Nonlinearity in cooperative systems-dynamical Bethe-Ising model

The dynamics of the short-range order as well as the long-range order in the nonlinear cooperative system is investigated specifically for a kinetic Ising model in the Bethe approximation. The phenomena of critical slowing down near the transition temperatureT _c and anomalous fluctuation belowT _c are directly related to the instability of the long-range order. The dynamics of the short-range order is essentially a fast mode and is noncritical. However, through the nonlinear coupling the short-range order is also influenced by the critical behavior of the long-range order.     

Three-dimensional ordering of impurity-doped TMMC in a magnetic field

The three-dimensional ordering temperatures of the quasi-one-dimensional antiferromagnet (CH₃)₄NMnCl₃ containing 0, 0.13, 0.25 and 0.48 at% Cu²⁺ ions were studied as a function of an external magnetic field up to 60 kOe by means of proton NMR. A strong increase of T_N(H) with H was found even for the impure system. The T_N versus H curve for the pure system cam be explained by introducing a simple form for the effect of the field on the correlation length, ξ(T, H) = ξ(T) × [1+(λJ/kT)H²], while that for the impure system is unlike the one for the pure system.     

Surface structure of SrTiO3(100)-(square root of 5 x square root of 5) - R26.6 degrees

Atomic and electronic structures of the SrTiO3(100)-(square root of 5 x square root of 5) - R26.6 degrees surface are studied by using scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM). Instead of the well established oxygen vacancy model, it is found that a structural model, consisting of an ordered Sr adatom at the oxygen fourfold site of a TiO2 terminated layer, can explain the experimental results very well. We theoretically simulate the model cluster with the first-principles total-energy calculation. Calculated density of states and images for STM and NC-AFM are in good agreement with the experimental results.     

Preferential formation of cis-4,5-dihydrooxazole derivatives via photoinduced electron transfer-initiated cyclization of N-acyl-α-dehydroarylalanine alkyl esters

The alkyl, aryl, and acyl substituent effects on the photoinduced electron transfer-initiated cyclization reaction of the title compounds (1) were investigated in polar solvents from mechanistic and synthetic points of view. The irradiation of (Z)-1 in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (cis-2 and trans-2). In addition to thermodynamic considerations for electron transfer and fluorescence quenching in the presence of TEA, acyl and aryl substituent effects on the emission intensity and photoreactivity of 1 confirmed the involvement of consecutive electron transfer reactions that form (E)-arylmethylene radical anion and (E)-N-acyl radical anion intermediates. It was also confirmed that the cyclization of the latter intermediate eventually leads to 2. On the basis of the finding that the selectivity for cis-2 is greatly increased with increasing the steric bulkiness of alkyl and aryl substituents in 1, it was concluded that steric hindrance of these substituents toward hydrogen shift in the cyclized biradical intermediate, precursor of 2, is responsible for the kinetically controlled hydrogen shift in this intermediate. A product composition analysis showed that the protic polar solvent, methanol, of hydrogen-bonding solvation ability is a most suitable solvent for the photocyclization reactions examined.     

Second hyperpolarizabilities of singlet polycyclic diphenalenyl radicals: effects of the nature of the central heterocyclic ring and substitution to diphenalenyl rings

