Tetrameric Clusters Research Articles

Role of the breathing mode in a strongly coupled Jahn-Teller system

The theoretical absorption spectrum of a vibronic system exhibits two main peaks of different height and width, and, at very strong Jahn-Teller energy, other peaks at higher energies (cone resonances or Slonczewski resonances). Tetrameric cations of the group V elements can be pictured as strongly coupled Jahn-Teller systems; however, high resolution photoelectron spectra available in the literature exhibit only two broad spectral bands. To explain the disappearance of the cone resonances, we have considered a two-mode vibronic model, that, besides a strong linear interaction with the mode , also includes a linear interaction with the total symmetric mode (breathing mode); we have also considered separately off-diagonal second-order effects on the Jahn-Teller system. The analysis has been carried out in the framework of the recursion Lanczos technique, properly implemented. We have verified that the Slonczewski resonances are quenched by the breathing mode, even in the case of moderate coupling; we have then compared our model with the available experimental spectra of group V tetrameric clusters.

Coupled activation of the donor and the acceptor side of photosystem II during photoactivation of the oxygen evolving cluster.

Photoactivation of photosystem II has been studied in the FUD 39 mutant of Chlamydomonas reinhardtii that lacks the 23 kDa extrinsic subunit of photosystem II. We have taken advantage of the slow photoactivation rate of FUD 39, earlier demonstrated in Rova, E. M., et al. [(1996) J. Biol. Chem. 271, 28918-28924], to study events in photosystem II during intermediate stages of the process. By measuring the EPR multiline signal, the decay of the variable fluorescence after single flashes, and electron transfer from water to the QB site, we found a good correlation between the building of a tetrameric Mn cluster, longer recombination times between QA- and the donor side of photosystem II, and the achievement of water splitting ability. An increased rate of electron transfer from QA- to the QB site on the acceptor side of photosystem II, mainly due to enhanced efficiency of binding of QB to its site, was found to precede the building of the Mn cluster. We also showed that TyrD was oxidized simultaneously with this increase in electron-transfer rate. Thus, it appears that photoactivation is sequential, with an increased rate of electron transfer on the acceptor side occurring together with the oxidation of TyrD in the first step, followed by the assembly of the Mn cluster. We suggest that a conformational change of photosystem II is induced early in the photoactivation process facilitating electron transfer from the primary donor to the acceptor side. As a consequence, TyrD, an auxiliary electron donor to P680+/TyrZ*, is oxidized. That this occurs before the Mn cluster is fully functional serves to protect photosystem II against donor side induced photodamage.

ESR and optical studies of cationic silver clusters in zeolite rho

Silver cationic clusters formed in γ-irradiated AgCs-rho zeolites in hydrated and dehydrated forms have been studied by electron spin resonance (ESR) spectroscopy and diffuse reflectance spectrophotometry. It was found that tetrameric silver clusters, Ag 4 3+ in dehydrated zeolites and Ag 4 2+ in hydrated samples, are stabilized at room temperature. Tetrameric silver is trapped inside octagonal prism of zeolite framework and does not show any noticeable decay as far as samples are evacuated.

MoV–MoVIcationic ordering in the layered molybdate (C2H10N2)[Mo4O12

The mixed valence MoV–MoVI layered tetramolybdate (C2H10N2)[Mo4O12 ] has been hydrothermally synthesized (180 °C, 3 weeks, autogenous pressure). It crystallizes in the monoclinic system (space group C2) with a=11.4090(2),b=11.8227(2), c=10.2305(1) A, β=117.549(1)° and Z=4 at 20 °C. The structure was solved from single crystal X-ray diffraction data. Full matrix anisotropic least-squares techniques led to a conventional R1(Fo) factor of 0.044 [wR2(Fo2)=0.107] for 2885 reflections [I>2sigma;(I)]. Its structure consists of [Mo4O12 ]2– layers perpendicular to the c* axis, made up of an ordered organization of isolated tetrameric MoV clusters linked by edges to tetrameric MoVI ones. Ethylenediamine cations are located between the inorganic layers and ensure the connectivity of the structure via hydrogen bonds.

