Azacalix[4]arene cation radicals: spin-delocalised doublet- and triplet-ground states observed in the macrocyclic m-phenylene system connected with nitrogen atoms

Electron paramagnetic resonance spectroscopy has unmasked for the first time the spin-delocalised doublet- and triplet-ground states of azacalix[4]arene cation radicals.     

Electron paramagnetic resonance of the excited states of the three-spins-1/2 ZnTPP-3-NOPy complex

EPR spectra of the excited quartet and doublet molecular states of (tetraphenylporphinato)zinc(II) covalently bounded to 3-(N-nitronyl-notroxide) pyridine stable radical are modeled in terms of the spin-Hamiltonian given by the sum of the contributions from the radical and triplet moieties, and the interaction between them. The later is represented by anisotropic point dipolar and isotropic exchange electron spin-spin interactions. It is shown that the high field (W-band) EPR spectra depend on energy separation between the electronic doublet (D) and quartet (Q) states. This dependence was utilized to estimate the upper limit of the intensity of exchange interaction between the radical and porphyrin moieties.     

Thermal and Photooxidative Degradation Behaviors of Poly(propylene)/SiO2 Nanocomposites with Various Polymer Morphologies

Photo‐ and thermal stabilities of poly(propylene) (PP)/SiO₂ nanocomposites were studied by varying the particle size of the SiO₂ nanoparticles. It was found that smaller SiO₂ nanoparticles improved the stabilities of the nanocomposites by depressing the size of spherulites. The phenomenon was successfully explained within the infectious spreading model, where the spatial spreading of oxidation was delayed at the interfacial region between the spherulites.

     

Photodissociation of CH2I2 and Subsequent Electron Transfer in Solution

We studied photoinduced reactions of diiodomethane (CH₂I₂) upon excitation at 268 nm in acetonitrile and hexane by subpicosecond–nanosecond transient absorption spectroscopy. The transient spectra involve two absorption bands centered at around 400 (intense) and 540 nm (weak). The transients probed over the range 340–740 nm show common time profiles consisting of a fast rise (<200 fs), a fast decay (≈500 fs), and a slow rise. The two fast components were independent of solute concentration, whereas the slow rise became faster (7–50 ps) when the concentration in both solutions was increased. We assigned the fast components to the generation of a CH₂I radical by direct dissociation of the photoexcited CH₂I₂ and its disappearance by subsequent primary geminate recombination. The concentration‐dependent slow rise produced the absorption bands centered at 400 and 540 nm. The former consists of different time‐dependent bands at 385 and 430 nm. The band near 430 nm grew first and was assigned to a charge‐transfer (CT) complex, CH₂I₂^δ+???I^δ?, formed by a photofragment I atom and the solute CH₂I₂ molecule. The CT complex is followed by full electron transfer, which then develops the band of the ion pair CH₂I₂⁺???I^? at 385 nm on the picosecond timescale. On the nanosecond scale, I₃^? was generated after decay of the ion pair. The reaction scheme and kinetics were elucidated by the time‐resolved absorption spectra and the reaction rate equations. We ascribed concentration‐dependent dynamics to the CT‐complex formation in pre‐existing aggregates of CH₂I₂ and analyzed how solutes are aggregated at a given bulk concentration by evaluating a relative local concentration. Whereas the local concentration in hexane monotonically increased as a function of the bulk concentration, that in acetonitrile gradually became saturated. The number of CH₂I₂ molecules that can participate in CT‐complex formation has an upper limit that depends on the size of aggregation or spatial restriction in the neighboring region of the initially photoexcited CH₂I₂. Such conditions were achieved at lower concentrations in acetonitrile than in hexane.     

Rational Design of Mesoporous Metals and Related Nanomaterials by a Soft‐Template Approach

We review recent developments in the preparation of mesoporous metals and related metal‐based nanomaterials. Among the many types of mesoporous materials, mesoporous metals hold promise for a wide range of potential applications, such as in electronic devices, magnetic recording media, and metal catalysts, owing to their metallic frameworks. Mesoporous metals with highly ordered networks and narrow pore‐size distributions have traditionally been produced by using mesoporous silica as a hard template. This method involves the formation of an original template followed by deposition of metals within the mesopores and subsequent removal of the template. Another synthetic method is the direct‐template approach from lyotropic liquid crystals (LLCs) made of nonionic surfactants at high concentrations. Direct‐template synthesis creates a novel avenue for the production of mesoporous metals as well as related metal‐based nanomaterials. Many mesoporous metals have been prepared by the chemical or electrochemical reduction of metal salts dissolved in aqueous LLC domains. As a soft template, LLCs are more versatile and therefore more advantageous than hard templates. It is possible to produce various nanostructures (e.g., lamellar, 2D hexagonal (p6mm), and 3D cubic (Ia d)), nanoparticles, and nanotubes simply by controlling the composition of the reaction bath.     