Adopting density functional theory and a hybrid exchange-correlation functional, the relationship between the second hyperpolarizability (gamma) and the diradical character has been investigated for diphenalenyl-based compounds containing different heterocyclic five-membered central rings (C(4)H(4)X, where X = NH, PH, O, S, CH(2), SiH(2), BH, GaH, C=O, C=S, and C=Se) or substituted by donor (NH(2))/acceptor(NO(2)) groups. It turns out that these structural modifications can tune the diradical character from 0.0 to 0.968 and lead to variations of gamma over more than 1 order of magnitude, demonstrating the controllability of gamma in this family of compounds. In particular, when the central ring is strongly aromatic, the diradical character is larger than 0.7, which is associated with pretty large gamma values except for almost the pure diradical case (y approximately 1). On the other hand, when the aromaticity decreases--or the antiaromaticity increases--the diradical character and the second hyperpolarizability get smaller. These relationships are correlated to structural (bond length alternation) and charge distribution (charge transfer between the phenalenyl rings and the central ring) properties, which account for the relative importance of the resonance diradical, zwitterionic, and quinoid forms. Therefore, the diradical character and the second hyperpolarizability can be controlled by the aromaticity of the ring while the paradigm of the enhancement of gamma for intermediate diradical character is globally verified. Then, upon introducing donor groups, the zwitterionic character increases, leading to closed-shell species and small second hyperpolarizabilities. In the case of substitution by acceptor groups, the charge transfer is reduced but the diradical character and the second hyperpolarizability hardly changes.     

Theoretical study on the second hyperpolarizabilities of phenalenyl radical systems involving acetylene and vinylene linkers: diradical character and spin multiplicity dependences

We have investigated the static second hyperpolarizabilities (gamma) of the singlet diradical systems with intermediate diradical character involving phenalenyl radicals connected by acetylene and vinylene pi-conjugated linkers, 1 and 2, using the hybrid density functional theory. For comparison, we have also examined the gamma values of the closed-shell and pure diradical systems with almost the same molecular size as 1 and 2. In agreement with our previous prediction of the diradical character dependence of gamma, it turns out that the gamma values of 1 and 2 are significantly enhanced compared to those of the closed-shell and pure diradical systems. In the present case, distinct differences in gamma values are not observed between the two pi-conjugated linkers, though the diradical character is found to depend on the kind of linker. Furthermore, we have investigated the spin multiplicity effect on gamma. Changing from the singlet to the triplet state, the gamma values of the systems with intermediate diradical character in the singlet state are quite reduced, though those of the pure diradical systems are hardly changed. Such spin multiplicity dependence of gamma is understood by considering the difference of diradical character between their singlet states together with the Pauli principle. The present results provide a possibility of a novel control scheme of gamma for phenalenyl radical systems involving pi-conjugated linkers by adjusting the diradical character through the change of the linked position of pi-conjugated linkers and the spin multiplicity.     

Salt-induced formation of uniform fiberlike SBA-15 mesoporous silica particles and application to toluene adsorption

Long, homogeneous fiberlike SBA-15 mesoporous silica particles are prepared by fine adjustment of three reaction conditions--stirring temperature, stirring time, and amount of inorganic salt added--using Pluronic P123 and TEOS under acidic conditions. The addition of NaCl along with a short stirring time played an important role in controlling the morphology and pore characteristics. Dynamic adsorption performance for gaseous toluene was systematically evaluated using four types of materials with different morphologies and pore characteristics synthesized with the addition of a different amount of NaCl. Breakthrough curves showed that the longer the fiberlike structure, the longer the breakthrough time. Furthermore, it was found from the breakthrough times and the total amounts adsorbed that long particles with high microporosity offer effective adsorption performance and should be useful in applications such as toluene adsorption.     

Acid-catalyzed rearrangement of aryl-substituted homobenzoquinone epoxides

The BF3-catalyzed reactions of diphenyl-substituted and endo-monophenyl-substituted homobenzoquinone epoxides proceeded through a regioselective oxirane ring opening followed by participation of a pi-aryl transannular cyclization to give the tricyclic diketo alcohols. The conformationally semirigid ethano-bridged diphenyl-substituted homologues also provided similar diketo alcohols and the subsequent ring-expanded cycloheptenedione (via a subsequent 1,2-acyl migration associated with cyclopropane ring opening), depending on the methyl-substitution pattern of the quinone frame. However, the exo-monophenyl-substituted and the rigid biphenyl-2,2'-diyl-substituted homobenzoquinone epoxides essentially remained unchanged.