Double exchange in tetrameric tetrahedral clusters with two-electron transfer: magnetic properties

A theoretical model is worked out to study the magnetic properties of mixed-valence tetrameric tetrahedral clusters d n -d n -d n+1 -d n+1 ( n = 0–4) with two-electron transfer. To calculate the energy levels of systems a computational procedure based on the second quantization technique and group theory approach is used. The exchange-tunnel states of d n -d n -d n+1 -d n+1 ( n = 0–4) tetrahedral tetrameric clusters with two-electron transfer are considered. The nature of the ground spin state of above mentioned systems is determined. The structures under consideration are shown to have the magnetic properties which differ in principle from the clusters with a single migrating electron (or hole).

Magnetic excitations in polyoxometalate tetrameric clusters

The metal-oxide clusters with formula [M 4(D 2O) 2(PW 9O 34) 2] 10− which contain a tetrameric magnetic cluster M 4O 16 provide an ideal series for the study of magnetic exchange interactions in polymetallic molecular clusters. To get a more direct information on the splitting of the spin states caused by the exchange interactions we have performed inelastic neutron scattering measurements on the Co, Mn and Ni clusters. Magnetic excitations have been observed in the range 0.5–6 meV. A tentative interpretation of these data from a Heisenberg exchange Hamiltonian and a single ion zero-field splitting is presented for Ni cluster.

Evidence for dimeric and tetrameric water clusters in HZSM-5

Temperature-programmed desorption shows that water desorbs from HZSM-5 (Si/Al = 13) · n H 2 O in a two-step process. The amount of water evolved at each step corresponds to about two H 2 O per acidic proton, indicating that the tetrameric cluster formed at full loading consists of a strongly bound pair and a weakly bound pair of water molecules. Quantum mechanical studies using cluster models of aluminosilicate systems indicate that the desorption energies for the first and second water molecules of a dimer adsorbed at the acid site are 9.7 and 12.6 kcal/mol, respectively. These values are within the energy resolution of the TPD experiment and are consistent with both water molecules from the dimer seeming to desorb simultaneously. Evidence for a tetramer is consistent with earlier findings on dealuminated mordenites.

Role of mixed-valence state in vanadium phosphates catalysts

An analysis of the electronic structure of the vanadyl pyrophosphate surface is given when vanadium atoms in different valence states are simultaneously present. A tetrameric vanadium-oxygen cluster is proposed to model the catalytic active site. Possible path for the interaction of 2,5-dihydrofuran with a lattice vanadyl oxygen leading to the formation of maleic anhydride is considered. It is shown that account of structural relaxation in mixed-valence state is needed to properly describe active site.

Magnetic Excitations in Tetrameric Clusters of Polyoxometalates Observed by Inelastic Neutron Scattering. Evidence for Anisotropic Exchange Interactions in Cobalt(II) Clusters.

Magnetic excitations in tetranuclear clusters of exchange-coupled metal ions encapsulated by polyoxotungstate ligands (1) have been observed by inelastic neutron scattering (INS) in the salts K10[M4(H2O)2(PW9O34)2]·nH2O, where M2+ = Mn, Co, and Ni. INS provides a much deeper and more detailed insight into the nature of the magnetic coupling in these clusters than bulk techniques. In particular, anisotropic exchange interactions are evidenced in the ferromagnetic Co4 cluster.

Direct synthesis and crystal structure of tetrameric copper(I) iodide with trimethylamine [CuI(NMe 3)] 4

The tetrameric cluster [CuI{N(CH 3) 3}] 4 has been synthesized directly from copper powder and trimethylammonium iodide in non-aqueous solutions (acetonitrile, N, N-dimethylformamide) and characterized by X-ray crystallography. The compound [CuI{N(CH 3) 3}] 4 possesses a molecular structure. A tetrahedral Cu 4 cluster with short CuCu distances (2.67 Å) is surrounded by an I 4 tetrahedron, thus each face of the Cu 4 tetrahedron is centred by a μ 3-bridging iodine atom.

Tunnel-exchange states of square-planar bielectron transfer clusters

The tunnel states of square-planar bielectron transfer dn-dn-dn+1-dn+1 (n=0–4) clusters are considered. The nature of the ground spin state is revealed in the limiting case of a strong double exchange. It is shown that the magnetic properties of these systems radically differ from those of clusters with one migrating electron (hole) and tetrameric tetrahedral bielectron transfer clusters.

