Time filtering of event based neutron scattering data: A pathway to study the dynamic structural responses of materials

Time-resolved diffraction has become a vital tool for probing dynamic responses to an applied stimulus. Such experiments traditionally use hardware solutions to histogram measured data into their respective bin. We will show that a major advantage of event-based data acquisition, which time-stamps measured diffraction data with 100 ns accuracy, is much preferred over hardware histogramming of the data by enabling postprocessing for advanced custom binning using a software solution. This approach is made even more powerful by coupling measured diffraction data with metadata about the applied stimuli and material response. In this work, we present a time-filter approach that leverages the power of event-based diffraction collection to reduce stroboscopic data measured over many hours into equally weighted segments that represent subsets of the response to a single cycle of the applied stimulus. We demonstrate this approach by observing ferroelectric/ferroelastic domain wall motion during electric field cycling of BaTiO₃. The developed approach can readily be expanded to investigate other dynamic phenomena using complex sample environments.

One Dimensional(1D)-to-2D Crossover of Spin Correlations in the 3D Magnet ZnMn2O4

We report on the intriguing evolution of the dynamical spin correlations of the frustrated spinel ZnMn₂O₄. Inelastic neutron scattering and magnetization studies reveal that the dynamical correlations at high temperatures are 1D. At lower temperature, these dynamical correlations become 2D. Surprisingly, the dynamical correlations condense into a quasi 2D Ising-like ordered state, making this a rare observation of two dimensional order on the spinel lattice. Remarkably, 3D ordering is not observed down to temperatures as low as 300 mK. This unprecedented dimensional crossover stems from frustrated exchange couplings due to the huge Jahn-Teller distortions around Mn³⁺ ions on the spinel lattice.

Determination of Hydrogen Bond Structure in Water versus Aprotic Environments To Test the Relationship Between Length and Stability

Hydrogen bonds profoundly influence the architecture and activity of biological macromolecules. Deep appreciation of hydrogen bond contributions to biomolecular function thus requires a detailed understanding of hydrogen bond structure and energetics and the relationship between these properties. Hydrogen bond formation energies (ΔGf) are enormously more favorable in aprotic solvents than in water, and two classes of contributing factors have been proposed to explain this energetic difference, focusing respectively on the isolated and hydrogen-bonded species: (I) water stabilizes the dissociated donor and acceptor groups much better than aprotic solvents, thereby reducing the driving force for hydrogen bond formation; and (II) water lengthens hydrogen bonds compared to aprotic environments, thereby decreasing the potential energy within the hydrogen bond. Each model has been proposed to provide a dominant contribution to ΔGf, but incisive tests that distinguish the importance of these contributions are lacking. Here we directly test the structural basis of model II. Neutron crystallography, NMR spectroscopy, and quantum mechanical calculations demonstrate that O-H···O hydrogen bonds in crystals, chloroform, acetone, and water have nearly identical lengths and very similar potential energy surfaces despite ΔGf differences >8 kcal/mol across these solvents. These results rule out a substantial contribution from solvent-dependent differences in hydrogen bond structure and potential energy after association (model II) and thus support the conclusion that differences in hydrogen bond ΔGf are predominantly determined by solvent interactions with the dissociated groups (model I). These findings advance our understanding of universal hydrogen-bonding interactions and have important implications for biology and engineering.

Integration of neutron time-of-flight single-crystal Bragg peaks in reciprocal space

The intensity of single-crystal Bragg peaks obtained by mapping neutron time-of-flight event data into reciprocal space and integrating in various ways is compared. These methods include spherical integration with a fixed radius, ellipsoid fitting and integration of the peak intensity, and one-dimensional peak profile fitting. In comparison to intensities obtained by integrating in real detector histogram space, the data integrated in reciprocal space result in better agreement factors and more accurate atomic parameters. Furthermore, structure refinement using integrated intensities from one-dimensional profile fitting is demonstrated to be more accurate than simple peak-minus-background integration.

