A Rare Cyclic Dimolybdenum Triad with Three Weakly Interacting Unpaired Electrons

Oxidation of α-[Mo₂(cis-DAniF)₂]₃(μ-F)₆ with an excess of FeCp₂BF₄ produces the triply oxidized species {β-[Mo₂(cis-DAniF)₂]₃(μ-F)₆}(BF₄)₃. During the oxidation process, the conformation in the α triangular species changes from an arrangement in which two dimetal units are parallel and the third one essentially orthogonal to a structure in which all three dimetal units are parallel. Furthermore, upon removal of three electrons, the Mo–Mo distances increase by about 0.05–0.06 Å and the Mo–F bond distances decrease by 0.04 Å. The structural data, as well as EPR, are consistent with an electronically localized system and a decrease in bond order from 4 to 3.5 for each dimetal unit.     

In-plane magnetization with high coercivity in terbium iron garnet thin films deposited on Pt/Si substrate by PLD

We report on the growth of terbium iron garnet (TbIG, Tb₃Fe₅O₁₂) thin films having anomalously large coercivity and in-plane easy axis of magnetization. The TbIG thin films were prepared at room temperature (RT) on Pt/Si(1 0 0) substrates by pulsed laser deposition technique. The films deposited at RT were X-ray amorphous and do not show any magnetic order. Annealing of the RT deposited film at 900 °C resulted into fully textured (532) TbIG film. Atomic force microscopy and cross-sectional scanning electron microscopy studies of the TbIG films showed good surface quality with an average surface roughness of 5.0 nm and thickness of about 300 nm, respectively. The M-H loops measured at 20 K for TbIG films, exhibit about an order of magnitude enhancement in the coercivity value (H_c) than the single crystal. In-plane and out-of-plane M-H loops revealed that the easy axis of the magnetization lies within the film’s plane. In-plane magnetization combining with large H_c value of the TbIG thin film may be of scientific interest for the possible applications.     

Consequence of doping mediated strain and the activation energy on the structural and optical properties of ZnO:Cr nanoparticles

We report on the sol-gel synthesis of Zn_1−xCr_xO (x=0.0, 0.05, 0.10, 0.15 and 0.20) nanoparticles. These nanoparticles were characterized by using thermogravimetry/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and Photoluminescence (PL). Electronegativity of Cr ions (Cr³⁺) reduces the final decomposition temperature by 40 °C and activation energy of the reaction when Cr is doped into ZnO. Doping of higher Cr concentration (x≥0.10) into ZnO shows formation of secondary spinel (ZnCr₂O₄) phase along with the hexagonal (ZnO) and is revealed by XRD. Formation of secondary phase changes the activation energy of the reaction and thus the strain in ZnO lattice. In Raman spectra, additional Raman modes have been observed for Zn_1−xCr_xO nanoparticles, which can be assigned to the modes generated due to Cr doping. The Cr doping into ZnO is also supported by PL, in which vacancies are formed with Cr ion incorporation and emission band shifts towards higher wavelength.     

A Poly(3,4‐ethylenedioxypyrrole)–Au@WO3‐Based Electrochromic Pseudocapacitor

A poly(3,4‐ethylenedioxypyrrole)–gold nanoparticle (Au)–tungsten oxide (PEDOP–Au@WO₃) electrochromic supercapacitor electrode capable of optically modulating solar energy while simultaneously storing/releasing energy (in the form of charge) was fabricated for the first time. WO₃ fibers, 50 to 200 nm long and 20 to 60 nm wide, were synthesized by a hydrothermal route and were electrophoretically deposited on a conducting substrate. Au nanoparticles and PEDOP were coated over WO₃ to yield the PEDOP–Au@WO₃ hybrid electrode. The inclusion of Au in the hybrid was confirmed by X‐ray diffraction, Raman spectroscopy, and energy‐dispersive X‐ray analyses. The nanoscale electronic conductivity, coloration efficiency, and transmission contrast of the hybrid were found to be significantly greater than those of pristine WO₃ and PEDOP. The hybrid showed a high specific discharge capacitance of 130 F g^?1 during coloration, which was four and ten times greater than the capacitance achieved in WO₃ or PEDOP, respectively. We also demonstrate the ability of the PEDOP–Au@WO₃ hybrid, relative to pristine PEDOP, to perform as a superior counter electrode in a solar cell, which is attributed to a higher work function. The capacitance and redox switching capability of the hybrid decreases insignificantly with cycling, thus establishing the viability of this multifunction hybrid for next‐generation sustainable devices such as electrochromic psuedocapacitors because it can concurrently conserve and store energy.     

