Recent progress of overall water splitting by photocatalysts and mechanocatalysts is briefly reviewed. In photocatalytic reactions, several new materials have been introduced, and the catalyst preparation has also been improved. As a result, a high quantum efficiency for overall water splitting of about 30% has been accomplished. In addition, recent progress in the understanding of the reaction mechanism is briefly introduced. On the other hand, mechano-catalytic overall water splitting reaction has been recently discovered by the authors. The phenomenological aspects of this novel reaction are summarized, and some implications of the reaction mechanism are also described.
Some of the 4-arylmethyleneamino-TEMPO radicals (TEMPO = 2,2,6,6-tetramethyl-1-piperidyloxyl), such as 4-(4-chlorobenzylideneamino)TEMPO (1) and 4-(benzylideneamino)TEMPO (2), showed intermolecular ferromagnetic interactions at an extremely low temperature. The experimental electron-density distributions of these compounds were investigated by the X-ray diffraction method. Intensity data were measured at 100 K up to 2θmax (Mo Kα) = 120° and 95° for 1 and 2, respectively. Deformation electron-density distributions were calculated from the structure factors after the refinements by the multipole expansion method. These electron-density distributions suggested that the unpaired electron of the O radical atom was not located on the O atom, but was rather located in the π*-orbital of the N-O bond. Lone-pair densities of the radical O atom are located in the >N-O plane showing the sp2 hybridization. From the crystal structures, it has been proposed that the ferromagnetic interactions are transferred through CH…O interactions between radical O atoms and β-H atoms of the neighboring TEMPO rings. The β-H atoms of the neighboring TEMPO rings contact with the O atom in the direction toward the π*-orbital of the N-O bond, not in the direction of the lone-pair densities of the O atom. The present studies support the mechanism of the intermolecular ferromagnetic interactions through the β-H atoms. Studies of electron densities from the view point of the intermolecular interactions are important.
To elucidate the origin of differences in the monomeric properties of chlorophyll a and a′ (Chl a/a′; C132-(R/S)-epimers of Chl a), a series of Zn-Chl derivatives possessing a variety of C134 substituents were prepared. The (S)-epimers gave absorption and emission bands at slightly longer wavelengths than the corresponding (R)-epimers. The (S)-epimers had more intense CD peaks than the (R)-epimers. In comparison with Zn-Chl a/a′ (C134-Me), Zn-Chl with a bulky 2,4-dimethyl-3-pentyl moiety at the C134-position showed a greater difference between the 13C NMR chemical shifts of the epimers. These spectral differences between the (R)- and (S)-epimers were attributed to ruffling of the chlorin macrocycle, arising from a steric repulsion between the C132 and C17 moieties. The trend of the rate constant of epimerization, isomerization at the C132 position, in the C134-substituted Zn-Chls is in line with the above interpretation. The relationship between the spectroscopic and kinetic results and the distortion of the macrocycle is schematically represented.
Some MCM-41 materials substituted by trivalent metal ion (Met-MCM-41; Met = Al, Fe, and Ga) were prepared, and their methane adsorption sites were investigated by IR spectroscopy. Three adsorption sites on Met-MCM-41 with different strengths of electric field were observed. They were assigned to pore wall site (0 V m-1), silanol site (ca. 4×109 V m-1), and Na+ site (ca. 6×109 V m-1), respectively. The amounts of silanol site in Met-MCM-41 were estimated to 2—3 mmol g-1, while the amounts of the Na+ site were 0.1—0.2 mol g-1. Methane molecules were preferentially adsorbed on the electrostatic sites (silanols and Na+sites) at low methane pressures, but as methane pressure increased, the amount of adsorption on the pore wall site became comparable to that on the silanol sites. Since the amounts of Na+ site estimated from methane adsorption on the Met-MCM-41 samples were less than the theoretical ones calculated from the compositions of starting materials, most of the Na+ sites were thought to be inaccessible for methane molecules.
Some quinolinol derivatives possessing suitable triplet state energy were examined to sensitize the luminescence of neodymium ion (Nd(III)) as coordinating ligands. Photosensitized luminescence in the near-infrared region of four neodymium(III) complexes: tris(8-quinolinolato)neodymium(III) ([Nd(q)3]), tris(5,7-dichloro-8-quinolinolato)neodymium(III) ([Nd(Clq)3]), tris(5,7-dibromo-8-quinolinolato)neodymium(III) ([Nd(Brq)3]) and tris(5,7-diiodo-8-quinolinolato)neodymium(III) ([Nd(Iq)3]), were observed by selective excitation of the ligand moieties in DMSO-d6. Highly efficient intramolecular energy transfer from the excited ligand moieties to the central Nd(III) ion was successfully achieved by using the halogenated ligands.
