Special pair dance and partner selection: elementary steps in proton transport in liquid water

Conditional and time-dependent radial distribution functions reveal the details of the water structure surrounding the hydronium during the proton mobility process. Using this methodology for classical multistate empirical valence bond (MS-EVB) and ab initio molecular dynamics trajectories, as well as quantal MS-EVB trajectories, we supply statistical proof that proton hops in liquid water occur by a transition from the H3O+[3H2O] Eigen-complex, via the H5O2+ Zundel-complex, to a H3O+[3H2O] centered on a neighboring water molecule. In the "resting period" before a transition, there is a distorted hydronium with one of its water ligands at a shorter distance and another at a longer distance than average. The identity of this "special partner" interchanges rapidly within the three first-shell water ligands. This is coupled to cleavage of an acceptor-type hydrogen bond. Just before the transition, a partner is selected by an additional translation of the H3O+ moiety in its direction, possibly enabled by loosening of donor-type hydrogen bonds on the opposite side. We monitor the transition in real time, showing how the average structure is converted to a distorted H5O2+ cation constituting the transitional complex for proton hopping between water molecules.     

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.     

Ruthenium-Catalyzed Hydroarylation of Methylenecyclopropanes through CH Bond Cleavage: Scope and Mechanism

Intermolecular hydroarylation reactions of highly strained methylenecyclopropanes 2-phenylmethylenecyclopropane (1), 2,2-diphenylmethylenecyclopropane (2), methylenespiropentane (3), bicyclopropylidene (4), (dicyclopropylmethylene)cyclopropane (5), and benzhydrylidenecyclopropane (6) through C?H bond functionalization of 2-phenylpyridine (7?a) and other arenes with directing groups were studied. The reaction was very sensitive to the substitution on the methylenecyclopropanes. Although these transformations involved (cyclopropylcarbinyl)-metal intermediates, substrates 1 and 4 furnished anti-Markovnikov hydroarylation products with complete conservation of all cyclopropane rings in 11-93?% yield, whereas starting materials 3 and 5 were inert toward hydroarylation. Methylenecyclopropane 6 formed the products of formal hydroarylation reactions of the longest distal C?C bond in the methylenecyclopropane moiety in high yield, and hydrocarbon 2 afforded mixtures of hydroarylated products in low yields with a predominance of compounds that retained the cyclopropane unit. As byproducts, Diels-Alder cycloadducts and self-reorganization products were obtained in several cases from substrates 1-3 and 5. The structures of the most important new products have been unambiguously determined by X-ray diffraction analyses. On the basis of the results of hydroarylation experiments with isotopically labeled 7?a-[D(5) ], a plausible mechanistic rationale and a catalytic cycle for these unusual ruthenium-catalyzed hydroarylation reactions have been proposed. Arene-tethered ruthenium-phosphane complex 53, either isolated from the reaction mixture or independently prepared, did not show any catalytic activity.     

Tuning the spin-transition properties of pyrene-decorated 2,6-bispyrazolylpyridine based Fe(II) complexes

Two 2,6-bispyrazolylpyridine ligands (bpp) were functionalized with pyrene moieties through linkers of different lengths. In the ligand 2,6-di(1H-pyrazol-1-yl)-4-(pyren-1-yl)pyridine (L1) the pyrene group is directly connected to the bpp moiety via a C-C single bond, while in the ligand 4-(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzyl-4-(pyren-1-yl)butanoate (L2) it is separated by a benzyl ester group involving a flexible butanoic chain. Subsequent complexation of Fe(II) salts revealed dramatic the influence of the nature of the pyrene substitution on the spin-transition behaviour of the resulting complexes. Thus, compound [Fe(L1)(2)](ClO(4))(2) (1) is blocked in its high spin state due to constraints caused by a strong intermolecular π-π stacking in its structure. On the other hand, the flexible chain of ligand L2 in compounds [Fe(L2)(2)](ClO(4))(2) (2) and [Fe(L2)(2)](BF(4))(2)·CH(3)CN·H(2)O (3) prevents structural constraints allowing for reversible spin transitions. Temperature-dependent studies of the photophysical properties of compound 3 do not reveal any obvious correlation between the fluorescence of the pyrene group and the spin state of the spin transition core.     

