The structure and EPR properties of (CH2OHCH2NH3)2CuCl4

The title compound is a member of the (RNH₃)₂CuCl₄ layer perovskite family, space group C2/c, witha=21.624(5),b=7.511(1),c=7.431(2)Å, =89.56(2)° andV=1206.9(4)ų withZ=4. In the antiferrodistortive structure, the Cu(II) ion assumes an elongated octahedral coordination (unique Cu–Cl distances=2.274(2), 2.288(2), and 3.010(2)Å). The octahedra share corners with four neighbors to form a two-dimensional network. The –NH₃ moieties hydrogen bond to the layers, so that the –C₂H₅OH moieties form sheathes about each metal halide layer. A twofold disorder of the –C₂H₅OH groups is observed. The EPR spectrum is investigated to help understand the magnetic properties of the systems. Theg-values are consistent with strong intralayer exchange coupling. The EPR linewidths show evidence of spin diffusion effects at liquid nitrogen temperature. However, at room temperature, the spin anisotropies lead to broadening of the EPR lines through a phonon modulation mechanism.     

Nonexponential statistics of fluorescence photobleaching

In this paper, I consider theoretical models of the decay via photobleaching of a sample of surface-immobilized fluorescent molecules excited by a spatially varying laser intensity profile. I show that, with mild restrictions on the photobleaching mechanism, the fluorescence decay measured in a nonuniform excitation profile is always nonexponential. Under the same conditions, the fluorescence decay can always be approximated by a discrete sum of exponentials. A particular example is given in which a homogeneous population of fluorophores with a single (intensity-dependent) photobleaching lifetime, when illuminated by a Gaussian laser, exhibits power law fluorescence decay at long times. These results indicate that the observation of multiple exponentials in single molecule or ensemble photobleaching lifetime measurements can arise solely as an artifact of a spatially varying laser profile and is not necessarily indicative of heterogeneity in molecular internal states, conformation, or local environment.     

Computer system and flow cell for automated potentiometric and constant-current stripping analysis

A flow cell and a personal computer-based system for potentiometric and constant-current stripping analysis are described. The thin-layer cell is designed for teflon-embedded glassy carbon electrodes in a spring-regulated holder, allowing simple instalment and leak-free behaviour. The personal computer is interfaced to hardware components such as potentiostat, amplifier, peristaltic pump, six inlet valves for different solutions, autosampler and strip-chart recorder. User-accessible commands allow electrolysis, stripping current, pump flow rates, valves, stripping transient recording, derivation, filtration and integration to be controlled, and through a number of user-defined programs, to be performed in unattended operation. This is described by an example. Through a software interface to a high-level language (BASIC), results from any number of analytical runs can be further evaluated by standard addition, or from calibration curves, or by statistical methods.     

Electron donor-acceptor dyads and triads based on tris(bipyridine)ruthenium(II) and benzoquinone: synthesis,characterization, and photoinduced electron transfer reactions

Two electron donor-acceptor triads based on a benzoquinone acceptor linked to a light absorbing [Ru(bpy)(3)](2+) complex have been synthesized. In triad 6 (denoted Ru(II)-BQ-Co(III)), a [Co(bpy)(3)](3+) complex, a potential secondary acceptor, was linked to the quinone. In the other triad, 8 (denoted PTZ-Ru(II)-BQ), a phenothiazine donor was linked to the ruthenium moiety. The corresponding dyads Ru(II)-BQ (4) and PTZ-Ru(II) (9) were prepared for comparison. Upon light excitation in the visible band of the ruthenium moiety, electron transfer to the quinone occurred with a rate constant k(f) = 5 x 10(9) s(-)(1) (tau(f) = 200 ps) in all the quinone containing complexes. Recombination to the ground state followed, with a rate constant k(b) approximately 4.5 x 10(8) s(-)(1) (tau(b) approximately 2.2 ns), for both Ru(II)-BQ and Ru(II)-BQ-Co(III) with no indication of a charge shift to generate the reduced Co(II) moiety. In the PTZ-Ru(II)-BQ triad, however, the initial charge separation was followed by a rapid (k > 5 x 10(9) s(-)(1)) electron transfer from the phenothiazine moiety to give the fairly long-lived PTZ(*)(+)-Ru(II)-BQ(*)(-) state (tau = 80 ns) in unusually high yield for a [Ru(bpy)(3)](2+)-based triad (> 90%), that lies at DeltaG degrees = 1.32 eV relative to the ground state. Unfortunately, this triad turned out to be rather photolabile. Interestingly, coupling between the oxidized PTZ(*)(+) and the BQ(*)(-) moieties seemed to occur. This discouraged further extension to incorporate more redox active units. Finally, in the dyad PTZ-Ru(II) a reversible, near isoergonic electron transfer was observed on excitation. Thus, a quasiequilibrium was established with an observed time constant of 7 ns, with ca. 82% of the population in the PTZ-Ru(II) state and 18% in the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state. These states decayed in parallel with an observed lifetime of 90 ns. The initial electron transfer to form the PTZ(*)(+)-Ru(II)(bpy(*)(-)) state was thus faster than what would have been inferred from the Ru(II) emission decay (tau = 90 ns). This result suggests that reports for related PTZ-Ru(II) and PTZ-Ru(II)-acceptor complexes in the literature might need to be reconsidered.     

