Excited-state hydrogen-atom transfer along solvent wires: water molecules stop the transfer

Excited-state hydrogen-atom transfer (ESHAT) along a hydrogen-bonded solvent wire occurs for the supersonically cooled n = 3 ammonia-wire cluster attached to the scaffold molecule 7-hydroxyquinoline (7HQ) [Tanner, C.; et al. Science 2003, 302, 1736]. Here, we study the analogous three-membered solvent-wire clusters 7HQ.(NH3)n.(H2O)m, n + m = 3, using resonant two-photon ionization (R2PI) and UV-UV hole-burning spectroscopies. Substitution of H2O for NH3 has a dramatic effect on the excited-state H-atom transfer: The threshold for the ESHAT reaction is approximately 200 cm(-1) for 7HQ.(NH3)3, approximately 350 cm(-1) for both isomers of the 7HQ.(NH3)2.H2O cluster, and approximately 600 cm(-1) for 7HQ.NH3.(H2O)2 but increases to approximately 2000 cm(-1) for the pure 7HQ.(H2O)3 water-wire cluster. To understand the effect of the chemical composition of the solvent wire on the H-atom transfer, the reaction profiles of the low-lying electronic excited states of the n = 3 pure and mixed solvent-wire clusters are calculated with the configuration interaction singles (CIS) method. For those solvent wires with an NH3 molecule at the first position, injection of the H atom into the wire can occur by tunneling. However, further H-atom transfer is blocked by a high barrier at the first (and second) H2O molecule along the solvent wire. H-atom transfer along the entire length of the solvent wire, leading to formation of the 7-ketoquinoline (7KQ) tautomer, cannot occur for any of the H2O-containing clusters, in agreement with experimentally observed absence of 7KQ fluorescence.     

Influence of micromixer characteristics on polydispersity index of block copolymers synthesized in continuous flow microreactors

The influence of interdigital multilamination micromixer characteristics on monomer conversions, molecular weights and especially on the polydispersity index of block copolymers synthesized continuously in two microtube reactors is investigated. The micromixers are used to mix, before copolymerization, a polymer solution with different viscosities and the second monomer. Different geometries of micromixer (number of microchannels, characteristic lengths) have been studied. It was found that polydispersity indices of the copolymers follow a linear relationship with the Reynolds number in the micromixer, represented by a form factor. Thus, beside the operating conditions (nature of the first block and comonomer flow rate), the choice of the micromixer geometry and dimension is essential to control the copolymerization in terms of molecular weights and polydispersity indices. This linear correlation allows the prediction of copolymer features. It can also be a new method to optimize existing micromixers or design other geometries so that mixing could be more efficient.     

Validation of a new method using the reactivity of electrogenerated superoxide radical in the antioxidant capacity determination of flavonoids

This article lays out a new method to measure the antioxidant capacity of some flavonoids. The methodology developed is based on the kinetics of the reaction of the antioxidant substrate with the superoxide radical (O(2)(*-)). A cyclic voltammetric technique was used to generate O(2)(*-) by reduction of molecular oxygen in aprotic media. In the same experiment the consumption of the radical was directly measured by the anodic current decay of the superoxide radical oxidation in the presence of increasing concentrations of antioxidant substrate. The method was statistically validated on flavonoid monomers and on the standard antioxidants: trolox, ascorbic acid and phloroglucinol. The linear correlations between the anodic current of O(2)(*-) and the substrate concentration allowed the determination of antioxidant index values expressed by the substrate concentration needed to consume 30% (AI(30)) and 50% (AI(50)) of O(2)(*-) in given conditions of oxygen concentration and scanning rate. The fidelity of the method was examined intraday and interlaboratories.     

