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Optische Aerosolspektrometer werden bei der zeitaufgelösten Bestimmung der Exposition gegenüber alveolengängigem Staub (A-Staub) an Arbeitsplätzen, aber auch bei Aerosolmessungen in Innenräumen, z. B. im Zusammenhang mit der COVID-19-Pandemie, zunehmend eingesetzt. Diese Geräte messen das von Aerosolpartikeln gestreute Licht als Maß für deren Größe. Um aus den Messergebnissen z.B. die A-Staub-Massenkonzentration bestimmen zu können, sind somit Annahmen hinsichtlich der Partikeleigenschaften, insbesondere des Brechungsindizes und der Dichte, erforderlich. Für die Bestimmung coronarelevanter Aerosole sind vor allem flüssige Aerosolpartikel von Bedeutung. Im Rahmen der hier vorgestellten Untersuchungen wurden Aerosolspektrometer verschiedener Hersteller im Labor sowie in Ringversuchen mit unterschiedlichen monodispersen und polydispersen sowie festen und flüssigen Prüfaerosolen beaufschlagt und die Messwerte den Ergebnissen von Referenzverfahren gegenübergestellt. Es zeigte sich, dass die Aerosolspektrometer generell für die genannten Einsatzzwecke geeignet sind. Um möglichst genaue Ergebnisse zu erzielen, ist aber ggf. eine individuelle Gerätekalibrierung für den jeweiligen Einsatzzweck erforderlich.

This article presents a novel approach towards tackling the balance between filter efficiency and pressure drop by layering micro and nano fibres using melt and solution electrospinning. Polyamide fibre mats were fabricated by a prototype fibre printing apparatus, based on a combination of melt and solution electrospinning. Fibrous mats with favorable filtration performance characteristics were developed by optimizing the process parameters. The measurements were performed at a face velocity of 5.3 cm/s. High filtration quality factors (such as 0.068–0.085 Pa-1 for PN1 fraction) were reached due to low values of pressure drop (15.92–50.17 Pa). The filtration efficiencies ranged from 93.7 (PN1) to 98.5% (PN10). Higher ratios of nano-sub micrometer (

Novel methods have been developed to measure the fractional deposition efficiency for nanoparticles of full-scale HVAC and HEPA filter cassettes down to a particle size of 3.5 nm. The methods use a flame spray nanoparticle generator to produce NaCl test aerosols with narrow size distributions and very high concentrations. While the efficiency curves of lower efficiency filters of classes F7 and E10 were still able to be determined by measuring the size distributions of the polydisperse test aerosols upstream and downstream of the filter, two new testing procedures were developed for high efficiency filters of class H13. One considers the narrow size distributions of the test aerosols as quasi-monodisperse and follows a similar approach like EN 1822 for flat sheet media. The second one evaluates mobility classified fractions of the quasi-monodisperse test aerosols. A dedicated multiple charge correction scheme was developed to account for the effect of multiply charged particles. While the latter procedure allows to extend the particle size range, the prior significantly reduces the measurement time. All tests delivered meaningful results, which were very comparable with the results from flat sheet media tests.

Exposure assessment is a key stage in the risk assessment/management of engineered nanomaterials. Although different sampling strategies and instruments have been used to define the occupational exposure to nano-scale materials, currently there is no international consensus regarding measurement strategy, metrics and limit values. In fact, the assessment of individual exposure to engineered nanomaterials remains a critical issue despite recent innovative developments in personal monitors and samplers. Hence, we used several of these instruments to evaluate the workers' personal exposure in a large research laboratory where engineered nanomaterials are produced, handled, and characterized in order to provide input data for nanomaterial exposure assessment strategies and future epidemiological studies. The results obtained using personal monitors showed that the workplace concentrations of engineered nanomaterials (lung deposited surface area and particle number concentrations) were quite low in all the different workplaces monitored, with short spikes during the execution of some specific job tasks. The sampling strategy was been adopted on the basis of an Organisation for Economic Cooperation and Development (OECD) suggestion for a tiered approach and was found to be suitable for determining the individual exposure and for identifying possible sources of emission, even those with very low emission rates. The use of these instruments may lead to a significant improvement not only in the exposure assessment stage but, more generally, in the entire risk assessment and management process.

