and a streak camera at different excitation wavelengths, 370, 400, and 425 nm. Fluorescence spectra showed a shift to longer wavelength with longer delay time after photoexcitation. The peak position of the fluorescence spectrum at each delay time was evaluated by simulating the spectrum by a log-normal function. It was found that the peak shift of the fluorescence at the early delay time was larger for the shorter excitation wavelength (blue side of the absorption), and in the longer delay time they coincide to the same value. The time profile of the peak position was well simulated by the sum of an exponential function and a stretched-exponential function. The average relaxation times evaluated by the integration of the longer relaxation function were weakly dependent on the excitation wavelength, and they were correlated with the viscosity of the solvent. The relation with the excitation wavelength dependence of the excited state intramolecular proton transfer reaction of 4′-N,N-diethylamino-3-hydroxyflavon is discussed.
Solvation dynamics of in ILs is strongly dependent on the excitation wavelength. The number of alkyl-carbon of cation affects the initial dynamics of the solvation.
The key tertiary interactions in the riboswitch aptamer that are critical for flavin mononucleotide binding were identified by structure-guided mutations. Binding affinity was reduced significantly (more than 16-fold) after disruption of non-canonical G·G base pair and enhanced moderately (up to 2.1-fold) after mutation in the base triple or T-loop like motif.
Humans can sense water on a solid substrate without sliding their fingertips. In this study, 20 subjects were presented with small amounts of pure water and aqueous solutions containing various amounts of a thickener. Many subjects distinguished a watch glass containing water from those containing aqueous solutions of the thickener. The most characteristic tactile texture of water reported by participants was a non-slimy feel. Furthermore, friction evaluations and high-speed observations identified frictional resistance as a distinguishing characteristic between water and the thickened aqueous solutions. Notably, no significant fingertip movements were observed when using a high-speed camera, although the frictional coefficients for water (0.18), the 1.00 wt % thickened solution (0.10) and the 5.00 wt % thickened solution (0.05) were significantly different. A regression analysis of the relationship between the tactile evaluation and friction properties suggested that many subjects evaluated the degree of sliminess based on the fluctuation of the frictional coefficient during contact of their fingertips with the watch glasses.
The formation of insulin amyloid fibrils under acidic conditions was accelerated in the liquid–liquid systems, especially in the toluene/water system. High interfacial adsorptivity of insulin monomer was confirmed by interfacial tension measurement. Fluorescence microscope observation using a thioflavine T (ThT) probe proved directly the interfacial formation and adsorption of the amyloid fibril at the dodecane/water interface. Far-UV-circular dichroism (CD) spectra of insulin in the dodecane/water system suggested the conversion of α-helix form to the β-sheet in the aqueous phase. Furthermore, the total internal reflection-induced CD measurement of the amyloid fibril–ThT complex at the dodecane/water interface led to the conclusion that the amyloid formation at the interface is faster than the conversion of α-helix form in the aqueous phase.
Introduction of an electron-donating methoxy group to 2-[2-(2-pyrrolyl)ethenyl]pyridine, which undergoes photoinduced intramolecular hydrogen atom transfer and one-way trans-to-cis photoisomerization, induced significant changes in these photochemical behaviors.
Optical resolution of C2-symmetric racemic 1,4-diols, 1,2-bis(1-hydroxyalkyl)benzene, was examined using (S)-5-allyl-2-oxabicyclo[3.3.0]octene ((S)-ALBO-V) as chiral resolving agent. Diastereomeric acetals obtained from the 1,4-diols with (S)-ALBO-V were easily separated by silica-gel column chromatography. After removal of the resolving agent, both enantiomers of the 1,4-diols were obtained with high enantiomeric excesses.
Two-dimensional self-assembled structures formed by diketopyrrolopyrrole Pigment Red 254 (3,6-bis(4-chlorophenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) and its alkyl-derivatives at the liquid-HOPG interface were investigated by scanning tunneling microscope (STM). Pigment Red 254 is found to form two different surface structures (crystal-like structure and solvated crystal-like structure). Furthermore, in order to investigate the effect of hydrogen bonding, alkyl-derivatized Pigment Red 254 (PR-2Cn, n: alkyl chain length) were synthesized, and their self-assembled structures were analyzed. We found that surface structures of PR-2Cn varied depending on the intermolecular and molecule–substrate van der Waals interactions arising from the n-alkyl chains introduced instead of hydrogen bonding. Shorter alkyl chains of PR-2C10 resulted in the formation of multi domains. On the other hand, PR-2C12, PR-2C14, and PR-2C16 molecules that have longer alkyl chains formed stable single domain structures.
The decomposition of hydrogen peroxide (H2O2) was studied using a silica capillary tube (o.d. 0.45 mm, i.d. 0.33 mm, length 1000 mm) coated with a thin palladium (Pd) or platinum (Pt) layer. The inner walls of the capillary were coated with a homogenous thin layer of metal by electroless plating. This metal-coated capillary provided rapid and continuous decomposition of H2O2 at room temperature within a residence time of 9.6 s, simply by passing the solution through the capillary. A capillary without Pd or Pt coating was unable to decompose H2O2 even at 50 °C. Pt proved to be a better catalyst than Pd. We also studied the effect of temperature, pH, concentration of H2O2 and the residence time on decomposition of H2O2.
Generation of virtual chemical structures is applied in material, product, and drug designs to obtain structures having desired activity or properties. Quantitative structure–activity relationship (QSAR) models and quantitative structure–property relationship (QSPR) models are used to estimate values of activity and those of properties of structures, respectively. However, estimated values are unreliable when new structures are out of an applicability domain (AD) which is defined using a training data set. Data density around a new structure, which can handle nonlinearities between descriptors and multimodal data distributions, can be an index of ADs. We focus on one-class support vector machine (OCSVM) as a data density estimation method and propose a strategy of structure generation within ADs. The partial derivative of an OCSVM model with respect to each descriptor is used as a guideline to change each descriptor to get structures within ADs. It was confirmed that structures could be changed to increase data density around the structures, and could be generated within ADs through QSAR and QSPR data analyses.
Two coordination isomers, i.e., square planar and pseudo-tetrahedral forms of the title compound, were crystallographically identified and discussed in relation to their thermal behavior revealed by XRD-DSC and FTIR-DSC. Upon the fusion, the coordination geometry partly changes, resulting in the liquid, which is an equilibrium mixture of molecules with the two coordination geometries. In this system, the mixed nature of liquid, in addition to molecular flexibility, is responsible for realization of large supercooling and glass transition, both of which enables the cold crystallization.
1,4-Sorbitan is a precursor to environmentally benign surfactants, which can be produced from biomass via sorbitol. Currently, sulfuric acid-catalyzed dehydration of sorbitol is the most widely used route for industrial synthesis of 1,4-sorbitan. In this work, we have studied the mechanism of the sorbitol dehydration by sulfuric acid. Our results show that both thermodynamic and kinetic parameters play significant roles to determine the yield of 1,4-sorbitan. Sorbitol preferentially forms an adduct with sulfuric acid, thereby inhibiting the subsequent dehydration of 1,4-sorbitan to isosorbide. Furthermore, a reaction mechanism is proposed for the dehydration reaction, which involves an SN2 reaction on primary C1 of sorbitol attacked by OH of secondary C4.