An asymptotically consistent approximant method with application to soft- and hard-sphere fluids

A modified Padé approximant is used to construct an equation of state, which has the same large-density asymptotic behavior as the model fluid being described, while still retaining the low-density behavior of the virial equation of state (virial series). Within this framework, all sequences of rational functions that are analytic in the physical domain converge to the correct behavior at the same rate, eliminating the ambiguity of choosing the correct form of Padé approximant. The method is applied to fluids composed of "soft" spherical particles with separation distance r interacting through an inverse-power pair potential, φ = ε(σ∕r)(n), where ε and σ are model parameters and n is the "hardness" of the spheres. For n < 9, the approximants provide a significant improvement over the 8-term virial series, when compared against molecular simulation data. For n ≥ 9, both the approximants and the 8-term virial series give an accurate description of the fluid behavior, when compared with simulation data. When taking the limit as n → ∞, an equation of state for hard spheres is obtained, which is closer to simulation data than the 10-term virial series for hard spheres, and is comparable in accuracy to other recently proposed equations of state. By applying a least square fit to the approximants, we obtain a general and accurate soft-sphere equation of state as a function of n, valid over the full range of density in the fluid phase.

Double product and end-organ damage in African and Caucasian men: the SABPA study

BACKGROUND

Increasing urbanisation in sub-Saharan African countries is causing a rapid increase in cardiovascular disease. Evidence suggests that Africans have higher blood pressures and a higher prevalence of hypertension-related cardiovascular morbidity and mortality, compared to Caucasians. We investigated double product (systolic blood pressure × heart rate), a substantial measure of cardiac workload, as a possible cardiovascular risk factor in African and Caucasian men.

MATERIAL AND METHODS

The study consisted of 101 urbanised African and 101 Caucasian male school teachers. We measured 24h ambulatory blood pressure and the carotid cross-sectional wall area, and determined left ventricular hypertrophy electrocardiographically by means of the Cornell product. Urinary albumin and creatinine were analysed to obtain the albumin-to-creatinine ratio.

RESULTS

Africans had higher 24h, daytime and nighttime systolic- and diastolic blood pressure, heart rate and resultant double product compared to the Caucasians. In addition, markers of end-organ damage, albumin-to-creatinine ratio and left ventricular hypertrophy were higher in the Africans while cross-sectional wall area did not differ. In Africans after single partial and multiple regression analysis, 24h systolic blood pressure, but not double product or heart rate, correlated positively with markers of end-organ damage (cross-sectional wall area: β=0.398, P=0.005; left ventricular hypertrophy: β=0.455, P<0.001; albumin-to-creatinine ratio: β=0.280, P=0.012). No associations were evident in Caucasian men.

CONCLUSIONS

Double product may not be a good marker of increased cardiovascular risk when compared to systolic blood pressure in African and Caucasian men.

Sigma-borane complexes of iridium: synthesis and structural characterization

Reaction of NaBH4 with (tBuPOCOP)IrHCl affords the previously reported complex (tBuPOCOP)IrH2(BH3) (1) (tBuPOCOP = kappa(3)-C6H3-1,3-[OP(tBu)2]2). The structure of 1 determined from neutron diffraction data contains a B-H sigma-bond to iridium with an elongated B-H bond distance of 1.45(5) A. Compound 1 crystallizes in the space group P1 (Z = 2) with a = 8.262 (5) A, b = 12.264 (5) A, c = 13.394 (4) A, and V = 1256.2 (1) A(3) (30 K). Complex 1 can also be prepared by reaction of BH3 x THF with (tBuPOCOP)IrH2. Reaction of (tBuPOCOP)IrH2 with pinacol borane gave initially complex 2, which is assigned a structure analogous to that of 1 based on spectroscopic measurements. Complex 2 evolves H2 at room temperature leading to the borane complex 3, which is formed cleanly when 2 is subjected to dynamic vacuum. The structure of 3 has been determined by X-ray diffraction and consists of the (tBuPOCOP)Ir core with a sigma-bound pinacol borane ligand in an approximately square planar complex. Compound 3 crystallizes in the space group C2/c (Z = 4) with a = 41.2238 (2) A, b = 11.1233 (2) A, c = 14.6122 (3) A, and V = 6700.21 (19) A(3) (130 K). Reaction of (tBuPOCOP)IrH2 with 9-borobicyclononane (9-BBN) affords complex 4. Complex 4 displays (1)H NMR resonances analogous to 1 and exists in equilibrium with (tBuPOCOP)IrH2 in THF solutions.

