Zinc-zinc bonded zincocene structures. Synthesis and characterization of Zn2(eta5-C5Me5)2 and Zn2(eta5-C5Me4Et)2

While, in general, decamethylzincocene, Zn(C5Me5)2, and other zincocenes, Zn(C5Me4R)2 (R = H, But, SiMe3), react with dialkyl and diaryl derivatives, ZnR'2, to give the half-sandwich compounds (eta5-C5Me4R)ZnR', under certain conditions the reactions of Zn(C5Me5)2 with ZnEt2 or ZnPh2 produce unexpectedly the dizincocene Zn2(eta5-C5Me5)2 (1) in low yields, most likely as a result of the coupling of two (eta5-C5Me5)Zn* radicals. An improved, large scale (ca. 2 g) synthesis of 1 has been achieved by reduction of equimolar mixtures of Zn(C5Me5)2 and ZnCl2 with KH in tetrahydrofuran. The analogous reduction of Zn(C5Me4R)2 (R = H, SiMe3, But) yields only decomposition products, but the isotopically labeled dimetallocene 68Zn2(eta5-C5Me5)2 and the related compound Zn2(eta5-C5Me4Et)2 (2) have been obtained by this procedure. Compound 2 has lower thermal stability than 1, but it has been unequivocally characterized by low-temperature X-ray diffraction studies. As for 1 a combination of structural characterization techniques has provided unambiguous evidence for its formulation as the Zn-Zn bonded dimer Zn2(eta5-C5Me4Et)2, with a short Zn-Zn bond of 2.295(3) A indicative of a strong Zn-Zn bonding interaction. The electronic structure and the bonding properties of 1 and those of related dizincocenes Zn2(eta5-Cp')2 have been studied by DFT methods (B3LYP level), with computed bond distances and angles for dizincocene 1 very similar to the experimental values. The Zn-Zn bond is strong (ca. 62 kcal.mol-1 for 1) and resides in the HOMO-4, that has a contribution of Zn orbitals close to 60%, consisting mostly of the Zn 4s orbitals (more than 96%).     

Determination of polyethylene glycol in low-density polyethylene by large volume injection temperature gradient packed capillary liquid chromatography

Polyethylene glycol (PEG) 20000 in low-density polyethylene has been determined using column switching and inverse temperature programming in reversed-phase packed capillary liquid chromatography with evaporative light scattering detection. PEG 20000 was extracted into water from the polyethylene dissolved in toluene and PEG 35000 was added as an internal standard (I.S.). The samples in aliquots of 100 microl were reconcentrated on the enrichment column using a loading mobile phase of acetonitrile-water (3:97, v/v) at a flow-rate of 75 microl/min for 3 min, then back-flushed and separated on the analytical column with acetonitrile-THF-water (40:5:55, v/v) as mobile phase. The column temperature was reduced from 68 to 55 degrees C with a ramp of -1.5 degrees C/min, held constant for 3 min and then reduced further to 45 degrees C with a -1.5 degrees C/min ramp and kept constant for 1 min. The analysis runtime was 20 min. The recovery of PEG 20 000 was determined to 65.1% with 2.8% RSD and the mass limit of detection of PEG 20 000 was 1.25 microg. The within-assay and between day precision of the retention times of both PEG 20000 and PEG 35000 displayed RSD of less than 1.1% (n = 9), while the overall area ratio RSD of 100 microg/ml PEG 20000 over PEG 35000 was 1.3% (n = 9). The method was linear within the investigated concentration range 25-125 microg/ml (R2 = 0.9983). In addition, a mixture of PEG 4000, 8000, 10000, 20000 and 35000 was analysed on the system to demonstrate the possibility of analysing several PEGs in a sample with the use of temperature gradient elution.     

Density functional restricted-unrestricted approach for nonlinear properties: application to electron paramagnetic resonance parameters of square planar copper complexes

The density functional restricted-unrestricted approach for treatments of spin polarization effects in molecular properties using spin restricted Kohn-Sham theory has been extended from linear to nonlinear properties. It is shown that the spin polarization contribution to a nonlinear property has the form of a quadratic response function that includes the zero-order Kohn-Sham operator, in analogy to the lower order case where the spin polarization correction to an expectation value has the form of a linear response function. The developed approach is used to formulate new schemes for computation of electronic g-tensors and hyperfine coupling constants, which include spin polarization effects within the framework of spin restricted Kohn-Sham theory. The proposed computational schemes are in the present work employed to study the spin polarization effects on electron paramagnetic resonance spin Hamiltonian parameters of square planar copper complexes. The obtained results indicate that spin polarization gives rise to sizable contributions to the hyperfine coupling tensor of copper in all investigated complexes, while the electronic g-tensors of these complexes are only marginally affected by spin polarization and other factors, such as choice of exchange-correlation functional or molecular structures, will have more pronounced impact on the accuracy of the results.     

