Finite energy sectors of the XY model

Continuing the earlier work on soliton sectors, we determine all finite energy representations of the XY model for almost all parameter values. In the interior of unique vacuum regions of parameters (i.e. the large external magnetic field region ||>1), the unique irreducible vacuum representation is the only finite energy representation.At the critical values of the parameters (||=1 as well as theXY symmetric case =0, ||1), there is an infinite number of mutually nonequivalent irreducible finite energy representations. Apart from the unique irreducible ground state representation and another associated irreducible representation, these infinite number of representations arise from an infinite number of nearly zero energy excitations of the ground state with a finite total energy and may be called infrared representations.In the remaining cases, as have been studied earlier, there are two additional irreducible finite energy representations besides two irreducible ground state representations and they are topological soliton sectors with different ground state limits in positive and negative spatial infinity. (For two exceptional values of parameters (, )=(0, ±1), they also become ground state representations.)     

Amperometric detection of nitrite and nitrate at tetraruthenated porphyrin-modified electrodes in a continuous-flow assembly

The modification of a glassy carbon surface by coating with an electrostatically assembled film of tetraruthenated cobalt porphyrin/(meso-tetra(4-sulphonatephenyl)porphyrinate zinc(II) yields an indicator electrode that allows the determination of nitrite to be performed with a limit of detection of 0.1 μM in a flow injection configuration. The dynamic range extends up to 1000 μM and the repeatability of the measurements was evaluated to be 1.5% with a throughput of 50 samples per hour. The efficiency of the bilayered film to mediate the electron transfer allows the determinations to be performed at a less positive potential (+0.75 V) with enhanced sensitivity. The coating also prevents the surface poisoning and its stability is maintained over several weeks. The same detector was used for determination of nitrate after reduction to nitrite in a reductor column containing copperised cadmium. This method was used for the determination of nitrate and nitrite in mineral water, saliva and cured meats, the results being in agreement with certified values and those obtained by using recommended procedures.     

Supramolecular porphyrin-fullerene via 'two-point' binding strategy: axial-coordination and cation-crown ether complexation

A highly stable porphyrin-fullerene conjugate with defined distance and orientation, was formed using a newly developed 'two-point' binding strategy involving axial-coordination and cation-crown ether complexation; photochemical studies performed in benzonitrile revealed efficient charge separation and slow charge-recombination in the supramolecular complex.     

Chemo-enzymatic synthesis of 2′,3′-dideoxy-3′-fluoro-β-d-guanosine via 2,3-dideoxy-3-fluoro-α-d-ribose 1-phosphate

2,3-Dideoxy-3-fluoro-α-d-ribose 1-phosphate 2 was stereoselectively synthesized and converted to 2′,3′-dideoxy-3-fluoro-β-d-guanosine 1 by enzymatic reaction using purine nucleoside phosphorylase. This chemo-enzymatic strategy was first applied to the synthesis of 1.     

Effect of axial ligation or pi-pi-type interactions on photochemical charge stabilization in "two-point" bound supramolecular porphyrin-fullerene conjugates

Two types of structurally well-defined, self-assembled zinc porphyrin-fullerene conjugates were formed by "two-point" binding strategies to probe the effect of axial ligation or pi-pi-type interactions on the photochemical charge stabilization in the supramolecular dyads. To achieve this, meso-tetraphenylporphyrin was functionalized to possess one or four [18]crown-6 moieties at different locations on the porphyrin macrocycle while fullerene was functionalized to possess an alkyl ammonium cation, and a pyridine or phenyl entities. As a result of the crown ether-ammonium cation complexation, and zinc-pyridine coordination or pi-pi-type interactions, stable zinc porphyrin-fullerene conjugates with defined distance and orientation were formed. Evidence for the zinc-pyridine complexation or pi-pi-type interactions was obtained from the spectral and computational studies. Steady-state and time-resolved emission studies revealed efficient quenching of the zinc-porphyrin singlet excited state in these dyads, and the measured rates of charge separation, k(CS) were found to be slightly better in the case of the dyads held by axial coordination and crown ether-cation complexation. Nanosecond transient absorption studies provided evidence for the electron transfer reactions, and these studies also revealed charge stabilization in these dyads. The lifetimes of the radical ion pairs were found to depend upon the type of porphyrins utilized to form the dyads, that is, porphyrin possessing the crown ether moiety at the ortho position of one of the phenyl rings yielded prolonged charge stabilized states. Addition of pyridine to the supramolecular dyads eliminated the zinc-pyridine coordination or pi-pi-type interactions of the "two-point" bound systems due to the formation of a new zinc-pyridine axial bond thus giving a unique opportunity to probe the effect of axial coordination or pi-pi interactions on k(CS) and k(CR). Under these conditions, the measured electron transfer rates revealed faster k(CS) and slower k(CR) as compared to those obtained in the absence of added pyridine. The evaluated lifetimes of the radical ion-pairs were found to be hundreds of nanoseconds and were longer in the presence of pyridine.     