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by Mössbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The Mössbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.

Magnetic and orbital ordering in the spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.

Neutron and X-ray structural studies of short hydrogen bonds in photoactive yellow protein (PYP)

Photoactive yellow protein (PYP) from Halorhodospira halophila is a soluble 14 kDa blue-light photoreceptor. It absorbs light via its para-coumaric acid chromophore (pCA), which is covalently attached to Cys69 and is believed to be involved in the negative phototactic response of the organism to blue light. The complete structure (including H atoms) of PYP has been determined in D(2)O-soaked crystals through the application of joint X-ray (1.1 A) and neutron (2.5 A) structure refinement in combination with cross-validated maximum-likelihood simulated annealing. The resulting XN structure reveals that the phenolate O atom of pCA accepts deuterons from Glu46 O(epsilon2) and Tyr42 O(eta) in two unusually short hydrogen bonds. This arrangement is stabilized by the donation of a deuteron from Thr50 O(gamma1) to Tyr42 O(eta). However, the deuteron position between pCA and Tyr42 is only partially occupied. Thus, this atom may also interact with Thr50, possibly being disordered or fluctuating between the two bonds.

Terminal Gold-Oxo Complexes

In contradiction to current bonding paradigms, two terminal Au-oxo molecular complexes have been synthesized by reaction of AuCl3 with metal oxide-cluster ligands that model redox-active metal oxide surfaces. Use of K10[alpha2-P2W17O61].20H2O and K2WO4 (forming the [A-PW9O34]9- ligand in situ) produces K15H2[Au(O)(OH2)P2W18O68].25H2O (1); use of K10[P2W20O70(OH2)2].22H2O (3) produces K7H2[Au(O)(OH2)P2W20O70(OH2)2].27H2O (2). Complex 1 crystallizes in orthorhombic Fddd, with a=28.594(4) A, b=31.866(4) A, c=38.241(5) A, V=34844(7) A3, Z=16 (final R=0.0540), and complex 2 crystallizes in hexagonal P6(3)/mmc, with a=16.1730(9) A, b=16.1730(9) A, c=19.7659(15) A, V=4477.4(5) A3, Z=2 (final R=0.0634). The polyanion unit in 1 is disorder-free. Very short (approximately 1.76 A) Au-oxo distances are established by both X-ray and 30 K neutron diffraction studies, and the latter confirms oxo and trans aqua (H2O) ligands on Au. Seven findings clarify that Au and not W is present in the Au-oxo position in 1 and 2. Five lines of evidence are consistent with the presence of d8 Au(III) centers that are stabilized by the flanking polytungstate ligands in both 1 and 2: redox titrations, electrochemical measurements, 17 K optical spectra, Au L2 edge X-ray absorption spectroscopy, and Au-oxo bond distances. Variable-temperature magnetic susceptibility data for crystalline 1 and 2 establish that both solids are diamagnetic, and 31P and 17O NMR spectroscopy confirm that both remain diamagnetic in solution. Both complexes have been further characterized by FT-IR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and other techniques.