Personal monitors based on unipolar diffusion charging (miniDiSC/DiSCmini, NanoTracer, Partector) can be used to assess the individual exposure to nanoparticles in different environments. The charge acquired by the aerosol particles is nearly proportional to the particle diameter and, by coincidence, also nearly proportional to the alveolar lung-deposited surface area (LDSA), the metric reported by all three instruments. In addition, the miniDiSC/DiSCmini and the NanoTracer report particle number concentration and mean particle size. In view of their use for personal exposure studies, the comparability of these personal monitors was assessed in two measurement campaigns. Altogether 29 different polydisperse test aerosols were generated during the two campaigns, covering a large range of particle sizes, morphologies and concentrations. The data provided by the personal monitors were compared with those obtained from reference instruments: a scanning mobility particle sizer (SMPS) for LDSA and mean particle size and a ultrafine particle counter (UCPC) for number concentration. The results indicated that the LDSA concentrations and the mean particle sizes provided by all investigated instruments in this study were in the order of ± 30% of the reference value obtained from the SMPS when the particle sizes of the test aerosols generated were within 20–400 nm and the instruments were properly calibrated. Particle size, morphology and concentration did not have a major effect within the aforementioned limits. The comparability of the number concentrations was found to be slightly worse and in the range of ± 50% of the reference value obtained from the UCPC. In addition, a minor effect of the particle morphology on the number concentration measurements was observed. The presence of particles > 400 nm can drastically bias the measurement results of all instruments and all metrics determined. © 2016 Elsevier B.V.

Personal inhalation exposures to airborne agents, including nanomaterials, are ideally measured in the breathing zone, using personal monitors or samplers. It is known from previous studies that the available personal monitors can measure airborne nanomaterial concentrations under laboratory conditions with an accuracy and comparability of ±30% or better. However, it is unclear whether this level of accuracy and comparability can also be achieved when these instruments are used as personal monitoring devices by individuals carrying out a wide variety of activities. In the present study, we investigated the reliability of DiSCmini and Partector during simulated exposure measurements. Two individuals were equipped with two identical instruments each, one mounted near the left and the other near the right collarbone. Both individuals went through a sequence of pre-determined and controlled activities, while simultaneously being exposed to well-defined NaCl aerosols within a 23 m3 chamber. A third specimen of both instruments was placed on a table in the middle of the chamber. The results of the Partector, mounted directly on the left or right side lapel within the personal breathing zone, agreed very well with each other and with the results from the third Partector on the table. The deviations were typically within ±10%. The scatter of the data was found to be larger when the individuals were walking than when they were sitting but the average concentrations remained unaffected by the activities. It can hence be concluded that the positioning of the sampling inlet within the breathing zone does not affect the measurement result, independent of personal activities and whether the carrying person is left- or right-handed. In contrast, the DiSCmini results showed very large deviations of up to a factor of three. However, this was caused by the use of silicone tubes in order to sample air from the personal breathing zone and transport to the belt mounted instruments. Siloxanes degas from the tubes into the airflow and are ionized in the unipolar diffusion charger of the DiSCmini and hence change the charging characteristics significantly affecting the measurement results.

Aerosol instrumentation based on unipolar diffusion charging has become popular in the recent years. These instruments can be made very small, making them suitable as personal monitors. In many app...

During occupational exposure studies, the use of conventional scanning mobility particle sizers (SMPS) provides high quality data but may convey transport and application limitations. New instruments aiming to overcome these limitations are being currently developed. The purpose of the present study was to compare the performance of the novel portable NanoScan SMPS TSI 3910 with that of two stationary SMPS instruments and one ultrafine condensation particle counter (UCPC) in a controlled atmosphere and for different particle types and concentrations. The results show that NanoScan tends to overestimate particle number concentrations with regard to the UCPC, particularly for agglomerated particles (ZnO, spark generated soot and diesel soot particles) with relative differences >20%. The best agreements between the internal reference values and measured number concentrations were obtained when measuring compact and spherical particles (NaCl and DEHS particles). With regard to particle diameter (modal size), results from NanoScan were comparable < [± 20%] to those measured by SMPSs for most of the aerosols measured. The findings of this study show that mobility particle sizers using unipolar and bipolar charging may be affected differently by particle size, morphologies, particle composition and concentration. While the sizing accuracy of the NanoScan SMPS was mostly within ±25%, it may miscount total particle number concentration by more than 50% (especially for agglomerated particles), thus making it unsuitable for occupational exposure assessments where high degree of accuracy is required (e.g., in tier 3). However, can be a useful instrument to obtain an estimate of the aerosol size distribution in indoor and workplace air, e.g., in tier 2., This work was supported by grants from the European Community's FP7 (FP7-PEOPLE-2012-ITN) no. 315760 (HEXACOMM project). Additional support was provided by European project nanoIndEx which is supported by the French National Funding Agency for Research (ANR), the German Federal Ministry of Education and Research (BMBF, IUTA grant no.: 03×0127A), the British Technology Strategy Board (TSB) and the Swiss TEMAS AG, under the frame of SIINN, the ERA-NET for a Safe Implementation of Innovative Nanoscience and Nanotechnology.

