Contributions of short-range and excluded-volume interactions to unperturbed polymer chain dimensions

A Monte Carlo study is made of the mean-square radius of gyration for the freely rotating chain with such fictitious excluded-volume interactions that the Lennard-Jones 6-12 potentials at the Theta temperature act only between the fourth-through (3+Delta)th-neighbor beads (Delta > or = 1) along the chain. The behavior of the asymptotic value (/n)infinity of the ratio /n as a function of the number n of bonds in the chain in the limit of n --> infinity is examined as a function of Delta. It is shown that the approach of (/n)infinity to its value for the real unperturbed chain with Delta = infinity is so slow that the interactions between even up to about 100th-neighbor beads should be taken into account in order to reproduce nearly its dimension. The result implies that the unperturbed polymer chain dimension as experimentally observed at the Theta temperature depends not only on short-range interactions but also to a considerable extent on the long-range excluded-volume interactions, and that the asymptotic value Cinfinity of the characteristic ratio Cn for the rotational isomeric state model in the limit of n --> infinity, which is determined only by the very local conformational energy, cannot be directly compared with the corresponding experimental value.     

Synthesis of all stereoisomers of sulfinylcalix[4]arenes

All four stereoisomers of p-tert-butylsulfinylcalix[4]arene [4(rccc), 4(rcct), 4(rctt), and 4(rtct)], arising from the disposition of the four sulfinyl groups with respect to the mean plane of the four bridging sulfur atoms, have been prepared via oxidation of p-tert-butylthiacalix[4]arene (2) or its tetra-O-benzyl ethers 5 of defined conformations. Thus, treatment of 2 with 4.4 molar equiv of NaBO(3).4H(2)O gave the rtct and rctt isomers in 27% and 17% yields, respectively, while oxidation of cone 5 (5(C)) and partial cone 5 (5(PC)) proceeded stereoselectively to give, after debenzylation of the resulting tetrasulfoxides 12 and 15, the rccc and rcct isomers in 56% and 28% yields, respectively, based on 5. The sulfinylcalix[4]arenes 4 were treated with iodomethane in the presence of a base to give the corresponding tetramethyl ethers 16, the structures of which in regard to the disposition of the sulfinyl groups and the conformation of the phenol units were determined by X-ray crystallography. Also reported is the synthesis of all four conformational isomers of tetra-O-benzyl ether of 2 (5(C), 5(PC), 5(1,2-A), and 5(1,3-A)).     

Acid-catalyzed transformation of rubrene to indenonaphthacene and its paired interacting orbital (PIO) analysis

Treatment of rubrene (5,6,11,12-tetraphenylnaphthacene) with trifluoroacetic acid in dichloromethane gives 4b,9,10-triphenyl-4b,9-dihydroindeno[1,2,3-fg]naphthacene (1) as the sole isolable product. Paired interacting orbital (PIO) analysis indicates that the reaction is initiated by preferential attack of H⁺ at C(11) position of rubrene.     

Temperature effect on solubilization of n-alkylbenzenes into sodium cholate micelles

The solubilization of n-alkylbenzenes (benzene, toluene, ethylbenzene, n-propylbenzene, n-butylbenzene, n-pentylbenzene, n-hexylbenzene) into an aqueous micellar solution of sodium cholate was carried out. Solubilizate concentrations at equilibrium were determined spectrophotometrically at 293.2, 298.2, 303.2, 308.2, and 313.2 K. The first stepwise association constants (K(1)) between solubilizate monomers and vacant micelles were evaluated from the equilibrium concentrations and found to increase with increasing hydrophobicity of the solubilizate molecules. From the Gibbs energy change for solubilization at different micelle aggregation numbers and from the molecular structure of the solubilizates, the function of sodium cholate micelles as solubilizer was discussed. Enthalpy and entropy changes of solubilization were calculated from the temperature dependence of the K(1) values, and the solubilization was found to be enthalpy-driven for the solubilizates with shorter alkyl chains. The results obtained were also compared with those for conventional aliphatic micelles.     

Ullmann CO coupling of sterically hindered secondary alcohols using excess amount of strongly coordinating monodentate ligands

A new effective copper catalyzed CO coupling reaction using excess amount of strongly coordinating monodentate ligands was successfully developed. Among the DMAP-type monodentate ligands, 4-pyrrolidinopyridine afforded the best results. The developed reaction is widely applicable for the synthesis of various hindered or acyclic secondary alkyl-aryl ethers. In this study, a novel and remarkable acceleration of the coupling reaction using excess amount of monodentate ligands was discovered.     

Mechanism of silver particle formation during photoreduction using in situ time-resolved SAXS analysis

Formation mechanisms of silver (Ag) particles in an aqueous ethanol solution of poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction of AgClO(4) were investigated by means of in situ small-angle X-ray scattering (SAXS) measurements. The kinetics of association process (nucleation, growth, and coalescence) of Ag(0) atoms to produce Ag particles was successfully revealed by the quantitative SAXS analysis for the number-average of radius (R(0)), number of particles (n(Ag)), reduced standard deviation (σ(R)/R(0)), and volume fraction (?(Ag)) of Ag particles produced by the photoreduction. The rate of nucleation and growth process during Ag particle formation strongly depend on the initial metal concentration. The time evolution of radius and number of Ag particles indicates that a mechanism of Ag particle formation is composed of different three processes, that is, reduction-nucleation, Ostwald ripening, and particle coalescence. In a rapid reduction-nucleation process, small nuclei or particles (average radius ～2.5 nm) are produced by an autocatalytic reduction. After the formation of small nuclei or particles proceeds, Ostwald ripening and particle coalescence, predicted by the Lifshitz-Slyozov-Wagner theory (LSW theory), subsequently occur, resulting in the particle growth (average radius ～11.5 nm).     

硫化砷非晶态波导光阻断效应的机理研究

在实验研究了As₂S₈和掺杂As₂S₈薄膜波导光阻断效应的基础上,讨论了反常电子组态的形成机理和转移机理,提出了光阻断效应切断过程和回复过程的动力学模型,数值分析结果与实验数据十分符合,理论预期与实验现象一致,表明该模型抓住了光阻断效应的基本特征,揭示了过程本质,反映了实验现象的内在机理. 关键词： 光波导技术 硫属化合物玻璃 光阻断效应 动力学模型