Photorearrangement of alpha-azoxy ketones and triplet sensitization of azoxy compounds

[reaction: see text] Although some aspects of azoxy group radical chemistry have been investigated, unhindered alpha-azoxy radicals remain poorly understood. Here we report the generation of alpha-azoxy radicals under mild conditions by irradiation of alpha-azoxy ketones 4a,b. These compounds undergo alpha-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives alpha-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.     

Selective dispersion of single-walled carbon nanotubes with specific chiral indices by poly(N-decyl-2,7-carbazole)

Physico-chemical methods to sort single-walled carbon nanotubes (SWNTs) by chiral index are presently lacking but are required for in-depth experimental analysis and also for potential future applications of specific species. Here we report the unexpected selectivity of poly(N-decyl-2,7-carbazole) to almost exclusively disperse semiconducting SWNTs with differences of their chiral indices (n - m) ≥ 2 in toluene. The observed selectivity complements perfectly the dispersing features of the fluorene analogue poly(9,9-dialkyl-2,7-fluorene), which disperses semiconducting SWNTs with (n - m) ≤ 2 in toluene. The dispersed samples are further purified by density gradient centrifugation and analyzed by photoluminescence excitation spectroscopy. All-atom molecular modeling with decamer model compounds of the polymers and (10,2) and (7,6) SWNTs suggests differences in the π-π stacking interaction as origin of the selectivity. We observe energetically favored complexes between the (10,2) SWNT and the carbazole decamer and between the (7,6) SWNT and the fluorene decamer, respectively. These findings demonstrate that subtle structural changes of polymers lead to selective solvation of different families of carbon nanotubes. Furthermore, chemical screening of closely related polymers may pave the way toward simple, low-cost, and index-specific isolation of SWNTs.     

Utilization of cyanobacteria in photobioreactors for orthophosphate removal from water

The effectiveness of photosynthetic free-living and polyurethane foam (PU) immobilized Anabaena variabilis cells for, removal of orthophosphate (P) from water in batch cultures and in a photobioreactor was studied. Immobilization in PU foams was found to have a positive effect on P uptake by cyanobacteria in batch cultures. The efficiency of P uptake by immobilized cells was higher than by free-living cells. A laboratory scale photobioreactor was constructed for removal of P from water by the immobilized cyanobacteria. The photobioreactor was designed so that the growth medium (water) from a reservoir was pumped through a photobioreactor column with immobilized cyanobacteria and back to the reservoir. This created a closed system in which it was possible to measure P uptake. No leakage of cells into the photobioreactor medium reservoir was observed during the operation. The immobilized cells incorporated into a photobioreactor column removed P continuously for about 15 d. No measurable uptake was demonstrated after this period. Orthophosphate uptake efficiency of 88–92% was achieved by the photobioreactor.     

4-Nitro-2-(1H-tetrazol-1-yl)­phenol

There are two symmetry-independent mol­ecules in the unit cell of the title compound, C₇H₅N₅O₃. The tetrazole and phenyl rings are essentially planar and are not coplanar in either mol­ecule [dihedral angles 30.2 (1) and 7.0 (1)°]. In the structure, four mol­ecules are connected by O—H⃛N bridges, forming four-membered molecular aggregates which are linked together by a complex three-dimensional hydrogen-bond network.     

CdBiO2Cl: synthesis and powder structure solution

The title compound, cadmium bismuth dioxide chloride, CdBiO₂Cl, was obtained as a white powder by reaction of solid BiOCl with CdO at 973 K. Ab initio crystal structure determination was carried out using X-ray powder diffraction techniques, including direct methods for atom location and Rietveld fitting for the final refinement. Being monoclinic, the crystal structure can be related to tetragonal Sillen layered phases. The main structural elements present are CdBiO₂⁺ pleated metal–oxy­gen layers alternating with Cl layers along the c axis, whereas along the b axis, all atoms are on mirror planes. The formation of a strong Cd—Cl bond draws the layers together, causing layer deformation and a monoclinic distortion in the layer arrangement.