Exposure to airborne fine particles has been associated with increased morbidity [1] and mortality [2]. In the EU, fine dust is defined as airborne respirable particles with aerodynamic diameters below 10 μm (defined via a pre-separator that has an efficiency of 50% for 10 μm particles). Fine dust is therefore also termed PM10 (particulate matter < 10 μm). PM10 sources can be of both natural or anthropogenic origin.

Die Exposition gegenuber Feinstaub wird mit einer Vielzahl von Krankheitsfallen [1] und vorzeitigen Todesfallen [2] in Verbindung gebracht. Als Feinstaub bezeichnet man luftgetragene, lungengangige Partikel mit einem aerodynamischen Durchmesser kleiner als 10 μm (definiert uber einen Vorabscheider, der bei 10 μm eine Abscheidung von 50% aufweist). Feinstaub wird auch als PM10 (particulate matter < 10 μm) bezeichnet. Feinstaubquellen konnen sowohl naturlichen als auch anthropogenen Ursprungs sein.

Because nanomaterials have been increasingly developed and used in many technology and industry sectors over the last 20 years, an increasing number of workers is likely to be exposed to airborne nanoparticles. In addition, the question of the nanomaterial characteristics that should be assessed in epidemiological studies remains open. Thus, assessing occupational exposure to airborne nanoparticles will not only rely on mass concentration and chemical composition. Rather, key parameters, such as particle size, have to be included in measurement strategies. We previously proposed a methodology to estimate the Count Median Diameter (CMD) of an aerosol based on the simultaneous size-integrated measurement of two particle concentrations, lung-deposited surface area, and number, thanks to field-portable, commercially available aerosol instruments (Nanoparticle Surface Area Monitor/Condensation Particle Counter combination). In addition to previous work, this study investigates the case of various polydisperse metal oxides, organic oil, and salt particles with CMDs ranging from 16-410 nm. Once corrected, the CMDs derived from the NSAM/CPC agree within ±20% with regard to the reference electrical mobility equivalent diameter, regardless of aerosol composition, morphology, or geometric standard deviation (GSD). Furthermore, the field-applicability of the method was tested through 6 sets of experimental data stemming from workplace measurement campaigns where different materials were produced and handled (TiO2, SiO2, Ag, Multi-Walled Carbon Nanotubes-MWCNT), covering a range of CMDs between 40 and 190 nm. All situations considered, the approach based on the combination of a NSAM and a CPC leads to a satisfying estimation of particle CMD, within ±20% compared to reference CMD.

The accuracy of instruments measuring the alveolar lung deposited surface area (LDSA) concentration based on unipolar diffusion charging has been assessed. Monodisperse particles with sizes between 10 nm and 700 nm were used. The results indicate that the LDSA concentration can be measured with at least ±30% accuracy for particle sizes between 20 nm and 400 nm. The LDSA concentrations of particles 400 nm increasingly underestimated by the instruments. The LDSA concentration of 685 nm particles was on average underreported by a factor of approximately 2. LDSA concentrations measured with agglomerated particles were consistently higher than those measured with spherical particles of the same electrical mobility diameter. The accuracy was, however, still mostly within ±30% for a size range from 20 nm to 400 nm. Due to the strong and increasing deviation for large particles, the use of an appropriate preseprator is recommended when measuring LDSA concentrations.