Double exchange in polynuclear mixed-valence clusters. 2. Iron-sulfur proteins [Fe4S4]+ and [Fe4S4]3+

The energy states of tetrameric iron clusters with d6-d6-d6-d5 and d5-d5-d5-d6 electronic configurations, which form the metal frameworks of the [Fe4S4]+ and [Fe4S4]3+ iron-sulfur proteins, are calculated using a general approach. It is revealed that the structural distortions of these systems significantly affect the composition of the magnetic states. The effect of the isotropic Heisenberg exchange is considered.

Tetrameric silver clusters in rho zeolite stable above room temperature — ESR studies

Cationic silver clusters formed in γ-irradiated Ag +-exchanged zeolite rho have been studied by electron spin resonance (ESR) spectroscopy. It was found that dehydrated Ag-rho zeolites show a remarkable tendency for stabilization of Ag 4 3+ clusters. In dehydrated samples tetrameric silver is stable for months at room temperature and is still observed at 100°C. It is postulated that Ag 4 3+ clusters are located in distorted octagonal prisms which appear to be very effective trapping sites. In hydrated rho zeolite framework distortions are removed and silver tetramers decay below room temperature.

Photoelectron investigations and density functional calculations of anionic Sbn− and Bin− clusters

We recorded photoelectron spectra of antimony Sb−N=2–9 and of bismuth clusters Bi−N=2–9 with a photon energy of 4.03 eV, as well as of Bi−N=2–21 with a photon energy of 5.0 eV. The experimentally determined photoelectron thresholds and peak positions of Sb−N=2–5 and Bi−N=2–5 are compared with the results of ab initio density-functional (LCAO) calculations. The agreement between the experimental thresholds and the calculated adiabatic electron affinities, as well as between the first maxima in the spectra and the calculated vertical detachment energies is fair to good for the antimony clusters and qualitative for the bismuth systems. For the calculation of the ionization (detachment) energies we determined for the neutral and anionic clusters the most stable structures by LCAO calculations. In particular, the tetramer cluster anions have a ‘‘roof’’ structure, while the negatively charged pentamers are planar rings [with similarities to the (C5H5)− anion]; positive and negative trimers are nonlinear. Furthermore, the ionization energies and affinities of larger antimony and bismuth clusters are discussed qualitatively and compared to jellium calculations of Seidl and Brack.

Magnetic properties of tetrameric tetrahedral clusters with two-electron transfer

Tunnel-exchange states of tetrahedral tetrameric clusters dn-dn-dn+1-dn+1 with double transfer are considered to calculate the energy levels and wave functions. A method based on the second quantization technique and group-theoretical approach is developed. The nature of the ground spin state of the above systems is determined. It is shown that the structures have magnetic properties that radically differ from the properties of clusters with one migrating electron (hole).

CO2 Activation. 7. Formation of the Catalytically Active Intermediate in the Hydrogenation of Carbon Dioxide to Formic Acid Using the [{(COD)Rh(μ-H)}4]/Ph2P(CH2)4PPh2 Catalyst: First Direct Observation of Hydride Migration from Rhodium to Coordinated 1,5-Cyclooctadiene

The nature of the catalytically active intermediate formed in situ from the tetrameric cluster [{(COD)Rh(μ-H)}4] (COD = 1,5-cyclooctadiene; 1) and the bidentate phosphane Ph2P(CH2)4PPh2 (dppb) during hydrogenation of CO2 to formic acid was investigated. Kinetic measurements suggest the initial formation of a catalyst precursor that reacts with dihydrogen to give the actual active species. NMR spectroscopic investigations of the reaction of 1 with dppb in THF-d8 reveal three phosphorus-containing products that were fully characterized by one- and two-dimensional techniques, including 2D-(31P,1H)-COLOC spectra. The tetrameric hydride cluster [{(dppb)Rh(μ-H)}4] (2) and the double-phosphane-substituted monomeric rhodium hydride [(dppb)2RhH] (3) are formed as byproducts in low yield. The (phosphane)rhodium η3-cyclooctenyl complex [(dppb)Rh(η3-C8H13)] (4), arising via hydride transfer from rhodium to coordinated COD, is the major product, containing about 80% of the dppb. Complex 4 was isolated from the mixture...

