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The syntheses, crystal structures and magnetic characterizations of three new hexanulear iron(III) compounds are reported. The large difference in ground state spin was identified as resulting from a single structural difference between the two types of complexes, and the magnetostructural correlation was established between the J values and both the Fe-O bond distances and the Fe-O-Fe angles at the bridging ligands.

Research was conducted to examine the determination of the Ce+3 bonding in the Ca0.5Sr0.5Ga2S4:Ce phosphor via x-ray photoelectron spectroscopy. The addition of oxygen in the phosphor was observed to nearly double the luminance and to blue shift the emission spectrum of the Ce+3 4f-5d transition. SCF/CI potential energy surface calculations and thermodynamic data revealed that the formation of Ce-O bonds are favored over Ce-Sd, Sr-O or Ca-O bonds.

Ground state spin-orbit effects in the molecules V[sub 2], VNb, and Nb[sub 2] have been investigated using self-consistent field configuration interaction (SCF/CI) calculations based on the intermediate neglect of differential overlap Hamiltonian parameterized for optical spectroscopy (INDO/S). Accurate results for the spin-orbit splittings of the X [sup 3]Σ[sup -] ground state of each molecule were obtained using a CI treatment designed to isolate essential correlation of the ground state and the isoconfigurational [sup 1]Σ[sup +] state causing the splittings. Energies computed for the [sup 1]Σ[sup +] states are compared to energies obtained from simple two-state perturbative models. It is observed that the consideration of only two states is a large source of error. Previous assignment of a low-lying excited state observed for V[sub 2] as the [sup 1]Σ[sub g][sup +] state is not supported. Additional results on excited states of VNb have also been obtained. A [sup 3]Σ[sup -]←X [sup 3]Σ[sup -] transition observed experimentally for VNb is assigned as a σ[sup *]←σ promotion, and the presence of a second excited [sup 3]Σ[sup -] state of VNb that has not been observed is suggested. An analysis of correlation effects in the INDO/S model is given based on a comparison of results obtained with the approximate essential correlation CI calculations and with larger restricted active space full CI calculations. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A convenient and high yield procedure to obtain the [F.sub.10] family of single-strand 'ferric wheels' involving the methanolysis under basic conditions of [[[Fe.sub.3]O[([O.sub.2]CR).sub.6][([H.sub.2]O).sub.3].sup.+] salts in MeOH is developed. The carboxylate substitution on preformed [Fe.sub.10] wheels, phenolysis, and reaction with the N-based chelate ethylenediamine could be used to yield [Fe.sub.2] and two [Fe.sub.8] products with 'Christmas star' like topology.

The density functional theory calculations were used to explore the S=11 ground states of the [Mn.sup.7] family of mixed-valence complexes with a metal-centered hexagonal topology. The topology was observed to arise by spin frustration involving small differences in the magnitudes of the two weakest interactions controlling the alignment of the central spin which lead to formation of complex with maximum S = 16 ground state.

A study was conducted to explore the application of both azide and alkoxide-containing groups in Mn cluster chemistry. The results demonstrated the synthetic use of alcohol-based chelates and azido ligands when used together and the synthesis of two isomeric [Mn.sup.III.sub.6][Mn.sup.II.sub.4] cores that differ in spin by a remarkable 22 units.

The syntheses, crystal structures, magnetochemical characterization, and theoretical calculations of three new iron clusters [[Fe.sub.6][O.sub.2][(N[O.sub.3]).sub.4][(hmp).sub.8][([H.sub.2]O).sub.2]][(N[O.sub.3]).sub.2], [[Fe.sub.4][([N.sub.3]).sub.6][(hmp).sub.6]], and [[Fe.sub.8][O.sub.3](OMe)[(pdm).sub.4][(pdmH).sub.4][(MeOH).sub.2][(Cl[O.sub.4]).sub.5] are reported. The findings demonstrate the ligating flexibility of pyridyl-alcohol chelates and their application in the synthesis of new polynuclear [Fe.sub.x] clusters without requiring the carboxylate ligands as catalyst.

The syntheses of various [Ce.sup.IV][Mn.sup.IV] clusters by using mixed transition metal-lanthanide complexes at high oxidation states are described.

The intermediate neglect of differential overlap method for optical spectroscopy (INDO/S0) is used by the spin-orbit configuration interaction (SOCI) calculations to investigate the ground-state spin-orbit splittings in V2 plus, VNb plus and Nb2 plus. Analysis of correlation effects in the INDO/S model indicates that a charge-iterative method could be more effective for systematically correlated semiempirical calculations on metal cluster ions.

Four new manganese (Mn) compounds are synthesized with the use of the dicarboxylate group. These new Mn compounds are of different nuclearities and oxidation states.

Presents information on a study which investigated the local chemistry of cerium luminescent center in the undoped and oxygen-doped calcium-strontium-gallium sulphide phosphor using x-ray photoelectron spectroscopy analysis. Experimental procedure; Results and discussion; Conclusions.

Research has been conducted on preformed hexanuclear and tetranuclear iron(III) clusters. The authors describe the use of the clusters' alcoholysis for the synthesis of higher-nuclearity clusters.

A new, diolate-containing class of ferric wheel [[Fe.sub.12](pd).sub.12][([O.sub.2]CEt).sub.12] is described. The N-based chelates are shown to convert molecular wheels into molecular squares and rectangles.