Quality specifications for evaluation and comparison of performance among external quality assessment schemes in occupational and environmental laboratory medicine

Quality specifications (QS) are proposed for lead in blood and for aluminium, copper, selenium and zinc in serum as part of the aim to set standards of performance for laboratories so that results can be demonstrated to be fit for the purpose to which they are applied. The QS were established taking account of the analytical state-of-the-art, physiological variations in the concentrations of the analyte and the clinical purpose for which the assay is to be used. A procedure was devised that uses these QS to give equivalence of assessment among external quality assessment schemes (EQAS), thus avoiding conflicting information which has been demonstrated in the past. Advantages of this procedure are: to provide direct comparison of performance of laboratories taking part in different schemes, to provide equivalence of assessment of laboratory performance necessary to establish mutual recognition agreements, and to demonstrate the fitness for purpose of results from participants.Presented at the Eurachem PT Workshop September 2005, Portorož, Slovenia     

Femtosecond photolysis of aqueous formamide

In this work, we investigate the primary photodynamics of aqueous formamide. The formamide was photolyzed using 200 nm femtosecond pulses, and formation of products and their relaxation was followed with approximately 300 fs time resolution using probe pulses covering the range from 193 to 700 nm. Following excitation, the majority of formamide molecules (approximately 80%) converts the electronic excitation energy to vibrational excitation, which effectively is dissipated to the solvent through vibrational relaxation in just a few picoseconds. The vibrational relaxation is observed as a distinct modulation of the electronic absorption spectrum of formamide. The relaxation process is modeled by a simple one-dimensional wavepacket calculation. A smaller fraction of the excited formamide molecules dissociates to the CHO and NH2 radical pairs, of which 50% escape recombination. In addition to the electronic excitation of formamide, we also observe a small contribution from one-photon ionization of formamide and two-photon ionization and dissociation of the water solvent.     

Unusual process of water formation on RuO2(110) by hydrogen exposure at room temperature

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.     

RNA fragmentation in MALDI mass spectrometry studied by H/D-exchange: Mechanisms of general applicability to nucleic acids

To reveal the gas-phase chemistry of RNA and DNA fragmentation during MALDI mass spectrometry in positive ion mode, we performed hydrogen/deuterium exchange on a series of RNA and DNA tetranucleotides and studied their fragmentation patterns on a high-resolution MALDI TOF-TOF instrument. We were specifically interested in elucidating the remarkably different fragmentation behavior of RNA and DNA, i.e., the characteristic and abundant production of c- and y-ions from RNA versus a dominating generation of (a-B)- and w-ions from DNA analytes. The analysis yielded important information on all significant backbone cleavages as well as nucleobase losses. Based on this, we suggest common fragmentation mechanisms for RNA and DNA as well as an important RNA-specific reaction requiring a 2'-hydroxyl group, leading to c- and y-ions. The data is viewed and discussed in the context of previously published data to obtain a coherent picture of the fragmentation of singly protonated nucleic acids.     

Atmospheric chemistry of perfluorinated aldehyde hydrates (n-C(x)F(2x+1)CH(OH)2, x = 1, 3, 4): hydration, dehydration, and kinetics and mechanism of Cl atom and OH radical initiated oxidation

Smog chamber/Fourier transform infrared (FTIR) techniques were used to measure k(Cl+C(x)F(2x+1)CH(OH)(2)) (x = 1, 3, 4) = (5.84 +/- 0.92) x 10(-13) and k(OH+C(x)F(2x+1)CH(OH)(2)) = (1.22 +/- 0.26) x 10(-13) cm(3) molecule(-1) s(-1) in 700 Torr of N(2) or air at 296 +/- 2 K. The Cl initiated oxidation of CF(3)CH(OH)(2) in 700 Torr of air gave CF(3)COOH in a molar yield of 101 +/- 6%. IR spectra of C(x)F(2x+1)CH(OH)(2) (x = 1, 3, 4) were recorded and are presented. An upper limit of k(CF(3)CHO+H(2)O) < 2 x 10(-23) cm(3) molecule(-1) s(-1) was established for the gas-phase hydration of CF(3)CHO. Bubbling CF(3)CHO/air mixtures through liquid water led to >80% conversion of CF(3)CHO into the hydrate within the approximately 2 s taken for passage through the bubbler. These results suggest that OH radical initiated oxidation of C(x)F(2x+1)CH(OH)(2) hydrates could be a significant source of perfluorinated carboxylic acids in the environment.     

Kinetics of the reduction of the Rh(111) surface oxide: linking spectroscopy and atomic-scale information

The reduction of the surface oxide on Rh(111) by H(2) was observed in situ by scanning tunneling microscopy (STM) and high-resolution core level spectroscopy (HRCLS). At room temperature, H(2) does not adsorb on the oxide, only in reduced areas. Reduction starts in very few sites, almost exclusively in stepped areas. One can also initiate the reduction process by deliberately creating defects with the STM tip allowing us to examine the reduction kinetics in detail. Depending on the size of the reduced area and the hydrogen pressure, two growth regimes were found. At low H(2) pressures or small reduced areas, the reduction rate is limited by hydrogen adsorption on the reduced area. For large reduced areas, the reduction rate is limited by the processes at the border of the reduced area. Since a near-random distribution of the reduction nuclei was found and the reduction process at defects starts at a random time, one can use Johnson-Mehl-Avrami-Kolmogoroff (JMAK) theory to describe the process of reduction. The microscopic data from STM agree well with spatially averaged data from HRCLS measurements.     

Crude Oil Model Emulsion Characterised by means of Near Infrared Spectroscopy and Multivariate Techniques

Water-in-oil emulsions are investigated by means of multivariate analysis of near infrared (NIR) spectroscopic profiles in the range 1100 — 2250 nm. The oil phase is a paraffin-diluted crude oil from the Norwegian Continental Shelf. The influence of water absorption and light scattering of the water droplets are shown to be strong. Despite the strong influence of the water phase, the NIR technique is still capable of predicting the composition of the investigated oil phase.