Effect of Acetylene on the γ-Ray Crosslinking Polymerization of Various Kinds of Acrylic Monomers

The γ-radiation-induced crosslinking polymerization of methyl methacrylate (MMA), methacrylic acid (MAA), 2-hydroxyethyl methacrylate (HEMA), methyl acrylate (MA), and acrylic acid (AA) was carried out in bulk. The polymerization rates of AA and HEMA were much larger than those of other monomers. Acetylene had no influence on the polymerization rate in the initial stage, and the rate was dependent only on the kinds of monomers. In the absence of acetylene, gelation took place in PMA, PAA, and PHEMA obtained at complete conversion, but not in PMMA and PMAA. By the addition of acetylene, complete gelation of PMMA was observed, but no gel formation was observed for PMAA. Volume and weight swelling ratios were measured for PMA, PAA, PHEMA, and PMMA with complete gelation. It was found that by the addition of acetylene the molar concentration of crosslinks increased as much as about twice for PMA and PHEMA, and over about one hundred times for PMMA AND PAA. The relation between volume S_v and weight swelling ratio S_w was studied theoretically. S_v was expressed by the the equation, S_v = (p_p /p+)S_w+ (1 -p_p/p_s), as a function of S_w and the densities of pure polymer (p_p) and solvent (p_s) in swollen polymer.     

The Role of Hydrogen Bond and Crystal Solvent in the Solid-state (β-α) Photoisomerization of Novel 2-(N-phenylcarbamoyl)ethyl Cobaloxime Complexes [1]

Abstract

The 2-(phenylcarbamoyl)ethyl group bonded to the cobalt atom in cobaloxime complexes was isomerized on exposure to visible light in the solid-state. The three complexes with different amines as axial base ligands were prepared and their crystal structures were determined by X-ray analysis. The reaction rates in the solid-state were mainly affected by the hydrogen bonds of the reactive group with the neighboring molecules and the occupation of crystal solvent molecules around the reactive group.     

Improved Upper Bounds for Gallai–Ramsey Numbers of Paths and Cycles

Given a graph G and a positive integer k, define the Gallai–Ramsey number to be the minimum number of vertices n such that any k‐edge coloring of contains either a rainbow (all different colored) triangle or a monochromatic copy of G. In this work, we improve upon known upper bounds on the Gallai–Ramsey numbers for paths and cycles. All these upper bounds now have the best possible order of magnitude as functions of k.     

Holonomic gradient descent for the Fisher–Bingham distribution on the d-dimensional sphere

We propose an accelerated version of the holonomic gradient descent and apply it to calculating the maximum likelihood estimate (MLE) of the Fisher–Bingham distribution on a \(d\) -dimensional sphere. We derive a Pfaffian system (an integrable connection) and a series expansion associated with the normalizing constant with an error estimation. These enable us to solve some MLE problems up to dimension \(d=7\) with a specified accuracy.     

The holonomic rank of the Fisher–Bingham system of differential equations

The Fisher–Bingham system is a system of linear partial differential equations satisfied by the Fisher–Bingham integral for the n  -dimensional sphere Sⁿ$S^n$. The system is given in [4, Theorem 2] and it is shown that it is a holonomic system [1]. We show that the holonomic rank of the system is equal to 2n+2$2n+2$.     

Analysis of Photoexcitation Energy Dependence in the Photoluminescence of Firefly Luciferin

The whole pathways for photoluminescence, which include absorption, relaxation and emission, of firefly luciferin in aqueous solutions of different pH values with different photoexcitation energies were theoretically investigated by considering protonation/deprotonation. It is experimentally known that the color of fluorescence changes from green to red with a decrease in the photoexcitation energy. We confirmed with the theoretical analysis that the peak energy shift in the fluorescence spectra with varying photoenergies is due to a change in photoluminescence pathway. When the photoexcitation energy is decreased, the red emission from a monoanion form of firefly luciferin with carboxylate and phenolate groups and N‐protonated thiazoline ring occurs irrespective of the pH values. However, because the species abundant in the solution and those excited by the photon depend on the solution pH, the pathway leading to the monoanion form changes with the solution pH.     

Direct Imaging of Light Emission Centers in Two‐Dimensional Crystals and Their Luminescence and Photocatalytic Properties

To fully understand the fundamental properties of light‐energy‐converting materials, it is important to determine the local atomic configuration of photofunctional centers. In this study, direct imaging of one‐ and two‐Tb‐atom emission centers in a two‐dimensional Tb‐doped Ca₂Ta₃O₁₀ nanocrystal was carried. The emission centers were located at the Ca sites in the perovskite structure, and no concentration‐based quenching was observed even when the emission centers were in close proximity to each other. The relative photoluminescence efficiency for green emission of the nanosheet suspension was 38.1 %. Furthermore, the Tb‐doped Ca₂Ta₃O₁₀ nanocrystal deposited co‐catalyst showed high photocatalytic activity for hydrogen production from water (quantum efficiency: 71 % at 270 nm). Tb³⁺ dopants in the two‐dimensional crystal might have the potential to stabilize the charge separation state.     

Good linear relationship between logarithms of Eigen’s water exchange constants for several divalent metal ions and activation energies of corresponding metal-catalyzed alkoxysilane hydrolysis in ethylene-propylene copolymer system

Catalytic efficiencies of seven divalent metal acetylacetonate complexes [M(acac)₂; M = Cd(II), Co(II), Cu(II), Fe(II), Ni(II), Pb(II), and Zn(II)] with respect to the water-crosslinking kinetics of vinyltrimethoxysilane-grafted ethylene-propylene copolymer (EPR-g-VTMS) were investigated to examine the effects of progressive changes in metal ion using ATR-FTIR spectroscopy. The hydrolysis activation energies of EPR-g-VTMS follows the order: No catalyst ≈ Ni(acac)₂ > Co(acac)₂ > Fe(acac)₂ ≈ Zn(acac)₂ > Cd(acac)₂ ≈ Cu(acac)₂ > Pb(acac)₂. Interestingly, the kinetics results revealed that the plots of hydrolysis activation energies of EPR-g-VTMS containing M(acac)₂ complexes and Eigen’s water exchange constants for corresponding metal ions showed a excellent linear relationship, suggesting that the reaction pathway for the silane water-crosslinking with hydrous M(acac)₂ complex in EPR-g-VTMS system may be similar to that for water exchange of the metal ion in an aqueous system. Based on the knowledge of traditional kinetics studies by Eigen and Wilkins and hybrid sol-gel chemistry, the plausible catalytic mechanism for M(acac)₂ complexes in EPR-g-VTMS system was proposed.     

Theory of chemical bonds in metalloenzymes. VII. Hybrid‐density functional theory studies on the electronic structures of P450

A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low‐lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 ( cpd1 ) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)?O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)?O and a doublet ligand radical. This excited state lies 20.8 kcal mol^?1 above the ground spin state, which is a non‐negligible energy level as compared with the activation energy barrier of ΔE^# = 26.6 kcal mol^?1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: ³O(³p) + CH₄. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (³O‐model) and insertion (¹O‐model) mechanisms, respectively. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008