Reactions of [o-MePyNN+]PF6 and [p-MePyNN+]PF6 with [n-Bu4N]2[Cu(ox)2] in acetonitrile yielded dark-blue crystals of [o-MePyNN+]2[Cu(ox)2]·2CH3OH (1) and [p-MePyNN+]2[Cu(ox)2]·2H2O (2), respectively, where [o-MePyNN+] or [p-MePyNN+] represents the 2-(2 or 4-N-methylpyridinium)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazo-1-oxyl 3-oxide radical cation. X-ray structural analyses showed that the crystals of 1 and 2 are isomorphous. The coordination geometry of the Cu2+ ions in 1 and 2 is an elongated octahedron, and the equatorial and apical coordination sites of the Cu2+ ions are occupied by two oxalato groups and two radial cations, respectively. Magnetic susceptibility measurements for 1 and 2 revealed that the copper and radical centers are ferromagnetically coupled with J = +8.2(1) and +7.8(1) cm-1 (H = -2J∑SCu·Srad), respectively.
Complexation of Cd(NO3)2 with 4,4′-bipyridine (4,4′-bpy) gave, depending on the L/M ratios and concentrations, two-dimensional, one-dimensional or zero-dimensional coordination assemblies, which were characterized by X-ray crystallography. The two dimensional structure was shown to be a non-interpenetrated square grid network in which the square cavities were occupied by water and nitrate ions. The one-dimensional polymer also formed a two-dimensional network with the assistance of O-H…N hydrogen bonds. The zero-dimensional structure was composed of two Cd2+ centers, four unidentate 4,4′-bpy, and a bridging bidentate 4,4′-bpy. The networks and the packing of these structures were analyzed in terms of coordination and hydrogen bonds.
The crystal structure of the aqua(butyl)cobaloxime/α-cyclodextrin inclusion complex was determined by single X-ray diffraction. It crystallizes in the orthorhombic space group P212121 with a = 14.083(3), b = 19.237(4), c = 24.517(5) Å and Z = 4. A thermogravimetric analysis has indicated that the thermal stability of the aqua(butyl) cobaloxime was increased in its α-cyclodextrin inclusion complex.
The genetic information for local structures along helices, encoded in local sequences of protein chains, was statistically investigated for tri- and tetrapeptide sequences (3- and 4-letter words) using the 411 analyzed protein chains. The 18 kinds of local structures adopted by tri- and tetrapeptide sequences were represented 1-dimensionally by using a single helix element and pairs of helix elements in parentheses, respectively. The local sequences have propensities for none to some specific local structures along helices. A local structure (LS)-value is introduced for the evaluation of the normalized preference (NP)-value of a local sequence for a particular local structure. It should be emphasized that N-capping tetrapeptide sequences of helices do not necessarily prevent helix elongation. Local structures adopted by tetrapeptide sequences are plastic, and a particular local structure along helices is determined by the sequence context of helices.
Enzymatic oxidative polymerization of phenol using peroxidase as catalyst has been carried out in a mixture of alcohol and buffer. Hydrogen peroxide (oxidizing agent) was added dropwise to the reaction mixture. Effects of the reaction parameters have been systematically investigated. The polymer solubility strongly depended on the buffer pH and content, and a polymer soluble in N,N-dimethylformamide (DMF) was obtained in good yields under appropriate reaction conditions. The solvent composition also affected the regioselectivity of the polymer. Purity and amount of the enzyme also affected the polymerization behaviors. By changing the concentration and addition rate of hydrogen peroxide, the molecular weight and solubility of the polymer could be controlled. Some solid properties of the resulting polyphenol were evaluated. From thermal analysis, the polymer was found to possess relatively high thermal stability.
H-Arg-Phe-Arg-Pro-Pro-Ile-Arg-(Arg-Pro-Pro-Phe)n-NH2 (n = 2—5 and 7), models of antibacterial bactenecin 5, were synthesized. The CD measurement showed that they took polyproline II-like conformations in neutral lipid vesicles at 25 and 50 °C, and in acidic ones at 25 °C. However, their conformations changed in acidic lipid vesicles at 50 °C. The membrane perturbation activity of the peptides was also found only for acidic lipid vesicles at 50 °C. The perturbation activity increased with increasing in the peptide chain length. All peptides exhibited negligible hemolytic activity for rabbit erythocytes, whereas they, except for the shortest peptide, had moderate or strong antibacterial activity against Gram-positive and Gram-negative bacteria. Together with the previous results, the N-terminal cationic portion and a certain peptide chain length were found to be important for antibacterial activity.