Synthesis, Structure, and Properties of BaAl2Si2

Single crystals of BaAl2Si2 were grown from an Al molten flux and characterized using single-crystal X-ray diffraction at 10 and 90 K and neutron diffraction at room temperature. BaAl2Si2 crystallizes with the alpha-BaCu2S2 structure type (Pnma), is isostructural with alpha-BaAl2Ge2, and is an open 3D framework compound, where Al and Si form a covalent cagelike network with Ba2+ cations residing in the cages. BaAl2Si2 has a unit cell of a=10.070(3) A, b=4.234(1) A, and c=10.866(3) A, as determined by room-temperature single-crystal neutron diffraction (R1=0.0533, wR2=0.1034). The structure as determined by single-crystal neutron and X-ray diffraction (10 and 90 K) indicates that BaAl2Si2 (Pnma) is strictly isostructural to other (alpha)-BaCu2S2-type structures, requiring site specificity for Al and Si. Unlike BaAl2Ge2, no evidence for an alpha to beta (BaZn2P2-type, I4/mmm) phase transition was observed. This compound shows metallic electronic resistivity and Pauli paramagnetic behavior.

Better Understanding of the Species with the Shortest Re26+ Bonds and Related Re27+ Species with Tetraguanidinate Paddlewheel Structures

A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) A for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(hpp)4Cl2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re2(hpp)4F](TFPB)2, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re2(hpp)4Cl2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re2(hpp)4Cl2]+ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.

Syntheses and Structures of Asymmetric Bis(silyl) Niobocene Hydrides

This paper deals with the preparation and structural investigation of asymmetric bis(silyl) niobocene hydrides, Cp2Nb(SiHMe2)(H)(SiXMe2) (2; X = F (a), Cl (b), Br (c), I (d)) and Cp2Nb(SiXMe2)(H)(SiYMe2) (X,Y= F-I; X not equal Y). Complexes 2a-d were prepared by selective electrophilic activation of the Si-H bond in Cp2Nb(SiHMe2)2(H). The Cp2Nb(SiXMe2)(H)(SiYMe2) complexes were prepared by electrophilic activation of the Si-H bond in 2a-d and, in some cases, by electrophilic exchange of the X halides in Cp2Nb(SiXMe2)2(H) (1) for other halides, Y. The structures of complexes 2b and 2c have been studied by X-ray and neutron diffraction (ND). The ND results unequivocally established that the hydride ligand in 2c is shifted toward the SiBrMe2 ligand and that in 2b is positioned symmetrically between two nonequivalent silyl groups, with the H...SiClMe2 distance being shorter because of the shorter Nb-SiClMe2 bond length. Analysis of the X-ray structures of complexes 2a-d and complexes Cp2Nb(SiXMe2)(H)(SiYMe2) shows that the largest structural distortions are observed for the silyl groups substituted by heavy halogen atoms. These trends are rationalized in terms of stronger interligand hypervalent interactions (IHI) Nb-H...Si-X for heavy atoms X from Group 7.

Structure and Thermoelectric Characterization of Ba8Al14Si31

A molten Al flux method was used to grow single crystals of the type I clathrate compound Ba8Al14Si31. Single-crystal neutron diffraction data for Ba8Al14Si31 were collected at room temperature using the SCD instrument at the Intense Pulsed Neutron Source, Argonne National Laboratory. Single-crystal neutron diffraction of Ba8Al14Si31 confirms that the Al partially occupies all of the framework sites (R1 = 0.0435, wR2 = 0.0687). Stoichiometry was determined by electron microprobe analysis, density measurements, and neutron diffraction analysis. Solid-state (27)Al NMR provides additional evidence for site preferences within the framework. This phase is best described as a framework-deficient solid solution Ba8Al14Si31, with the general formula, Ba(8)Al(x)Si(42-3/4x)[](4-1/4x) ([] indicates lattice defects). DSC measurements and powder X-ray diffraction data indicate that this is a congruently melting phase at 1416 K. Temperature-dependent resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers.