We here report on a study that examines the performance of a water based personal condensation particle counter (CPC) PUFP C100 (Enmont, LLC) when sampling a variety of different test aerosols. Three additional CPCs were included in the study, namely, the general purpose water based CPC model 3787 (TSI) as well as the more than 20 year old CPC model 3022 (TSI) and ultrafine CPC model 3025A (TSI), both based on butanol. The responses of these CPCs were compared to a reference ultrafine CPC model 3776 (TSI). Test aerosols included hydrophilic NaCl, moderately hydrophobic carbon and highly hydrophobic DEHS aerosols of various sizes and concentrations. The CPC PUFP C100 measured according to its specifications with high precision and accuracy as long as the particles were not highly hydrophobic, but failed to adequately measure the concentrations of highly hydrophobic DEHS aerosols. Its detection efficiency for very small particles, e.g., 5.4 nm, was decreased, which was expected based on its particle size detection limit. The TSI water CPC model 3787 showed a similar behavior for all aerosols, except that this CPC was able to accurately measure the concentration of 5.4 nm NaCl particles. The concentrations measured with the two old and unserviced butanol CPCs were highly correlated with the concentrations delivered by the reference CPC. Nevertheless, the concentrations measured with both old CPCs were clearly below the reference concentrations.

Upon standing in air at room temperature, acidified aqueous solutions (pH 2) obtained by dissolving sodium molybdate, iron(II) chloride, acetic acid and potassium dodecatungstoborate led to the formation of greenish thin plate crystals (1) containing discrete clusters. Upon drying 1 at room temperature cross-linking of the clusters occurs in a solid-state reaction which leads to compound 2. Based on crystallographic studies of 1 and on extensive physical characterization (vibrational, NMR, Mossbauer spectra and magnetic susceptibility measurements) of 2, it was shown that compounds 1 and 2 contain reduced spherical {Mo72Fe30} Keplerate-type shells encapsulating non-reduced tungstoborate Keggin-type anions. The properties and structures of these compounds are compared to related hybrid structures containing in principle easier reducible molybdophosphate or molybdosilicate cores. According to the results the present type of encapsulation can be considered as a general phenomenon. (C) 2012 Elsevier B.V. All rights reserved.

The investigation of hydrophobic interactions under confined conditions is of tremendous interdisciplinary interest. It is shown that based on porous capsules of the type {(pentagon)}(12){(linker)}(30) ≡ {(Mo)Mo(5)(12){Mo(2)(ligand)}(30), which exhibit different hydrophobic interiors-achieved by coordinating related ligands to the internal sites of the 30 {Mo(2)} type linkers-there is the option to study systematically interactions with different uptaken/encapsulated hydrophobic molecules like long-chain alcohols as well as to prove the important correlation between the sizes of the related hydrophobic cavities and the option of water encapsulations. The measurements of 1D- and 2D-NMR spectra (e.g. ROESY, NOESY and HSQC) allowed the study of the interactions especially between encapsulated n-hexanol molecules and the hydrophobic interior formed by propionate ligands present in a new synthesized capsule. Future detailed studies will focus on interactions of a variety of hydrophobic species with different deliberately constructed hydrophobic capsule interiors.

The present study refers to a variety of reduced metal-oxide core-shell hybrids, which are unique with regard to their electronic structure, their geometry, and their formation. They contain spherical {Mo(72)Fe(30)} Keplerate-type shells encapsulating Keggin-type polyoxomolybdates based on very weak interactions. Studies on the encapsulation of molybdosilicate as well as on the earlier reported molybdophosphate, coupled with the use of several physical methods for the characterization led to unprecedented results (see title). Upon standing in air at room temperature, acidified aqueous solutions obtained by dissolving sodium molybdate, iron(II) chloride, acetic acid, and molybdosilicic acid led to the precipitation of monoclinic greenish crystals (1). A rhombohedral variant (2) has also been observed. Upon drying at room temperature, compound 3 with a layer structure was obtained from 1 in a solid-state reaction based on cross-linking of the shells. The compounds 1, 2, and 3 have been characterized by a combination of methods including single-crystal X-ray crystallography, magnetic studies, as well as IR, Mossbauer, (resonance) Raman, and electronic absorption spectroscopy. In connection with detailed studies of the guest-free two-electron-reduced {Mo(72)Fe(30)}-type Keplerate (4) and of the previously reported molybdophosphate-based hybrids (including (31)P NMR spectroscopy results), it is unambiguously proved that 1, 2, and 3 contain non-reduced Keggin ion cores and reduced {Mo(72)Fe(30)}-type shells. The results are discussed in terms of redox considerations (the shell as well as the core can be reduced) including those related to the reduction of "molybdates" by Fe(II) being of interdisciplinary including catalytic interest (the Mo(VI)/Mo(V) and Fe(III)/Fe(II) couples have very close redox potentials!), while also referring to the special formation of the hybrids based on chemical Darwinism.

