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Achim Müller

Publications and source records attributed to Achim Müller.

At least 19 recordsLinked to original sources
PubMed2007

Static magnetization of V15 cluster at ultra-low temperatures: precise estimation of antisymmetric exchange.

In this article, the low-temperature static (adiabatic) magnetization data of the nanoscopic V15 cluster present in K6[VIV15As6O42(H2O)] x 8H2O is analyzed. The cluster anion, which attracted much attention in the past, contains a triangular VIV3 array causing frustration as a function of applied field and temperature. In the analysis, a three-spin (S = 1/2) model of V15 was employed that includes isotropic antiferromagnetic exchange interaction and antisymmetric (AS) exchange in the most general form compatible with the trigonal symmetry of the system. It was shown that, along with the absolute value of AS exchange, the orientation of the AS vector plays a significant physical role in spin-frustrated systems. In this context, the role of the different components of the AS in the low-temperature magnetic behavior of V15 was analyzed, and we were able to reach a perfect fit to the experimental data on the staircaselike dependence of magnetization versus field in the whole temperature range including extremely low temperature. Furthermore, it was possible for the first time to precisely estimate the two components of the AS vector coupling constant in a triangular unit, namely, the effective in-plane component, Dperpendicular, and the perpendicular part, Dn.

Journal Article↗
PubMed2006

Deprotonations and charges of well-defined {Mo72Fe30} nanoacids simply stepwise tuned by pH allow control/variation of related self-assembly processes.

The solution behavior of the largest inorganic acid known thus far, the neutral, spherical iron/molybdenum/oxide nanocluster {Mo72Fe30} ([triple bond{(MoVI) MoVI5}12FeIII30 1a), including the pH-controlled deprotonation, is reported. The acidic properties are due to the 30 peripheral, weakly acidic FeIII(H2O) groups that form a unique Archimedean solid with all edges and dihedral angles being equal, the icosidodecahedron, and therefore an "isotropic" surface. Interestingly, the aqueous solutions are stable even for months because of the inertness of the spherical solutes and the presence of the hard FeIII and MoVI centers. The stability can be nicely proven by the very characteristic Raman spectrum showing, because of the (approximately) icosahedral symmetry, only a few lines. Whereas the {Mo72Fe30} clusters exist as discrete, almost neutral, molecules in aqueous solution at pH < 2.9, they get deprotonated and self-associate into single-layer blackberry-type structures at higher pH while the assembly process (i.e., the size of the final species) can be controlled by the pH values; this allows the deliberate generation of differently sized nanoparticles, a long-term goal in nanoscience. The average hydrodynamic radius (Rh) of the self-assembled structures decreases monotonically with increasing number of charges on the {Mo72Fe30} macroanions (from approximately 45 nm at pH approximately 3.0 to approximately 15 nm at pH approximately 6.6), as studied by laser light scattering and TEM techniques. The {Mo72Fe30} macroions with high-stability tunable charges/surfaces, equal shape, and masses provide models for the understanding of more complex polyelectrolyte solutions while the controllable association and dissociation reported here of the assembled soft magnetic materials with tuneable sizes could be interesting for practical applications.

Journal Article↗
PubMed2006

Low temperature EPR spectra of the mesoscopic cluster V15: the role of antisymmetric exchange.

The low temperature EPR spectra of the unique nanometer-scale molecular magnet V(15) are analyzed within the three-spin model preserving trigonal symmetry. The Hamiltonian includes isotropic and antisymmetric (AS) exchange interactions introduced by Dzyaloshinsky (Zh. Eksp. Teor. Fiz. 32, 1547 (1957) [Sov. Phys. JETP 5, 1259 (1957)]) and [Moria Phys. Rev. 120, 91 (1960)]. With the aid of pseudoangular momentum representation the exact selection rules for the EPR transitions are deduced. AS exchange in the frustrated triangular system gives rise to a first order zero-field splitting of two low lying spin doublets and to a second order splitting of the excited quadruplet. This leads to a peculiar series of strong intramultiplet transitions and weak intermultiplet transitions. We show that the intramultiplet transitions are allowed only when the vector of the AS exchange is normal to the plane of vanadium triangle, but the series of weak intermultiplet transitions are a consequence of the in-plane part of the AS exchange. We have revealed a special temperature dependence of the EPR pattern with a peculiar shape of the full spectrum. Experimental data on high-frequency EPR of V15 at ultralow temperature are discussed.

Journal Article↗
PubMed2006

Reactions inside a porous nanocapsule/artificial cell: encapsulates' structuring directed by internal surface deprotonations.

In the cavities of unprecedentedly functionalised, spherical, porous capsules of the type {Pentagon}12{Linker}30 identical with [{(Mo)Mo5O21(H2O)6}12{Mo2O4(ligand)}30]n- reactions with the ligands -i.e. at the internal shell surfaces - can be performed, in the present case deliberate aquation/hydration and deprotonation reactions at the linker fragments {(Mo2O4)C2O4H}+ similar to that reported in the literature for [(NH3)5CoC2O4H]2+ in solution.

Journal Article↗
PubMed2006

Formation of a "less stable" polyanion directed and protected by electrophilic internal surface functionalities of a capsule in growth: [{Mo6O19}2- subset {Mo(VI)72Fe(III)30O252(ac)20(H2O)92}]4-.

The spherical capsule skeleton of the host-guest system [{Mo6O19}2- subset {Mo(VI)72Fe(III)30O252(CH3COO)20(H2O)92}]4- 1a--built up by 12 {(Mo(VI))Mo(VI)5} type pentagonal units linked by 30 Fe(III) 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.

Journal Article↗
PubMed2005

A small cavity with reactive internal shell atoms spanned by four {As(W/V)9}-type building blocks allows host-guest chemistry under confined conditions.