Structural Factors Influencing Linear M−H−M Bonding in Bis(dialkylphosphino)methane Complexes of Nickel

Structural data for four closely related dinuclear nickel hydride complexes have been compared in order to gain insight into the factors governing the Ni-H-Ni geometries. The derivatives [(dippm)2Ni2X2](mu-H) [dippm = 1,2-bis(diisopropylphosphino)methane] were found to contain a linear Ni-H-Ni bridge, whereas the derivatives [(dcpm)2Ni2X2](mu-H) [dcpm = 1,2-bis(dicyclohexylphosphino)methane] were found to contain a bent Ni-H-Ni bridge. The number of internal and interatomic CH-to-halide contacts of the former were much shorter and more numerous than the latter, suggesting an important role of external forces in bridging hydride geometries.

Molecular Hydrogen Occupancy in Binary THF−H2 Clathrate Hydrates by High Resolution Neutron Diffraction

We have determined the time-space average filling of hydrogen molecules in a binary tetrahydrofuran (THF)-d(8) + D(2) sII clathrate hydrate using high resolution neutron diffraction. The filling of hydrogen in the lattice of a THF-d(8) clathrate hydrate occurred upon pressurization. The hydrogen molecules were localized in the small dodecahedral cavities at 20 K, with nuclear density from the hydrogen approximately spherically distributed and centered in the small cavity. With a formation pressure of 70 MPa, molecular hydrogen was found to only singly occupy the sII small cavity. This result helps explain discrepancies about the hydrogen occupancy in the THF binary hydrate system.

Conceptual design of a macromolecular neutron diffractometer (MaNDi) for the SNS

This paper describes the design criteria, calculations and simulations for a high-resolution macromolecular neutron diffractometer (MaNDi) for the Spallation Neutron Source (SNS). MaNDi is optimized to achieve 1.5 Å resolution from crystals of 0.1–1 mm3with lattice repeats in the range of 150 Å. It was determined that locating MaNDi on a decoupled hydrogen moderator beamline with a curved guide will provide data of higher resolution and higher signal-to-noise than a coupled hydrogen moderator at the SNS. In addition, for an instrument with an initial flight path of 24 m at the 60 Hz source and a wavelength bandwidth of Δλ≃ 2.7 Å, bandwidth selection disk choppers can shift the wavelength range higher or lower for different experiments. With a wavelength range of 1.5–4.2 Å anddmin= 2.0 Å, simulations predict experiment duration times of 1–7 d, which is expected to revolutionize neutron macromolecular crystallography (NMC) for applications in the fields of structural biology, enzymology and computational chemistry.

Electronic and Steric Effects on Molecular Dihydrogen Activation in [Cp*OsH4(L)]+ (L = PPh3, AsPh3, and PCy3)

Single-crystal neutron diffraction, inelastic neutron scattering, and density functional calculations provide experimental and theoretical analyses of the nature of the osmium-bound, "elongated" dihydrogen ligands in [Cp*OsH(4)(L)][BF(4)] complexes (L = PPh(3), AsPh(3), or PCy(3)). The PPh(3) and AsPh(3) complexes clearly contain one dihydrogen ligand and two terminal hydrides; the H(2) ligand is transoid to the Lewis base, and the H-H vector connecting the central two hydrogen atoms lies parallel to the Ct-Os-L plane (Ct = centroid of Cp* ring). In contrast, in the PCy(3) complex the H-H vector is perpendicular to the Ct-Os-L plane. Not only the orientation of the central two hydrogen atoms but also the H-H bond length between them depends significantly on the nature of L: the H...H distance determined from neutron diffraction is 1.01(1) and 1.08(1) A for L = PPh(3) and AsPh(3), respectively, but 1.31(3) A for L = PCy(3). Density functional calculations show that there is a delicate balance of electronic and steric influences created by the L ligand that change the molecular geometry (steric interactions between the Cp* and L groups most importantly change the Ct-Os-L angle), changing the relative energy of the Os 5d orbitals, which in turn govern the H-H distance, preferred H-H orientation, and rotational dynamics of the elongated dihydrogen ligand. The geometry of the dihydrogen ligand is further tuned by interactions with the BF(4)(-) counterion. The rotational barrier of the bound H(2) ligand in [Cp*OsH(4)(PPh(3))](+), determined experimentally (3.1 kcal mol(-)(1)) from inelastic neutron scattering experiments, is in reasonable agreement with the B3LYP calculated H(2) rotational barrier (2.5 kcal mol(-)(1)).