Solid-Phase Synthesis of 2,4-diaminopyrimidines via Lewis acid-mediated aromatic nucleophilic substitution

Primary amines were immobilized on (4-formyl-3,5-dimethoxyphenoxy)methylpolystyrene resin via reductive amination. Attachment of two different 4-chloro-2-methylthiopyrimidines, followed by sulfide oxidation, led to their corresponding sulfone intermediates. Aromatic nucleophilic substitution with various anilines or heteroaromatic amines in the presence of trimethyl aluminum afforded the desired 2,4-diaminopyrimidines after acidic cleavage from the resin. The synthetic methodology described herein was validated with the synthesis of a small 162-member library.     

On the role of parallel architecture supercomputers in time-dependent approaches to quantum scattering

Summary Results of our initial study of the use of parallel architecture super-computers in solving time-dependent quantum scattering equations are reported. The specific equations solved are obtained from the time-dependent Lippmann-Schwinger integral equation by means of a quadrature approximation to the time integral. This leads to a modified Cayley transform algorithm in which the primary computational step is a matrix-vector multiplication. Implementation has been carried out both for the MasPar MP-1 and the NCUBE 6400 parallel machines. The codes are written in a modular form that greatly facilitates porting from one machine architecture to another. Both parallel machines prove to be more powerful for this application than the serial architecture VAX 8650. Specific analysis of machine performance is given.Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. 2-7405-ENG-82. This research was supported by the Division of Chemical Sciences and Applied Mathematical Sciences, Office of Basic Energy SciencesR.A. Welch Predoctoral Fellow under R.A. Welch Foundation Grant E-608Supported in part under National Science Foundation Grant CHE89-07429     

Fully localized mixed-valence oxidation products of molecules containing two linked dimolybdenum units: an effective structural criterion

Two previously reported compounds [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)] (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 4619), in which [Mo(2)] is an abbreviation for the quadruply bonded Mo(2)(4+) unit embraced by three (p-anisyl)NC(H)N(p-anisyl) anions and M = Zn (1) or Co (2), have been chemically oxidized. One-electron oxidation products [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)](PF(6)) (3, M = Zn; 4, M = Co) and the two-electron oxidation product [Mo(2)](CH(3)O)(2)Zn(CH(3)O)(OH)[Mo(2)](PF(6))(2) (5) have been isolated and structurally characterized. As expected, oxidations occur at the dimolybdenum units. The mono-charged cations in 3 and 4 have asymmetric molecular structures with two distinct [Mo(2)] units. In each case, one of the [Mo(2)] units has a lengthened Mo-Mo bond distance of 2.151[1] A, as expected for one-electron oxidation, whereas the other remains unchanged at 2.115[1] A. These correspond to bond orders of 3.5 (sigma(2)pi(4)delta(1)) and 4.0 (sigma(2)pi(4)delta(2)), respectively. The crystallographic results thus show unambiguously that in the crystalline state, the mixed-valence compounds (3 and 4) are electronically localized and the unpaired electron is trapped on one [Mo(2)] unit. These results are supported by the EPR spectra. The doubly oxidized compound 5 has two equivalent [Mo(2)] units, both with a Mo-Mo bond distance of 2.149[1] A. EPR and magnetic susceptibility measurements for 5 indicate that there is no significant ferromagnetic or antiferromagnetic spin coupling and the species is valence-trapped.     

Sandwich-type germanotungstates: structure and magnetic properties of the dimeric polyoxoanions [M4(H2O)2(GeW9O34)2]12 - (M = Mn2+, Cu2+, Zn2+, Cd2+)

The novel dimeric germanotungstates [M(4)(H(2)O)(2)(GeW(9)O(34))(2)](12)(-) (M = Mn(2+), Cu(2+), Zn(2+), Cd(2+)) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and (183)W-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na(12)[Mn(4)(H(2)O)(2)(GeW(9)O(34))(2)].38H(2)O (Na(12)()-1), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.0419(8) A, b = 17.8422(10) A, c = 21.1626(12) A, beta = 93.3120(10) degrees, and Z = 2; Na(11)Cs(2)[Cu(4)(H(2)O)(2)(GeW(9)O(34))(2)]Cl.31H(2)O (Na(11)()Cs-2) crystallizes in the triclinic system, space group P, with a = 12.2338(17) A, b = 12.3833(17) A, c = 15.449(2) A, alpha = 100.041(2) degrees, beta = 97.034(2) degrees, gamma = 101.153(2) degrees, and Z = 1; Na(12)[Zn(4)(H(2)O)(2)(GeW(9)O(34))(2)].32H(2)O (Na(12)()-3) crystallizes in the triclinic system, space group P, with a = 11.589(3) A, b = 12.811(3) A, c = 17.221(4) A, alpha = 97.828(6) degrees, beta = 106.169(6) degrees, gamma = 112.113(5) degrees, and Z = 1; Na(12)[Cd(4)(H(2)O)(2)(GeW(9)O(34))(2)].32.2H(2)O (Na(12)()-4) crystallizes also in the triclinic system, space group P, with a = 11.6923(17) A, b = 12.8464(18) A, c = 17.616(2) A, alpha = 98.149(3) degrees, beta = 105.677(3) degrees, gamma = 112.233(2) degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[GeW(9)O(34)](10)(-) Keggin moieties linked via a rhomblike M(4)O(16) (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. (183)W-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na(12)()-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at g(iso) = 1.9949, typical of Mn systems, and an upper limit of |D| = 20.0 mT; its magnetization studies still await further theoretical treatment. Detailed EPR studies on Na(11)()Cs-2 over temperatures down to 2 K and variable frequencies yield g( parallel ) = 2.4303 and g( perpendicular ) = 2.0567 and A( parallel ) = 4.4 mT (delocalized over the Cu(4) framework), with |D| = 12.1 mT. Magnetization studies in addition yield the exchange parameters J(1) = -11 and J(2) = -82 cm(-)(1), in agreement with the EPR studies.     