A set of agroclimatic indices representing Canadian climatic conditions for field crop production are analyzed for long-term trends during 1895–2007. The indices are categorized for three crop types: cool season, warm season, and overwintering. Results indicate a significant lengthening of the growing season due to a significantly earlier start and a significantly later end of the growing season. Significant positive trends are also observed for effective growing degree-days and crop heat units at most locations across the country. The occurrence of extremely low temperatures has become less frequent during the nongrowing season, implying a more favorable climate for overwinter survival. In addition, the total numbers of cool days, frost days, and killing-frost days within a growing season have a decreasing trend. This means that crops may also be less vulnerable to cold stress and injury during the growing season. Extreme daily precipitation amounts and 10-day precipitation totals during the growing season have been increasing. Significant trends associated with increased availability of water during the growing season are identified by the standardized precipitation index and seasonal water deficits. The benefit of the increased precipitation may have been offset by an upward trend in evaporative demand; however, this would depend on the amount of growth and productivity resulting from increased actual evapotranspiration. [ABSTRACT FROM AUTHOR]

The syntheses, crystal structures, and magnetic characterizations of three new hexanuclear iron(III) compounds are reported. Known [Fe6O2(OH)2(O2CBut)10(hep)2] (1)is converted to new [Fe6O2(OH)(O2CBut9(hep)4] (3) when treated with an excess of 2-(2-hydroxyethyl)-pyridine (hepH). Similarly, the new compound [Fe6O2(OH)2(O2CPh)10-(hep)2] (2), obtained from the reaction of [Fe3O(O2CPh)6(H2O)3] with hepH, is converted to [Fe6O2(OH)(O2CPh)9-(hep)4] (4) when treated with an excess of hepH. This can be reversed by recrystallization from MeCN. The cores of the four Fee complexes all comprise two triangular [Fe3(μ3-O)(O2CR)3(hep)]+3 units connected at two of their apices by two sets of bridging ligands. However, 1 and 2 differ slightly from 3 and 4 in the precise way the two Fe3 units are linked together. In 1 and 2, the two sets of bridging ligands are identical, consisting of one μ-hydroxo and two μ-carboxylate groups bridging each Fe2 pair, i.e., a (μ-OH-)(μ-O2CR-)2 set. In contrast, 3 and 4 have two different sets of bridging ligands, a (μ-OH-)(μ-O2CR-)2 set as in 1 and 2, and a (μ-OR-)2(μ-O2CR-) set, where RO- refers to the alkoxide arm of the hep- chelate. Variable-field and -temperature dc magnetization measurements establish that 1 and 2 have S = 5 ground states and significant and positive zero-field splitting parameters (D), whereas 3 and 4 have S = 0 ground states. This dramatic difference of 10 unpaired electrons in the ground state S values for near-isomeric compounds demonstrates an acute sensitivity of the magnetic properties to small structural changes. The factors leading to this have been quantitatively analyzed. The semiempirical method ZILSH, based on unrestricted molecular orbital calculations, was used to obtain initial estimates of the Fe2 pairwise exchange interaction constants (J). These calculated values were then improved by fitting the experimental susceptibility versus T data, using a genetic algorithm approach. The final J values were then employed to rationalize the observed magnetic properties as a function of the core topologies and the presence of spin frustration effects. The large difference in ground state spin value was identified as resulting from a single structural difference between the two types of complexes, the different relative dispositions (cis vs trans) of two frustrated exchange pathways. In addition, use of the structural information and corresponding J values allowed a magnetostructural correlation to be established between the J values and both the Fe-O bond distances and the Fe-O-Fe angles at the bridging ligands. [ABSTRACT FROM AUTHOR]

Discusses the use of alcoholysis of preformed tetranuclear and hexanuclear iron(III) clusters for the synthesis of four higher-nuclearity clusters. Solid-state variable-temperature magnetic susceptibility measurements; Spin-singlet ground states for complexes; Magnetic behavior of the molecules.

Molecular orbital calculations employing the PM3 model have been used to examine the bonding in the complexes CpCr(CO) 2 (N X ) ( X = O, S, Se, Te). The previously established trend of increasing Cr-N interaction as X changes from O to S is demonstrated by these calculations, and found to extend to Se and Te. Bond lengths, bond orders, vibrational frequencies, and heats of reaction are used to support the conclusion that metal to ligand π-backbonding increases down the periodic chart from NO to NTe. [ABSTRACT FROM AUTHOR]

The electronic structure and spectroscopy of the vanadium dimer has been studied with semiempirical self-consistent field-configuration interaction calculations using the intermediate neglect of differential overlap Hamiltonian parameterized for spectroscopy (INDO/S) including spin-orbit coupling effects. An approximate configuration interaction (CI) treatment is designed based on correlation effects observed in CI calculations in small active spaces, and yields good agreement with experimental observations of state energies and spin-orbit splittings. The location of a [sup 1]Σ[sub g][sup +] excited state isoconfigurational with the ground state was determined, and calls into question a previous assignment of an excited state observed near 1860 cm-1. The previously observed A [sup 3]Π[sub u]←X [sup 3]Σ[sub g][sup -] transition is assigned as a dδ[sub g]←dπ[sub u] promotion. In addition, an unassigned transition observed near 15 000 cm-1 has been assigned as B [sup 3]Σ[sub u][sup -]←X [sup 3]Σ[sub g][sup -]. Both this transition and the previously observed A[sup ′] [sup 3]Σ[sub u][sup -]←X [sup 3]Σ[sub g][sup -] transition are assigned as σ[sub u]←σ[sub g] promotions, in disagreement with previous assignments. A [sup 1]Σ[sub u][sup +] state isoconfigurational with the A[sup ′] [sup 3]Σ[sub u][sup -] state is suggested as a candidate for an unassigned transition in the range 11 250-12 500 cm-1. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]