Single-crystal neutron-diffraction study of 3.4% Zn-doped (ND4)2[Cu(D2O)6](SO4)2 at 20 K

Doping the perdeuterated ammonium copper Tutton salt (ND4)2[Cu(D2O)6](SO4)2 [perdeuterated diammonium hexaaquacopper(II) bis(sulfate)] with Zn leads to a change in the structure from dimorph A (low density) to dimorph B (high density). This change, which accompanies a switch in the direction of the Jahn-Teller distortion, had previously been observed to occur with substitution of Zn2+ at the Cu2+ site of between 1.3 (A) and 3.4% (B). In this study, the single-crystal neutron-diffraction analysis of (ND4)2[(Cu/Zn)(D2O)6](SO4)2 at 20 K, with 3.4% Zn doping and a deuterium substitution of 85% on the H-atom sites, reveals that the structure is entirely of type B, with the Cu/Zn site at an inversion centre and with no evidence of disorder or unusual atomic displacement parameters that might occur near a phase transition boundary.

A Late-Transition Metal Oxo Complex: K 7 Na 9 [O=Pt IV (H 2 O)L 2 ], L = [PW 9 O 34 ] 9-

Terminal mono-oxo complexes of the late transition metal elements have long been considered too unstable to synthesize because of repulsion between the oxygen electrons and the mostly filled metal d orbitals. A platinum(IV)-oxo compound flanked by two polytungstate ligands, K7Na9[O=Pt(H2O)L2], L = [PW9O34(9-)], has now been prepared and isolated at room temperature as air-stable brown crystals. X-ray and neutron diffraction at 30 kelvin revealed a very short [1.720(18) angstrom] Pt-O bond and no evidence of a hydrogen atom at the terminal oxygen, ruling out a better precedented Pt-OH complex. Density functional theory and spectroscopic data account for the stability of the Pt(IV)-oxo unit by electron withdrawal into delocalized orbitals of the polytungstates.

Silylene Hydride Complexes of Molybdenum with Silicon−Hydrogen Interactions: Neutron Structure of (η5-C5Me5)(Me2PCH2CH2PMe2)Mo(H)(SiEt2)

Reduction of CpMoCl(4) with 3.1 equiv of Na/Hg amalgam (1.0% w/w) in the presence of 1 equiv of dmpe and 1 equiv of trimethylphosphine afforded the molybdenum(II) chloride complex Cp(dmpe)(PMe(3))MoCl (1) (Cp = 1,2,3,4,5-pentamethylcyclopentadienyl, dmpe = 1,2-bis(dimethylphosphino)ethane). Alkylation of 1 with PhCH(2)MgCl proceeded in high yield to liberate PMe(3) and give the 18-electron pi-benzyl complex Cp(dmpe)Mo(eta(3)-CH(2)Ph) (2). Variable temperature NMR experiments provided evidence that 2 is in equilibrium with its 16-electron eta(1)-benzyl isomer [Cp(dmpe)Mo(eta(1)-CH(2)Ph)]. This was further supported by reaction of 2 with CO to yield the carbonyl benzyl complex Cp(dmpe)(CO)Mo(eta(1)-CH(2)Ph) (3). Complex 2 was found to react with disubstituted silanes H(2)SiRR' (RR' = Me(2), Et(2), MePh, and Ph(2)) to form toluene and the silylene complexes Cp(dmpe)Mo(H)(SiRR') (4a: RR' = Me(2); 4b: RR' = Et(2); 4c: RR' = MePh; 4d: RR' = Ph(2)). Reactions of 2 with monosubstituted silanes H(3)SiR (R = Ph, Mes, Mes = 2,4,6-trimethylphenyl) produced rare examples of hydrosilylene complexes Cp(dmpe)Mo(H)Si(H)R (5a: R = Ph; 5b: R = Mes; 5c: R = CH(2)Ph). Reactivity of complexes 4a-c and 5a-d is dominated by 1,2-hydride migration from metal to silicon, and these complexes possess H.Si bonding interactions, as supported by spectroscopic and structural data. For example, the J(HSi) coupling constants in these species range in value from 30 to 48 Hz and are larger than would be expected in the absence of H.Si bonding. A neutron diffraction study on a single crystal of diethylsilylene complex 4b unequivocally determined the hydride ligand to be in a bridging position across the molybdenum-silicon bond (Mo-H 1.85(1) A, Si-H 1.68(1) A). The synthesis and reactivity properties of these complexes are described in detail.