Structure of hydrated clusters of tetrahydroisoquinoline [THIQ-(H2O)(n=1,3)] investigated by jet spectroscopy

The hydrated clusters of tetrahydroisoquinoline have been investigated by laser-induced fluorescence (LIF), UV-UV hole burning, and IR-UV double-resonance spectroscopy in a seeded supersonic jet. Clusters of different sizes and isomeric structures have different 0-0 transitions (origins) in the LIF spectrum. UV-UV hole burning spectroscopy has been used to identify different cluster species and their vibrational modes. The structures of the clusters have been predicted by comparing the observed OH and NH frequencies in the IR-UV double-resonance spectra with the results calculated at different levels of sophistication. It is found that the water molecules form linear and six- and eight-membered cyclic H-bonded structures at the nitrogen center of 1:1, 1:2, and 1:3 clusters, respectively.     

Magnetism, electron paramagnetic resonance, electrochemistry, and mass spectrometry of the pentacopper(II)-substituted tungstosilicate [Cu5(OH)4(H2O)2(A-alpha-SiW9O33)2]10-, a model five-spin frustrated cluster

The dimeric, pentacopper(II)-substituted tungstosilicate [Cu(5)(OH)(4)(H(2)O)(2)(A-alpha-SiW(9)O(33))(2)](10)(-) (1) has been characterized by single-crystal X-ray diffraction, elemental analysis, IR, electrochemistry, magnetic measurements, electron paramagnetic resonance (EPR), and mass spectrometry (MS). Magnetization and high-field EPR measurements reveal that the pentameric copper core {Cu(5)(OH)(4)(H(2)O)(2)}(6+) of 1 exhibits strong antiferromagnetic interactions (J(a) = -51 +/- 6 cm(-)(1), J(b) = -104 +/- 1 cm(-)(1), and J(c) = -55 +/- 3 cm(-)(1)) resulting in a spin S(T) = (1)/(2) ground state. EPR data show that the unpaired electron spin density is localized on the spin-frustrated apical Cu(2+) ion with g(zz) = 2.4073 +/- 0.0005, g(yy) = 2.0672 +/- 0.0005, g(xx) = 2.0240 +/- 0.0005, and A(zz) = -340 +/- 20 MHz (-0.0113 cm(-)(1)). 1 can therefore be considered as a model system for a five-spin, electronically coupled, spin-frustrated system. Polyanion 1, which is stable over a wide pH domain (pH 1-7), was characterized by cyclic voltammetry (CV) in a pH 5 medium. Its CV was constituted by an initial two-step reduction of the Cu(2+) centers to Cu(0) through Cu(+), followed at more negative potential by the redox processes of the W centers. Controlled potential coulometry of 1 allows for the reduction of the five Cu(2+) centers, as seen by consumption of 10.05 +/- 0.05 electrons per molecule. Polyanion 1 triggers efficiently the electrocatalytic reduction of nitrate and nitrite, and it also catalyzes the reduction of N(2)O. To our knowledge, this is the first example of N(2)O catalytic reduction by a polyoxoanion. Fourier transform ion cyclotron resonance MS was used to unambiguously assign the molecular weight of the solution-phase species 1 and the oxidation states of the Cu atoms in the central {Cu(5)(OH)(4)(H(2)O)(2)}(6+) core. Infrared (IR) multiphoton dissociation MS/MS of 1 showed evidence of a condensation process similar to bronze formation at low irradiation intensity. Higher IR intensity resulted in the formation of stable fragments consistent with those previously observed in the solution chemistry of polyoxoanions.