The reaction of [H2As(III)W18O60]7- with VO2+ and SO4(2-) ions in aqueous solution leads to a V(IV)/V(V) mixed-valence cluster anion containing the {As4M40O140}-type cryptand which has a high formation tendency. An important result is that it exhibits a new type of reactive internal cavity shell. The correspondingly obtained compound Na(NH4)20[{(V(IV)O(H2O))(V(IV)O)2(SO4)2}{(As(III)W9O33)2(As(III)W7.5V(V)1.5O31)2(WO2)4}] x 40 H2O (1), which can also be synthesized from a precursor with the preorganized cryptand, was characterized by elemental and thermogravimetric analyses (determination of crystal water content), redox titrations (determination of the number of V(IV) centers), electronic absorption as well as vibrational spectra, single-crystal X-ray structure analysis (including bond valence sum calculations), and magnetic susceptibility measurements. The relatively small central cavity--formed by the linking of four {AsM9}-type lacunary units (M = W/V) by four WO6 octahedra--allows positioning of a variety of cationic as well as anionic "guests" under confined conditions according to a new approach: replacement of some of the W by V atoms leads to high reactivity of the internal cavity shell as a result of relatively weak VO bonds compared to the WO bonds. This allows an interesting "encapsulation chemistry" with new options. In the present case the cavity contains besides an arrangement of three V(IV) centers, two sulfate groups that replace O atoms of the {AsM9} units as well as an interesting hydrogen bond situation.

Journal Article↗
PubMed2005

Porous inorganic capsules in action: modelling transmembrane cation-transport parameter-dependence based on water as vehicle.

Insight into basic principles of cation transport through "molecular channels", and especially details of the related fundamental H2O vehicle function, could be obtained via7Li NMR studies of the Li+ uptake/release processes by the unique porous nanocapsule [{(MoVI)MoVI5O21(H2O)6}12{MoV2O4(SO4)}30]72- which behaves as a semi-permeable inorganic membrane open for H2O and small cations; channel traffic as well as internal cavity distribution processes show a strong dependence on "environmental" effects such as exerted by solvent properties, the amount of water present, and competing complexing ligands, and end up in a complex equilibrium situation as in biological leak channels.

Cation Transport Proteins↗
PubMed2005

A new type of metalloprotein: The Mo storage protein from azotobacter vinelandii contains a polynuclear molybdenum-oxide cluster.

Azotobacter vinelandii is a diazotrophic bacterium characterized by the outstanding capability of storing Mo in a special storage protein, which guarantees Mo-dependent nitrogen fixation even under growth conditions of extreme Mo starvation. The Mo storage protein is constitutively synthesized with respect to the nitrogen source and is regulated by molybdenum at an extremely low concentration level (0-50 nM). This protein was isolated as an alpha4beta4 octamer with a total molecular mass of about 240 kg mol(-1) and its shape was determined by small-angle X-ray scattering. The genes of the alpha and beta subunits were unequivocally identified; the amino acid sequences thereby determined reveal that the Mo storage protein is not related to any other known molybdoprotein. Each protein molecule can store at least 90 Mo atoms. Extended X-ray absorption fine-structure spectroscopy identified a metal-oxygen cluster bound to the Mo storage protein. The binding of Mo (biosynthesis and incorporation of the cluster) is dependent on adenosine triphosphate (ATP); Mo release is ATP-independent but pH-regulated, occurring only above pH 7.1. This Mo storage protein is the only known noniron metal storage system in the biosphere containing a metal-oxygen cluster.

Amino Acid Sequence↗
PubMed2003

Self-assembly in aqueous solution of wheel-shaped Mo154 oxide clusters into vesicles.

Surfactants and membrane lipids readily assemble into complex structures such as micelles, liposomes or hollow vesicles owing to their amphiphilic character-the fact that part of their structure is attracted to polar environments while another part is attracted to non-polar environments. The self-assembly of complex structures also occurs in polyoxometallate chemistry, as exemplified by the molybdenum blue solutions known for centuries. But while the presence of nanometre-sized metal oxide aggregates in these solutions has long been recognized, unravelling the composition and formation process of these aggregates proved difficult. Recent work has indicated that discrete, wheel-shaped mixed-valence polyoxomolybdate clusters of the type [Mo154] (refs 2-4) assemble into well-defined nanometre-sized aggregates, including spherical structures. Here we report light-scattering data and transmission electron microscopy images of hollow spherical structures with an average, almost monodisperse radius of about 45 nm and composed of approximately 1,165 [Mo154] wheel-shaped clusters. The clusters appear to lie flat and homogeneously distributed on the vesicle surface. Unlike conventional lipid vesicles, the structures we observe are not stabilized by hydrophobic interactions. Instead, we believe the polyoxomolybdate-based vesicles form owing to a subtle interplay between short-range van der Waals attraction and long-range electrostatic repulsion, with important further stabilization arising from hydrogen bonding involving water molecules encapsulated between the wheel-shaped clusters and in the vesicles' interior.

Journal Article↗
PubMed2003

Synergetic activation of "silent receptor" sites leading to a new type of inclusion complex: integration of a 64-membered ring comprising K+ and SO4(2-) ions into a molybdenum oxide-based nanoobject.

The new type of inclusion species [Mo(VI)114Mo(V)32O429(H2O)50(KSO4)16]30- comprising 16 encapsulated K+ and SO4(2-) ions shows an unusual 64-membered [K(SO4)]16 ring integrated into a wheel-shaped type nanocluster host; it is formed by a synergetically induced functional complementarity and represents a text-book example of a very complex non-biological material.

Chemical Phenomena↗