Ensemble and single-molecule studies on fluorescence quenching in transition metal bipyridine-complexes.

Beyond their use in analytical chemistry fluorescent probes continuously gain importance because of recent applications of single-molecule fluorescence spectroscopy to monitor elementary reaction steps. In this context, we characterized quenching of a fluorescent probe by different metal ions with fluorescence spectroscopy in the bulk and at the single-molecule level. We apply a quantitative model to explain deviations from existing standard models for fluorescence quenching.

Systematic study of the basis set superposition error in core-electron correlation effects.

In a recent paper, Xu et al. [J. Phys.

PERI-CC2: A Polarizable Embedded RI-CC2 Method.

We present a combination of the polarizable embedding (PE) method with the resolution-of-the-identity implementation of the approximate coupled-cluster singles and doubles method CC2. The new approach, termed PERI-CC2, allows one to study excited state phenomena of large solvated molecular systems with an accurate correlated wave function method. Central to the PE approach is the advanced description of the environmental electrostatic potential and inclusion of polarization, and the quintessence of RI-CC2 is efficient access to excited state properties while retaining the accuracy associated with CC theory.

Magnetic properties of paddlewheels and trinuclear clusters with exposed metal sites.

Magnetic exchange-coupling constants of tri- and dinuclear transition-metal complexes (paddlewheels) have been computed using various ab initio methods. The di- and trinuclear complexes under study may serve as secondary building units in metal-organic frameworks (MOFs). Multi-reference methods such as the complete-active-space self-consistent-field method (CASSCF) as well as second-order perturbation theory (CASPT2) yield spin ladders from which the magnetic exchange-coupling constants are obtained.

Highly sensitive plasmonic silver nanorods.

We compare the single-particle plasmonic sensitivity of silver and gold nanorods with similar resonance wavelengths by monitoring the plasmon resonance shift upon changing the environment from water to 12.5% sucrose solution. We find that silver nanoparticles have 1.

Coating colloidal carbon spheres with CdS nanoparticles: microwave-assisted synthesis and enhanced photocatalytic activity.

This manuscript describes the accurate coating of CdS nanoparticles on the surface of colloidal carbon spheres by a facile two-step, microwave-assisted method and the studies on the photocatalytic activity of the C@CdS core-shell spheres. For the coating of CdS nanoparticles, cadmium ions were incorporated into the hydrophilic shell of colloidal carbon spheres and reacted with an introduced sulfur source under a microwave field to obtain the C@CdS hybrid spheres. Using this process, the as-prepared hybrid structures preserved the good dispersity and uniformity of initial carbon spheres, and the thickness of the CdS nanoparticles shell could be varied or controlled by the irradiation time.

Quenched substrates for live-cell labeling of SNAP-tagged fusion proteins with improved fluorescent background.

Recent developments in fluorescence microscopy raise the demands for bright and photostable fluorescent tags for specific and background free labeling in living cells. Aside from fluorescent proteins and other tagging methods, labeling of SNAP-tagged proteins has become available thereby increasing the pool of potentially applicable fluorescent dyes for specific labeling of proteins. Here, we report on novel conjugates of benzylguanine (BG) which are quenched in their fluorescence and become highly fluorescent upon labeling of the SNAP-tag, the commercial variant of the human O(6)-alkylguanosyltransferase (hAGT).

Facile microemulsion route to coat carbonized glucose on upconversion nanocrystals as high luminescence and biocompatible cell-imaging probes.

Surface modification of lanthanide-doped upconversion nanocrystals is crucial to make them useful for various biological applications. Unfortunately, the current methods available to achieve a desirable surface exhibiting both high luminescence and good biocompatibility are limited. In this work, we present a facile microemulsion route to coat carbonized glucose on hydrophobic NaYF(4) nanocrystals.

Unexpected Trimerization of Pyrazine in the Coordination Sphere of Low-Valent Titanocene Fragments.

The titanium mediated trimerization of pyrazine leads to the formation of a tris-chelate complex employing a 4a,4b,8a,8b,12a,12b-hexahydrodiyprazino[2,3-f:2',3'-h]quinoxaline ligand (HATH6, 3). The driving force in the formation of the (Cp*2Ti)3(HATH6) complex 2 is attributed to the formation of six Ti-N bonds. We show that density functional theory (DFT) fails to predict quantitatively correct results.

Symmetry-Adapted Perturbation Theory Applied to Endohedral Fullerene Complexes: A Stability Study of H2@C60 and 2H2@C60.

Because of difficulties in a description of host-guest interactions, various theoretical methods predict different numbers of hydrogen molecules which can be inserted into the C60 cavity, ranging from one to more than 20. On the other hand, only one H2 molecule inside the C60 fullerene has been detected experimentally. Moreover, a recently synthesized H2@C70 complex prevails in the mixture formed with 2H2@C70.