Using incoherent quasi-elastic and inelastic neutron scattering, we have investigated the hydrogen relaxational dynamics and hydrogen vibrational modes in the polyoxomolybdate specie [Mo 72 Fe 30 O 252 (CH 3 COO) 12 [Mo 2 O 7 (H 2 O)] 2 [H 2 Mo 2 O 8 (H 2 O)](H 2 O) 91 ]· ≈ 150 H 2 O. The translational dynamics of the water molecules in the compound is profoundly different from that of bulk water at the same temperature showing a non-Debye relaxation behavior. The temperature dependence of the relaxation time can be described in terms of an Arrhenius law, indicating that the dynamics is triggered by the breaking of the bonds connecting the crystal water molecules with the hydrophilic nanocapsule surfaces. Inelastic neutron scattering spectra confirm the attenuation of water translational modes with respect to the bulk water case due to the strong destructuring effect imposed by the nanocage interface and the enhancement of the highest frequency librational mode as already found in hydrated Vycor or Gelsil matrix. Small angle X-ray scattering on freshly prepared aqueous solution evidences the presence of nanocapsule structures proper of the monomer (2.6 nm in diameter) that coexist with a small amount of oligomers. After 1 month the polyoxomolibdate specie self-assembles in a supramolecular structure with a polydisperse distribution of dimensions spanning from the monomer to the “blackberry” vesicular structure already reported in literature.

The effect of confinement and energy transfer on the dynamics of a molecular magnet, known as a model system to study quantum coherence, is investigated. For this purpose the well-known polyoxovanadate [V(15)As(6)O(42)(H(2)O)](6-) (V(15)) is incorporated into a protein (human serum albumin, HSA) cavity. Due to a huge overlap of the optical absorption spectrum of V(15) with the emission spectrum of a fluorescence center of HSA (containing a single tryptophan residue), energy transfer is induced and probed by steady-state and time-resolved fluorescence. The geometrical coordination and the distance of the confined V(15) to the tryptophan moiety of HSA are investigated at various temperatures. This effect is used as a local probe for the thermal denaturation of the protein at elevated temperatures.

Using quasielastic neutron scattering, we have investigated the water dynamics of powders of the compound with the stoichiometry of [Mo72Fe30O252(CH3COO)(12)[Mo2O7(H2O)](2)[H-2 Mo2O8(H2O)](H2O)(91)]center dot approximate to 150H(2)O. It contains about 150 crystal water molecules in the voids between the spherical {Mo72Fe30} nanocapsules, which are considered unique polytropic inorganic acids with well-defined hydrophilic surfaces due to the presence of H2O and O atoms. It has been proposed that {Mo72Fe30} can be used as a structurally well-constrained experimental model of oxide mineral surfaces for earth scientists.(I) In this respect, it is of fundamental importance to understand the dynamics of the water molecules at the surface of the nanoclusters. Our measurements show that the dynamics of these water molecules is as expected profoundly different from that of bulk water at the same temperature, especially because of the strong hydrogen bonding between the crystal and cluster surface water molecules. In fact, our data show a non-Debye relaxation behavior. The momentum transfer dependence of the dynamics is close to that expected for a purely diffusive motion. This suggests that the nonexponentiality of the dynamics originates from a distribution of relaxation times, probably related to the different local environments experienced by the water molecules. The dynamics of the crystal water in the voids between the well-defined and arrayed nanocages is significantly slower than that of bulk water at the same temperature that has often been reported for interfacial water. In the investigated range, the temperature dependence of the relaxation time can be described in terms of an Arrhenius law, indicating that the dynamics is triggered by the breaking of the bonds connecting the crystal water molecules with the hydrophilic nanocage surfaces.

International audience; A large variety of measurement methods for the characterization of airborne nanoparticles in indoor or outdoor air exist. The choice of an appropriate method depends strongly on the questions to be tackled. If the aerosol is to be characterized only for a single location, one may use stationary equipment that is rather bulky but provides the most details and is most accurate. Spatially resolved measurements can only be conducted with portable or personal measurement equipment which provide a limited dataset with lower accuracy. Furthermore, the metrics to be measured (e.g., number, surface area of mass concentration, chemical composition, etc.) determine the choice of measurement methods as no single method can do it all. Another determining factor is the time resolution of the instruments. While direct-reading monitors deliver the information with high time resolution (often 1 s) and hence allow for linking the measured concentration to certain activities, samplers collect the particles for subsequent analyses and therefore provide an average over the sampling time. Consequently, the choice of a measurement instrument for the characterization of airborne nanoparticles remains a compromise. In many practical applications, the combination of different techniques may be required.