Use of a temperature‐programmable injector coupled to gas chromatography–combustion–isotope ratio mass spectrometry for compound‐specific carbon isotopic analysis of polycyclic aromatic hydrocarbons

Compound‐specific isotopic analysis (CSIA) can provide information about the origin of analysed compounds; for instance, polycyclic aromatic hydrocarbons (PAHs) in aerosols. This could be a valuable tool in source apportionment of particulate matter (PM) air pollution. Because gas chromatography–combustion–isotope ratio mass spectrometry (GC‐C‐IRMS) analysis requires an amount of at least 10 ng of an individual PAH, a high concentration of PAHs in the injected extract is needed. When the concentration is low a large volume injector creates the possibility of introducing a satisfactory amount of individual PAHs. In this study a temperature‐programmable injector was coupled to GC‐C‐IRMS and injection parameters (solvent level, transfer column flow, transfers time) were optimised using six solid aromatic compounds (anthracene, fluoranthene, pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene) dissolved in n‐pentane and EPA 610 reference mixture. CSIA results for solid PAHs were compared with results obtained for the single components analysed by elemental analysis–isotope ratio mass spectrometry. The injection method was validated for two sample injection volumes, 50 and 100 µL. This method was also compared with commonly used splitless injection. To be included in the study, measurements had to have an uncertainty lower than 0.5‰ for and a minimum peak height of 200 mV. The lower concentration limits at which these criteria were fulfilled for PAHs were 30 mg/L for 1 µL in splitless injection and 0.3 and 0.2 mg/L for 50 and 100 µL, respectively, in large volume injection. Copyright © 2009 John Wiley & Sons, Ltd.     

Unexpected intermolecular pd-catalyzed cross-coupling reaction employing heteroaromatic carboxylic acids as coupling partners

Aryl-substituted five-membered heteroaromatics have attracted great interest over the past years due to their presence in a large number of pharmaceuticals and natural products. Recently, an advance in the preparation of these scaffolds was achieved by employing a C-H functionalization strategy. This method allows easy access to these biaryl motifs by precluding the necessity of preparing specific coupling partners, although poor regioselectivity is sometimes observed when more than one reactive C-H is present on the substrate. In an effort to circumvent this liability, we envisioned the use of a carboxylic acid moiety as a blocking group that could be later functionalized or removed. Remarkably, the coupling was found to occur exclusively at the position previously occupied by the acid, even in the presence of a reactive C-H group. This selective transformation was also found to proceed with other heteroaromatic carboxylic acids, allowing for the preparation of a variety of aryl-substituted heteroaromatics that would be difficult to obtain via other methods.     

Air-trapping on biocompatible nanopatterns

The occurrence of air-trapping inside poly-eta-caprolactone nanopits was investigated by measuring the contact angles of water droplets on a set of defined nanotopographies. It is shown that the advancing angles follow the Cassie-Baxter theory, thus revealing the presence of air bubbles inside the biodegradable nanopatterns. The importance of these observations for the definition of hydrophilicity/hydrophobicity and in the context of in vitro cell behavior is discussed.     

Preparation and characterisation of synthetic mixtures of lithium isotopes

Re-certification of the absolute isotopic composition of the natural lithium isotopic reference material (IRM), IRMM-016, requires measurements calibrated by means of synthetic mixtures of highly enriched lithium isotopes. Ten such mixtures were prepared by weighing and mixing of two well characterised, isotopically enriched, Li₂CO₃ compounds. The starting materials, 99.9981% enriched ⁶Li, and 99.9937% enriched ⁷Li, were purified by ion exchange, and the purified materials converted from LiOH to Li₂CO₃ by reaction with CO₂. Ten new mixtures were prepared by mixing different weighed amounts of these dissolved Li₂CO₃ carrier compounds. The compounds had an estimated level of impurities of 100 ± 100 μg · g^–1 (expanded uncertainty with a coverage factor of 2). In the ten mixtures, the n(⁶Li)/n(⁷Li) ratio varies from 0.025 to 14 and the achieved expanded relative uncertainty on the amount ratio prepared is typically 2 · 10^–4. These mixtures were then used to determine the correction factor, K, for mass discrimination of the measurement procedure and instrument concerned.