Contractible elements in k‐connected graphs not containing some specified graphs

In 15 , Thomassen proved that any triangle‐free k‐connected graph has a contractible edge. Starting with this result, there are several results concerning the existence of contractible elements in k‐connected graphs which do not contain specified subgraphs. These results extend Thomassen's result, cf., 2 , 3 , 9 - 13 . In particular, Kawarabayashi 12 proved that any k‐connected graph without K subgraphs contains either a contractible edge or a contractible triangle. In this article, we further extend these results, and prove the following result. Let k be an integer with k ≥ 6. If G is a k‐connected graph such that G does not contain as a subgraph and G does not contain as an induced subgraph, then G has either a contractible edge which is not contained in any triangle or a contractible triangle. © 2008 Wiley Periodicals, Inc. J Graph Theory 58:97–109, 2008     

Coordination structures of the silver ion: infrared photodissociation spectroscopy of Ag(+)(NH(3))(n) (n = 3-8)

Infrared (IR) spectra are measured for Ag(+)(NH(3))(n) with n = 3-8 in the NH-stretch region using photodissociation spectroscopy. The spectra of n = 3 and 4 exhibit absorption features only near the frequencies of the isolated NH(3), indicating that every NH(3) molecule is coordinated individually to Ag(+). For n >or= 5, the occupation of the second shell is evidenced by lower-frequency features characteristic of hydrogen bonding between NH(3) molecules. Density functional theory and MP2 calculations are carried out in support of the experiments. A detailed comparison of the experimental and theoretical IR spectra reveals the preference for a tetrahedral coordination in the n = 5 and 6 ions. Likewise, most of the features observed in the spectra of n = 7 and 8 can be assigned to isomers containing a tetrahedrally coordinated inner shell as the basic structural motif. These results signify that the ammonia-solvated Ag(+) ion has a propensity toward a coordination number of four and the resulting tetrahedral Ag(+)(NH(3))(4) complex forms the central core of further solvation process.     

Cholesterol location and orientation in aqueous suspension of large unilamellar vesicles of phospholipid revealed by intermolecular nuclear overhauser effect

The locational and orientational structure and the dynamics of cholesterol in the bilayer membrane were studied by using the solution-state NMR. The intermolecular nuclear Overhauser effect (NOE) was analyzed for large unilamellar vesicles (100 nm in diameter) composed of dimyristoylphosphatidylcholine (DMPC) and cholesterol at cholesterol concentrations of 9-33 mol %. The DMPC headgroups show (1)H-{(1)H}-NOEs with the methyl groups at the hydrophobic terminals of both cholesterol and DMPC, illustrating the significant fluctuation of the bilayer membrane in the vertical (bilayer normal) direction. Cholesterol was found to keep the hydroxyl (OH) group toward the outer water pool on the basis of the following observations: (1) the cross correlation between the DMPC headgroup and the cholesterol terminal methyl group is weaker than those between the DMPC headgroups and (2) the methyl group at the hydrophobic terminal of cholesterol shows strong correlation with the terminal group of the DMPC chain portion. The OH group plays a crucial role in orienting cholesterol with its OH group outward, since cholestane, which has a molecular structure similar to that of cholesterol except for the absence of the OH group, was found to have no orientational preference in the bilayer membrane. The dynamic slowdown at high cholesterol concentrations is demonstrated on the basis of the correlation times for NOE as well as the broadening of the proton linewidths.     

Environment-induced surface dynamics of a biomimetic ionomer studied using in situ second harmonic generation

The environmental-induced surface dynamics of the biomimetic phosphoryl choline (PC)-functionalized poly(trimethylene carbonate) ionomer has been studied and compared to its unfunctionalized counterpart using in situ second harmonic generation measurements. Whereas the nonpolar liquid n-hexane did not induce any surface dynamic processes in the ionomer under study, the presence of water initiated a Debye-type dynamic reaction at the surface of the PC ionomer, which had no equivalent in the unfunctionalized material. This first-order reaction was attributed to a surface enrichment process of the functionalized ionomer in the hydrophilic environment involving movement of the PC endgroups from aggregates in the bulk to the surface. The time constant of the process was found to be about 6 min, and the corresponding activation energy was 0.4 eV. The dehydration process of the PC-functionalized ionomer in nitrogen gas atmosphere could be described by two time constants, one slightly below 1 min and the other one just above 13 min. The results presented in this work show that SHG measurements are well suited for the study of polymer surface restructuring dynamics in response to environmental changes. Such information is very important for the successful design and implementation of biomimetic polymers intended for biomedical applications.