We report a self-recognition phenomenon based on an assembly process in a homogeneous dilute aqueous solution of two nano-scaled, spherical polyprotic metal oxide-based macroions (neutral species in crystals), also called Keplerates of the type [(linker)(30)(pentagon)(12)]=[{M(H2O)}(30){(Mo)Mo-5}(12)] where M is Fe-III or Cr-III. Upon deprotonation of the neutral species, the resulting macroions assemble into hollow "blackberry"-type structures through very slow homogeneous dimer-oligomerization processes. Although the geometrical surface structures of the two macroions are practically identical, mixtures of these form homogeneous superstructures, rather than mixed species. The phase separation is based on the difference in macroionic charge densities present during the slow homogeneous dimer or oligomer formation. The surface water ligands' residence times of Cr-III and Fe-III differ markedly and lead to very different interfacial water mobilities between the Keplerates.

The spherical capsule skeleton of the host–guest system [{Mo6O19}2−⊂{MoVI72FeIII30O252(CH3COO)20(H2O)92}]4−1a—built up by 12 {(MoVI)MoVI5} type pentagonal units linked by 30 FeIII centers which span the unique icosahedral Archimedean solid, the icosidodecahedron—can now be constructed deliberately and with a simpler composition than before from an acidified aqueous molybdate solution containing the mentioned (virtual) pentagonal units; the encapsulated hexamolybdate—normally not formed in water—is built up in an unprecedented way concomitant with capsule growth, while being directed by the corresponding internal electrophilic surface functionalities.

(NH4)42 [MoVI72MoV60O372 (OAc)30(H2O)72] ·300H2O·10NH4OAc (I) is synthesized by addition of the oxalato complex (NH4)2 [Mo2O4(C2O4)2 (H2O)2]·3H2O to an aqueous solution of (NH4)6Mo7O24, AcOH, and NH4OH at pH 4 (4 h, 16% yield).

The addition of dinuclear {Mo(2)} units to a dynamic library containing molybdates results in the spontaneous self-assembly of a giant spherical metal-oxide species of the type {(Mo)Mo(5)}(12){Mo(2)}(30) while the required pentagonal {(Mo)Mo(5)} building blocks are "immediately" formed.

The deliberate synthesis of the Keplerate [K(20) subset{(W)W(5)O(21)(SO(4))}(12)(VO)(30)(SO(4))(H(2)O)(63)](18-) with 20 pores all closed by K(+) in a supramolecular fashion proves that it is possible to follow new routes in polyoxotungstate chemistry based on pentagonal {(W)W(5)}-type units and to tune magnetic exchange couplings in {(M)M(5)}(12)M'(30) type Keplerates.

We report the self-assembly processes in solution of three Keplerate-type molybdenum-oxide based clusters {Mo72V30}, {Mo72Cr30} and {Mo72Fe30} (all with diameters of approximately 2.5 nm). These clusters behave as unique weak polyprotic acids owing to the external water ligands attached to the non-Mo metal centers. Whereas the Cr and Fe clusters have 30 water ligands attached at the 30 M3+ centers pointing outside, {Mo72V30} has 20 water ligands coordinated to vanadium atoms, of which only 10 are pointing outside. The self-assembly processes of the Keplerates leading to supramolecular blackberry-type structures are influenced by the effective charge densities on the cluster surfaces, which can be tuned by the pH values and solvent properties. As expected, {Mo72Cr30} and {Mo72Fe30} behave similarly in aqueous solution due to their analogous structures and in both cases the self-assembly follows the partial deprotonation of the external water ligands attached to the non-Mo metal centers. However, the M-OH2 functionalities differ not only in acidity but also lability, i.e. in different residence times of the H2O ligands. In contrast to {Mo72Cr30} and {Mo72Fe30}, the {Mo72V30} clusters carry a rather large number of negative charges so that their solution properties are different. They exist as discrete macroions in dilute aqueous solution, and form only in mixed water/organic solvent (like acetone) blackberry-type structures whose size increases with acetone content. The comparison of the properties of the clusters allows more general information about the interesting self-assembly phenomenon to be unveiled.