Spectral tuning by switching C-H[...]O hydrogen bonds: rotation-induced spectral shifts of 7-hydroxyquinoline HCOOH isomers

Spectral tuning effects on visible chromophores by hydrogen bonds are central to the chemistry of vision and of photosynthesis. A model for large spectral tuning effects by hydrogen bond switching is provided by the 7-hydroxyquinoline x HCOOH complex, which forms two isomers, CTN1 and CTN2, both with an HCOOH[...]N hydrogen bond but with different (quinoline)C-H[...]O=C hydrogen bonds. A 180 degrees rotation of the HCOOH moiety around the O-H[...]N hydrogen bond exchanges the C-H[...]O hydrogen bonds, rotates the dipole moment of HCOOH, and leads to an approximately 850 cm(-1) shift of the electronic spectrum. Mass-selected S1<--S0 resonant two-photon ionization, UV-UV holeburning, S1-->S0 fluorescence spectra, and photoionization efficiency curves of the two 7-hydroxyquinoline x HCOOH isomers were measured in supersonic expansions. Comparison to ab initio calculations allow us to determine the H-bond connectivity and structure of the two isomers and to assign their inter- and intramolecular vibrations. The Franck-Condon factors of the intermolecular shear vibration chi in the S1<--S0 spectra indicate that the weak C-H[...]O hydrogen bond contracts markedly in the CTN1 isomer but expands in the CTN2 isomer. These changes of H-bond lengths agree with the spectral shifts. In contrast, the strong O-H[...]N hydrogen bond undergoes little change upon S1[...]S0 excitation.     

Water substitution on iron centers: from 0D to 1D sandwich type polyoxotungstates

Four novel polyoxotungstates have been synthesized by reaction of the sandwich type compound [Fe (III) 4(H 2O) 10(B-beta-SbW 9O 33) 2] (6-) (noted Fe 4(H 2O) 10Sb 2W 18) with ethylenediamine (en) and/or oxalate (ox) ligands under various conditions. The one-dimensional (1D) compound [enH 2] 3[Fe (III) 4(H 2O) 8(SbW 9O 33) 2].20H 2O ( 1) is isolated at 130 degrees C and results from the elimination of two water molecules and the condensation of the polyoxotungstate precursor. The reaction of Fe 4(H 2O) 10Sb 2W 18 with oxalate ligands affords the molecular complex Na 14[Fe (III) 4(ox) 4(H 2O) 2(SbW 9O 33) 2].60H 2O ( 2) where two organic ligands substitute four water molecules, while the same reaction in the presence of en molecules at 130 degrees C leads to the formation of the functionalized 1D chain [enH 2] 7[Fe (III) 4(ox) 4(SbW 9O 33) 2].14H 2O ( 3) with protonated ethylenediamine counterions. Finally, at 160 degrees C a rearrangement of the Fe 4(H 2O) 10Sb 2W 18 polyoxotungstate is observed, and the sandwich type compound [enH 2] 5[Fe (II) 2Fe (II) 2(enH) 2(Fe (III)W 9O 34) 2].24H 2O ( 4) crystallizes. In 4, the heteroelement is a Fe (III) ion, and the water molecules on the two outer Fe (II) centers are bound to pendant monoprotonated en ligands. The four compounds have been characterized by IR spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. A detailed study of the magnetic properties of the mixed-valent hexanuclear iron complex in 4 shows evidence of an S = 5 ground-state because of spin frustration effects. A quantification of the electronic parameters characterizing the ground state ( D = +1.12 cm (-1), E/ D = 0.15) confirms that polyoxotungstate ligands induce large magnetic anisotropy.     

Self-assembly of fullerene-rich nanostructures with a stannoxane core

Fullerene derivatives bearing a carboxylic acid function undergo self-assembly with n-butylstannonic acid (nBuSn(O)OH) to produce fullerene-rich nanostructures with a stannoxane core in almost quantitative yields.     

Rapid synthesis of medium-ring fused polycarbocyclic systems by rearrangement of carbenoid-derived oxonium ylides

Tandem carbenoid generation, ylide formation and [2,3]-rearrangement is a powerful method for the construction of bicyclic and linearly fused tricyclic systems containing a seven-membered ring.     

Silver versus gold catalysis in tandem reactions of carbonyl functions onto alkynes: a versatile access to furoquinoline and pyranoquinoline cores

An efficient and versatile tandem process of acetalization and cycloisomerization reactions has been developed for the reactions of 1-alkynyl-2-carbonylquinoline substrates. The reaction occurs thanks to Au(I) and Ag(I) catalysis. Silver(I) catalysis has been extensively studied (11 different silver species) on a broad range of quinoline derivatives (variation of alkyne substituent, of carbonyl function and of nucleophiles), leading to a variety of furoquinoline and pyranoquinoline moieties. An insight is given for the presumed mechanism along with DFT-B3 LYP/6-31G** calculations to address the 6-endo and 5-exo regioselectivities observed.