Mechanisms of initial propane activation on molybdenum oxides: a density functional theory study.

We report the first detailed density functional theory study on the mechanisms of initial propane activation on molybdenum oxides. We consider 6 possible mechanisms of the C-H bond activation on metal oxides, leading to 17 transition states. We predict that hydrogen abstraction by terminal Mo=O is the most feasible reaction pathway.

Ag3SnCuP10: [Ag3Sn] tetrahedra embedded between adamantane-type [P10] cages.

Ag3SnCuP10 was synthesized from a stoichiometric mixture of copper, silver, tin, and red phosphorus, with SnI4 added as a mineralization agent. Cubic Ag3SnCuP10, space group F3m (No. 216) with lattice parameter a = 10.

Dynamics of bimolecular reactions of vibrationally highly excited molecules: quasiclassical trajectory studies.

Excitation functions from quasiclassical trajectory calculations on the H + H2O --> OH + H2, H + HF --> F + H2, and H + H'F --> H' + HF reactions indicate a different behavior at low and high vibrational excitation of the breaking bond. When the reactant tri- or diatomic molecule is in vibrational ground state or in a low vibrationally excited state, all these reactions are activated; i.e.

Mechanisms of methane activation and transformation on molybdenum oxide based catalysts.

We present a comprehensive survey of different C-H activation pathways over various kinds of active sites of terminal oxygens [=O] and bridge oxygens [-O-] by using Mo3O9 model systems. This allows us to provide some insights into fundamental issues concerning C-H activation by metal oxo species involved in many heterogeneous, homogeneous, and enzymatic processes. We show that H abstraction is the most feasible reaction pathway for the activation of a C-H bond on molybdenum oxides; and that [=O] is more active than [-O-].

Cation mobility and kinetics of ion exchange in zirconium hydrogen monothiophosphate hydrate, Zr(HPO(3)S)(2)x1.5H(2)O.

The ion conductivity of zirconium hydrogen monothiophosphate (Zr(HPO(3)S)(2)x1.5H(2)O) has been measured by impedance spectroscopy. The measured value of proton conductivity is 3 x 10(-5) S/cm at 298 K.

Edge dislocation in a vertical smectic-A film: line tension versus temperature and film thickness near the nematic phase.

The line tension of a dislocation is measured in a vertical smectic-A film as a function of temperature and film thickness. There are two contributions to the line tension: a bulk contribution that corresponds to the energy of the dislocation in an infinite medium and a surface correction that accounts for interactions with the two free surfaces. Both terms are measured in pure 8CB (octylcyanobiphenyl) as a function of temperature when the bulk nematic-smectic-A transition temperature T(c) is approached.

Spin-directed stereoselectivity of carbonyl-alkene photocycloadditions.

The concentration dependence of the Paterno-Buchi photocycloaddition of the two cyclic enolethers 2,3-dihydrofuran and 2,3-dihydropyran, respectively, with aromatic as well as aliphatic aldehydes was studied. For aliphatic aldehydes, a sharp transition from low to high diastereostereoselectivity was observed, indicating a switch from singlet to triplet photocycloaddition with different selectivity controlling mechanisms.

Photoinduced electron-transfer reactions with quinolinic and trimellitic acid imides: experiments and spin density calculations(1).

The regioselectivity of photoinduced electron-transfer (PET) reactions of unsymmetrical phthalimides is controlled by the spin density distribution of the intermediate radical anions. ROHF ab initio calculations were found to be most suitable for atomic spin density analysis. Intramolecular PET reactions of quinolinic acid imides were studied with the potassium butyrate and hexanoate 1a,b and a cysteine derivative 3.

Solution pH: A Selectivity Switch in Aqueous Organometallic Catalysis-Hydrogenation of Unsaturated Aldehydes Catalyzed by Sulfonatophenylphosphane-Ru Complexes.

The equilibribrium distribution of water-soluble ruthenium hydrides [HRuCl(tppms) ] and [H Ru(tppms) ] (tppms = (3-sulfonatophenyl)diphenylphosphane) in the reaction with H is governed by the pH value. As a consequence, the selectivity in the hydrogenation of cinnamaldehyde for reaction at C=C or C=O can be completely inverted by changing the pH value (see drawing below).

Synthesis, Reactivity, and Structure of [H(13)O(6)][PtCl(5)(H(4)O(2))].2(18-cr-6): A Crown Ether Complex of a Pentachloroaquaplatinic Acid with an [H(13)O(6)](+) Cation in a Cage of Three Crown Ether Molecules.

An aqueous solution of H(2)[PtCl(6)].6 H(2)O reacts with crown ether 18-crown-6 to give [H(13)O(6)][PtCl(5)(H(4)O(2))]. 2(18-cr-6)(1) (1) and (H(3)O)(2)[PtCl(6)].