Our measurements of the differential susceptibility $\ensuremath{\partial}M/\ensuremath{\partial}H$ of the frustrated magnetic molecules ${{\text{Mo}}_{72}{\text{Fe}}_{30}}$ and ${{\text{Mo}}_{72}{\text{Cr}}_{30}}$ reveal a pronounced dependence on magnetic field $(H)$ and temperature $(T)$ in the low $H$--low $T$ regime, contrary to the predictions of the existing models. Excellent agreement with experiment is achieved upon formulating a nearest-neighbor classical Heisenberg model where the 60 nearest-neighbor exchange interactions in each molecule, rather than being identical as has been assumed heretofore, are described by a two-parameter rectangular probability distribution of values of the exchange constant. We suggest that the probability distribution provides a convenient phenomenological platform for summarizing the combined effects of multiple microscopic mechanisms that disrupt the idealized picture of a Heisenberg model based on a single value of the nearest-neighbor exchange constant.

The 16-Fe-III-containing 48-tungsto-8-phosphate [P8W48O184Fe16(OH)(28)(H2O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li4K16[P8W48O184Fe16(OH)(28)(H2O)(4)]center dot 66H(2)O center dot 2KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9)angstrom, b = 13.9708(3) angstrom, c = 26.9140(7) angstrom, and Z=2) and on the corresponding mixed sodium-potassium salt Na9K11-[P8W48O184Fe16(OH)(28)(H2O)4]center dot 100H(2)O (NaK-1, monoclinic space group C2/c, a = 46.552(4) angstrom, b = 20.8239(18) angstrom, c = 27.826(2) angstrom, = 97.141(2)degrees and Z = 4). Polyanion 1 contains-in the form of a cyclic arrangement-the unprecedented {Fe-16(OH)(28)(H2O)}(20+) nanocluster, with 16 edge- and corner-sharing FeO6 octahedra, grafted on the inner surface of the crown-shaped [H7P8W48O184](33-) (P8W8) precursor. The synthesis of I was accomplished by reaction of different iron species containing Fell (in presence of O-2) or Fe-III ions with the P8W, anion in aqueous, acidic medium (pH approximate to 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH I through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe-III. centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mVs(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO., including nitrate. Magnetic and variable frequency EPR measurements on I suggest that the Fe-III ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.

Variable-temperature O-17 NMR experiments were conducted on the nanometer-sized Keplerate MO72Fe30 cluster, with the stoichiometry [MO72Fe30O252(CH3COO)(12)[MO2O7(H2O)](2)[H2MO2O8(H2O)(91)]center dot similar to(H2O)91]center dot similar to 150H(2)O. This molecule contains on its surface 30 Fe(H2O) groups forming a well-defined icosidodecahedron, and we estimated the rates of exchange of the isolated >Fe-III-OH2 waters with bulk aqueous solution, Both longitudinal and transverse O-17-relaxation times were measured, as well as chemical shifts, and these parameters were then fit to the Swift-Connick equations in order to obtain the rate parameters. Correspondingly, we estimate: k(ex)(298) = 6.7(+/- 0.8) x 10(6) s(-1), which is about a factor of similar to 4 x 10(4) times larger than the corresponding rate coefficient for the Fe(OH2)(6)(3+) ion of k(ex)(298) = 1.6 x 10(2) S-1 (Grant and Jordan, 1981; Inorg. Chem. 20, 55-60) and Delta H double dagger and Delta S double dagger are 26.3 +/- 0.6 kj mol(-1) and -26 +/- 0.9 J mol(-1) K-1, respectively. High-pressure 110 NMR experiments were also conducted, but the cluster decomposed slightly under pressure, which precluded confident quantitative estimation of the AV. However, the increase in the reduced transverse-relaxation time with pressure suggests a dissociative character, such as a D or Id mechanism. The enhanced reactivity of waters on the MO72Fe30 cluster is associated with an increase in the Fe-III-OH2 bond length in the solid state of similar to 0.1 angstrom relative to the Fe(OH2)(6)(3+) ion, suggesting that a correlation exists between the Fe-III-OH2 bond length and k(ex)(298). Although there are only few high-spin Fe(III) complexes where both exchange rates and structural data are available, these few seem to support a general correlation.