By X-ray crystallographic analysis, the molecular structure of 3,4,7,8-tetragermacycloocta-1,5-diyne 1b has been established to be planar and quite similar to that of silicon analog 3,4,7,8-tetrasilacycloocta-1,5-diyne 1a. The detailed comparison of structures of cyclic acetylene analogs including a group 14 element shows that ring strain induced by the ethynylene units may be released in a different manner by the carbocycle and by its group 14 metalloid analogs. In contrast to cycloocta-1,5-diynes, structures of the higher homologs, 3,4,7,8,11,12-hexasilacyclododeca-1,5,9-triyne 2a and 3,4,7,8,11,12-hexagermacyclododeca-1,5,9-triyne 2b were determined by X-ray crystallographic analyses to adopt a chair-like conformation. Ionization energies of these 3,4-dimetala-1-butyne-1,4-diyls in condensed phase were obtained by photoelectron spectroscopic methods.
The reaction of dimethyl(phenyl)silanol with butyl acrylate in the presence of a stoichiometric amount of Pd(OAc)2 or by a combined use of 0.1 molar amount of Pd(OAc)2 and Cu(OAc)2/LiOAc (molar ratio 3/2) gave butyl cinnamate in 76% or 57% yield, respectively. The similar reaction with tributyl(phenyl)tin also proceeded in 77% yield. The organotin compound was shown to react faster than the silanol, although the tin reagent sometimes induced undesirable homo-coupling, while the reaction with silanol did not give such by-product.
This work comprised the following parts: a) hydrolysis of Ti(OEt)4 and Zr(OPr)4 with different amounts of water, b) complexation of the same alkoxides with varying amounts of 2-methylbutanoic and 3-methylbutanoic acids, and c) hydrolysis of complexation products with varying amounts of water. All reactions were carried out at room temperature. The complex products were [Ti(OEt)3.2(2-MBA)0.8], [Ti(OEt)3.2(3-MBA)0.8], [Zr(OPr)2.5(2-MBA)1.5], and [Zr(OPr)3.3(3-MBA)0.7] (2-MBA: 2-methylbutanoate, 3-MBA: 3-methylbutanoate). The products were characterized by 1H NMR, FT-IR, and elemental analysis. While Ti(OEt)4, [Ti(OEt)3.2(2-MBA)0.8], and [Ti(OEt)3.2(3-MBA)0.8] were hydrolyzed with 1.30, 1.28, and 1.30 mol of water, Zr(OPr)4, [Zr(OPr)2.5(2-MBA)1.5], and [Zr(OPr)3.3(3-MBA)0.7] were hydrolyzed with 2.0, 0.96, and 1.10 mol of water, respectively. These products were characterized by gas chromatography (GC), Karl-Fischer coulometric titrations, and thermal analysis.
The transfer hydrogenation of aromatics containing various reducible groups has been carried out employing isopropanol as a hydrogen donor in the presence of different calcined LDHs. It was found that the NiII-AlIII (mol ratio 3) catalyst is highly active in bringing about chemoselective reduction compared to other calcined LDHs.
A C60 monolayer was constructed by a multi-step creep method for the first time. In the case that a dilute C60 benzene solution of 1×10-5 M (1 M = 1 mol dm-3) was spread on the water surface, C60 molecules formed into highly organized 2-dimensional crystalline domains. On the other hand, in the case of the conventional continuous compression method up to a low surface pressure below 15 mN m-1, the C60 monolayer-like aggregates were formed with a lot of vacancies. Each monolayer-like C60 aggregate was easily collapsed into small fragments and changed into 3-dimensional aggregates at a high surface pressure above 20 mN m-1. The C60 film could not full up the vacancies during continuous compression, since the C60 molecules formed rigid aggregates. These results clearly indicate that structural relaxation at a low surface pressure was required to construct the defect-diminished C60 monolayer. The occupied area of a C60 molecule in the monolayer prepared by the multi-step creep method was in good agreement with that of a 2-dimensional C60 crystal calculated from lattice constants of a 3-dimensional C60 crystal. The surface morphology of the C60 monolayer prepared by the multi-step creep method was homogeneous, and C60 molecules in the monolayer were closely and regularly packed in a hexagonal array. The hexagonal packing of the C60 molecules was confirmed on the basis of atomic force microscopic and electron diffraction observations.
Monomethylphenanthrenes (MPs) were isomerized by heating in the presence of Na-montmorillonite at 250—400 °C. Isomerization between 1- and 2-MP was found to proceed faster than those between the other sets of MPs. On the basis of this facile interconversion, the molar ratio of 1- to 2-MP was suggested to be a useful new parameter for the maturity assessment of sedimentary organic matter. This parameter was tested in a geochemical study on Miocene to Pliocene sediments in the Shinjo basin of Yamagata Prefecture. We determined the molar ratio in 12 depth-differing sediments, and found an almost linearly decreasing trend in the ratio with increasing depth, showing a good correlation of this parameter to the maturity of the sedimentary organic matter.