Silver atoms and clusters in molecular sieves and clays

Electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies on small paramagnetic silver clusters generated radiolytically in molecular sieves and clays are reviewed. Special attention is paid to the role of framework structure with respect to cluster nuclearity and stability. In general, geometrical size constraints play a crucial role and molecular sieves with cage-like structures with small openings between the cages such as A and rho zeolites stabilize cationic silver clusters more efficiently than channel-like zeolite structures. In addition, other factors like total cation capacity, type of cocation and/or molecular adsorbate in the cages can affect the silver agglomeration process to a great extent. The mechanism of silver agglomeration in irradiated molecular sieves and clays based on the ESR results is described as well as the reactivity of tetrameric and hexameric silver clusters with various molecular adsorbates.

Excited state absorption spectra of the tetranuclear cuprous iodide cluster Cu 4I 4(py) 4 and related species

Nanosecond flash photolysis studies have been carried out on the tetrameric clusters Cu 4I 4L 4 (L = pyridine (py), 4- t-butyl pyridine (4-Bu t-py)), 4-phenyl pyridine (Phpy) or piperidine (pip)). A broad transient absorption band in the visible region was observed for each of the clusters with aromatic heterocycle ligands upon excitation at 337 or 355 nm but no excited state absorption (ESA) was observed when L is the saturated amine piperidine. The ESA band maxima are dependent upon the nature of the pyridine ring substituent, occurring at 542 nm, 565 run, and 532 nm for L = py, Phpy, and 4-Bu t-py), respectively. These ESA bands are modestly solvent-sensitive, shifting to lower energy in solvents of lower polarity. The microsecond lifetimes of the ESA bands are comparable to those observed for emission from the triplet cluster centered excited states (3CC) previously assigned for these compounds (K.R. Kyle, C.K. Ryu, J.A. DiBenedetto and PC. Ford, J. Am. Chem. Soc., 113 (1991) 2954–2965). Consideration of this data and examination of the energetics of the states involved suggests that the transition responsible for the excited state absorption occurs from the 3CC state to a higher energy triplet halide-to-ligand charge transfer ( 3XLCT) state.

Gas-Phase Molecular, Molecular Pair, and Molecular Triplet Fe+ Affinities of Pyridines

The relative molecular, molecular pair, and molecular triplet Fe+ affinities of pyridines are determined by the kinetic method through examination of the competitive dissociation of Fe+-bound dimer, trimer, and tetramer cluster ions. The molecular Fe+ affinities vary strongly with the electron donating power of the substituent on the pyridine. Similar behavior is seen in the molecular pair and molecular triplet affinities, the first triplet affinity values determined by the kinetic method. The absolute Fe+ affinity of pyridine itself is estimated to be 49 ± 3 kcal/mol. A linear correlation (slope = 0.38) is demonstrated between the relative Fe+ affinities and the corresponding proton affinities of the meta- and para-substituted pyridines. The slope together with data from previous studies falls in the order of OCNCO+≈ SiCl3+ > Cl+ > CN+ > SiCl+ > Fe+, which indicates that the strength of binding to pyridines falls in the same order. The relative molecular pair and molecular triplet Fe+ affinities also correlate linearly with the number averaged proton affinities of the pyridines. Lower than expected affinities are observed for systems containing ortho-substituted pyridines due to intramolecular steric effects between the ortho-substituted alkyl group(s) and the central Fe+ cation. A set of gas-phase steric parameters (Sk), measured by the deviation of the experimental data from the regression line, shows much larger steric effects in the crowded trimers and tetramers.

Group theoretical classification of resonance states of mixed valence polynuclear clusters

A group theoretical method is suggested for determining allowed type states of multielectronic polyatomic mixed valence systems. The classification of resolved terms of tetrameric tetrahedral mixed valence clusters d n -d n -d n -d n+1 and d n -d n -d n+1-d n+1 is given.