Structure conversions of cellulose IIII crystal models in solution state: a molecular dynamics study

This paper re-examines our previous molecular dynamics (MD) study on cellulose III_I crystal models with finite dimensions solvated in explicit water molecules. Eight crystal models, differing in a constituent lattice plane and dimensions, were studied. One calculation allowed for O–H and C–H bond stretching, and had a small time step of 0.5 fs. The other calculation adopted non-scaling factors of the 1–4 non-bonded interactions. As in our previous study, in the former MD calculations, six of the eight crystal models exhibited structure conversion with cooperative chain slippages generated by a progressive fiber bend. This converted the initial non-staggered chain packing of cellulose III_I into a near one-quarter staggering and gave the crystal model a triclinic-like configuration. In contrast, in the non-1–4 scaling MD calculations, all of the eight crystal models retained the initial cellulose III_I crystal structure. Another series of non-1–4 scaling MD calculations were performed for the four crystal models containing chains with a degree of polymerization (DP) of 40 at 370 K, which simulated hot water treatment to convert cellulose III_I to Iβ. Some of the hydroxymethyl groups irreversibly rotated from gt into tg conformation. This accompanied exchange of the intrasheet hydrogen bonding scheme along the (1 ?1 0) lattice plane from O2–O6 to O3–O6. The original corrugated (1 ?1 0) chain sheet was partly converted into a cellulose I-like flat chain sheet.     

Low-temperature synthesis of niobium oxide nanoparticles from peroxo niobic acid sol

A peroxo niobic acid sol was prepared by peptization of the niobic acid precipitate (Nb2O5.nH2O) with a H2O2 aqueous solution. Crystallized Nb2O5 nanoparticles and niobic acid nanoparticles were obtained by heating the peroxo niobic acid sol. When peroxo niobic acid sol prepared by peptization of the niobic acid precipitate ([NH3]=0.3 mol/l) was heated at 348 K for 1 week, Nb2O5 nanoparticles with a diameter of 4.5 nm and a S(BET) of 275 m2/g were obtained. When peroxo niobic acid sol prepared by peptization of the niobic acid precipitate ([NH3]=1 mol/l) was heated at 348 K for 1 week, niobic acid nanoparticles with a diameter of less than 2 nm were obtained. The pore structure and degree of crystallinity of the nanoparticles prepared by heating the peroxo niobic acid sol greatly depended on the concentration of the ammonia solution used for preparing the niobic acid precipitate.     

Comparison of surface films formed on titanium by pulsed Nd:YAG laser irradiation at different powers and wavelengths in nitrogen atmosphere

The nitridation of titanium (Ti) caused by a Q-switched Nd:YAG laser under nitrogen gas atmosphere was investigated in situ using X-ray photoelectron spectroscopy (XPS). A laser having a wavelength of 1064 nm and 532 nm (SHG mode) was irradiated on a titanium substrate in an atmosphere-controlled chamber, and the substrate was then transported to an XPS analysis chamber without exposing it to air. The characteristics of the surface layer strongly depend on the laser power. When the power is relatively low, a titanium dioxide layer containing a small amount of nitrogen is formed on the substrate. Laser irradiation beyond a certain laser power is required to obtain a stoichiometric titanium nitride (TiN) layer. A TiN layer and an oxynitride layer with a TiO_xN_y-like structure are formed as the topmost and the lower surface layer, respectively, when the laser power exceeds this threshold value. The threshold laser power strongly depends on the wavelength of the laser, and this threshold value for the 532-nm laser is quite lower than that for the 1064-nm laser.     

Quantum critical point of an itinerant antiferromagnet in a heavy fermion

A quantum critical point of the heavy fermion Ce(Ru(1-x)Rh(x))2Si2, (x = 0,0.03) has been studied by single-crystalline neutron scattering. By accurately measuring the dynamical susceptibility at the antiferromagnetic wave vector k3 = 0.35c*, we have shown that the inverse energy width gamma(k3), i.e., the inverse correlation time, depends on temperature as gamma(k3) = c1 + c2T((3/2)+/-0.1), where c1 and c2 are x dependent constants, in a low temperature range. This critical exponent 3/2 +/- 0.1 proves that the quantum critical point is controlled by that of the itinerant antiferromagnet.     

Characterization of homoionic Fe-type montmorillonite: Potential chemical species of iron contaminant

Fe²⁺-montmorillonite with Fe²⁺ ions occupying cation exchange sites is an ideal transformation product in bentonite buffer material. In our previous study on preparation and characterization of Fe²⁺-montmorillonite, the montmorillonite sample that adsorbed Fe²⁺ ions on almost all of the cation exchange sites was prepared using a FeCl₂ solution under an inert gas condition [N. Kozai, Y. Adachi, S. Kawamura, K. Inada, T. Kozaki, S. Sato, H. Ohashi, T. Ohnuki, T. Banba, J. Nucl. Sci. Technol. 38 (2001) 1141]. In view of the unstable nature of iron(II) chemical species, this study attempted to determine the potential contaminant iron chemical species in the sample. Nondestructive elemental analysis revealed that a small amount of chloride ions remained dispersed throughout the clay particles. The chloride ion retention may be due to the adsorption of FeCl⁺ ion pairs in the initial FeCl₂ solution and the subsequent containment of the Cl⁻ ions that are dissociated from the FeCl⁺ ion pairs during excess salt removal treatment. Two explanations are advanced for the second process: the slow release of the remaining Cl⁻ ions from the collapsed interlayer of the montmorillonite, and the transformation of a minor fraction of the remaining FeCl⁺ ion pairs to iron(III) hydroxide chloride complexes having low solubility.     

Stereoselective cis-addition of aromatic C-H bonds to alkynes catalyzed by dinuclear palladium complexes

Reactions of alkynes with arenes proceeded in the presence of dinuclear palladium complexes and trialkylboranes to yield alkyne hydroarylation products with high stereoselectivity. In the reactions of monosubstituted benzenes, meta and para products were formed in statistical ratios, while no ortho isomers were detected.     

Mode-selective internal conversion of perylene

We observed fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation of jet-cooled perylene-h ₁₂ and perylene-d ₁₂, and carefully examined the vibrational structures of the S₀ ¹ A _g and S₁ ¹ B _2u states. We performed vibronic assignments on the basis of the results of ab initio calculation, and found that the vibrational energies in the S₁ state are very similar to those in the S₀ state, indicating that the potential energy curves are not changed much upon electronic excitation. We conclude that the small structural change is the main cause of its slow radiationless transition and high fluorescence quantum yield at the zero-vibrational level in the S₁ state. It has been already reported that the lifetime of perylene is remarkably short at specific vibrational levels in the S₁ state. Here, we show that the mode-selective nonradiative process is internal conversion (IC) to the S₀ state, and the ν₁₆(a _g ) in-plane ring deforming vibration is the promoting (doorway) mode in the S₁ state which enhances vibronic coupling with the high-vibrational level (b _2u ) of the S₀ state.