Isomerization polymerization of β-propiolactones carrying side-chain ester groups

Based on our recent discovery of the isomerization polymerization of β-(2-acetoxyethyl)-β-propiolactone into poly-δ-ester,^1,2 we examined the generality of this phenomenon by using two related monomers. The catalysts were (EtAlO)_n and Et(ZnO)₂ZnEt. The side-chains in the monomers selected were the (CH₃)₂CHCOO? CH₂CH₂? (2) and (CH₃)CICHCOO? CH₂CH₂? (3) groups in which steric effects are almost identical but electronic effects are in opposition. The monomers yielded isomerized poly-δ-ester units, depending on the terminal substituent groups in the side-chain. These observations can be interpreted with the bicyclic intermediate proposed in the earlier work. Monomer (2) was reactive and produced a poly-δ-ester structure most readily, probably because of the higher electron density at the side-chain ester group which coordinated with the catalyst. In contrast, monomer (3) was less reactive, and the probability of isomerization was the lowest, i.e., the electron deficient side-chain ester group apparently interfered with the formation of the intermediate, especially in the Zn-catalysis. Equibinary random copolymers were prepared from (2) and (3) according to the catalyst and polymerization conditions chosen.     

A new synthesis of pyrroles by the condensation of cyclopropenes and nitriles mediated by gallium(III) and indium(III) salts

Gallium(III) and indium(III) halides were found to mediate the condensation of cyclopropenes and nitriles to give pyrrole derivatives in moderate yields. When excess nitrile was present, diazepine was also formed.     

m-Diethynylbenzene macrocycles: syntheses and self-association behavior in solution

m-Diethynylbenzene macrocycles (DBMs), buta-1,3-diyne-bridged [4(n)]metacyclophanes, have been synthesized and their self-association behaviors in solution were investigated. Cyclic tetramers, hexamers, and octamers of DBMs having exo-annular octyl, hexadecyl, and 3,6,9-trioxadecyl ester groups were prepared by intermolecular oxidative coupling of dimer units or intramolecular cyclization of the corresponding open-chain oligomers. The aggregation properties were investigated by two methods, the (1)H NMR spectra and the vapor pressure osmometry (VPO). Although some discrepancies were observed between the association constants obtained from the two methods, the qualitative view was consistent with each other. The analysis of self-aggregation by VPO revealed unique aggregation behavior of DBMs in acetone and toluene, which was not elucidated by the NMR method. Namely, the association constants for infinite association are several times larger than the dimerization constant, suggesting that the aggregation is enhanced by the formation of dimers (a nucleation mechanism). In polar solvents, DBMs aggregate more strongly than in chloroform due to the solvophobic interactions between the macrocyclic framework and the solvents. Moreover, DBMs self-associate in aromatic solvents such as toluene and o-xylene more readily than in chloroform. In particular, the hexameric DBM having a large macrocyclic cavity exhibits extremely large association constants in aromatic solvents. By comparing the aggregation properties of DBMs with the corresponding acyclic oligomers, the effect of the macrocyclic structure on the aggregation propensity was clarified. Finally, it turned out that DBMs tend to aggregate more readily than the corresponding phenylacetylene macrocycles, acetylene-bridged [2(n)]metacyclophanes, owing to the withdrawal of the electron density from the aromatic rings by the butadiyne linkages which facilitates pi-pi stacking interactions.     

Starburst dendrimer-supported pseudostationary phases for electrokinetic chromatography

Pseudo-stationary phases for electrokinetic chromatography were prepared by the alkylation of starburst dendrimers (SBDs). The SBD-supported pseudo-stationary phase with dodecyl groups showed higher efficiency than short-akyl derivatives, and maintained the hydrophobic property inthe presence of methanol. The dodecyl-modified SBD provided wide migration time windows ar high methanol content to effect the separation of sixteen aromatic hydrocarbons, the priority pollutants designated by EPA, in 65% methanol. The selectivity of polymer-supported pseudo-stationary phase can be varied simply by changing the length of the alkyl groups. The dodecyl SBD showed relatively similar selectivity as sodium dodecyl culfate micelle, whereas short alkyl derivatives showed preference towards rigid and planar compounds based on the rigid and planar compounds based on the rigid polymer backbones. The selectivity of SBD-supported pseudo-stationary phases was dominated by the chain length of the alkyl groups, with the minor effect of the structure of the core and the generation of SBD where alkyl groups were attached.     

Synthesis of Indolones via Radical Cyclization of N‐(2‐Halogenoalkanoyl)‐Substituted Anilines

The radical reactions of N‐(2‐halogenoalkanoyl)‐substituted anilines (anilides) of type 1 have been investigated under various conditions. Treatment of compounds 1a – 1o with Bu₃SnH in the presence of (2,2′‐azobis(isobutyronitrile) (AIBN) afforded a mixture of the indolones (oxindoles) 2a – 2o and the reduction products 5a – 5o (Table 1). In contrast, the N‐unsubstituted anilides 1p – 1s, 1u , and 1v gave the corresponding reduction products exclusively (Table 1). Similar results were obtained by treatment of 1 with Ni powder (Table 2) or wth Et₃B (Table 3). Anilides with longer N‐(phenylalkyl) chains such as 6 and 7 were inert towards radical cyclization, with the exception of N‐benzyl‐2‐bromo‐N,2‐dimethylpropanamide ( 6b ), which, upon treatment with Ni powder in i‐PrOH, afforded the cyclized product 9b in low yield (Table 4). Upon irradiation, the extended anilides 6, 7, 10 , and 11 yielded the corresponding dehydrobromination products exclusively (Table 5).