Synthesis, Structure, and Reactivity of a Dinuclear Metal Complex with Linear M−H−M Bonding

Reaction of two equiv of 1-adamantylzinc bromide with (dippm)NiBr2 (dippm = bis(di-isopropylphosphino)methane) led to a dinuclear metal complex containing a unique linear bridging hydride ligand. The hydride was characterized by neutron diffraction methods, which confirmed a linear bonding mode. Preliminary reactivity studies of this unusual dimer are reported.

Box length search algorithm for molecular simulation of systems containing periodic structures

We have developed a box length search algorithm to efficiently find the appropriate box dimensions for constant-volume molecular simulation of periodic structures. The algorithm works by finding the box lengths that equalize the pressure in each direction while maintaining constant total volume. Maintaining the volume at a fixed value ensures that quantitative comparisons can be made between simulation and experimental, theoretical or other simulation results for systems that are incompressible or nearly incompressible. We test the algorithm on a system of phase-separated block copolymers that has a preferred box length in one dimension. We also describe and test a Monte Carlo algorithm that allows the box lengths to change while maintaining constant volume. We find that the box length search algorithm converges at least two orders of magnitude more quickly than the variable box length Monte Carlo method. Although the box length search algorithm is not ergodic, it successfully finds the box length that minimizes the free energy of the system. We verify this by examining the free energy as determined by the Monte Carlo simulation.

γ-ray and neutron diffraction studies of CoF2: magnetostriction, electron density and magnetic moments

Accurate structure factors up to sin theta/lambda = 1.6 A(-1) have been measured with 316.5 keV gamma-rays from CoF(2), both at room temperature and in the antiferromagnetic state at 10 K. The same crystal was used to collect extended time-of-flight neutron diffraction data in the two magnetic states, which allowed an accurate determination of the fluorine positional parameter. For room temperature, the standard structural parameters are reported. At 10 K, a complete charge-density study has been carried out. The total number of 3d electrons on Co is found to be 6.95 (3). The experimental populations of the d orbitals agree with expectation from crystal field theory. The fluorine valence region exhibits a strong dipolar deformation. Electronic properties at the bond critical points and integrated atomic properties are derived from the static model electron density, revealing the Co-F interactions as purely ionic. On magnetic ordering, a shift of the fluorine ions of 1.5 (4) x 10(-3) A is found which confirms a prediction from theory of optical birefringence. The effect of magnetostriction on the distortion of the ligand coordination octahedra is compared for the late members of the 3d transition-metal difluorides. From neutron powder diffraction, an ordered magnetic moment of 2.60 (4) mu(B) per cobalt ion is found. Despite the strong deviation from the ideal spin value of 3 mu(B), there is still an appreciable orbital contribution to the local magnetic moment.

REMGa3Ge and RE3Ni3Ga8Ge3 (M = Ni, Co; RE = Rare-Earth Element): New Intermetallics Synthesized in Liquid Gallium. X-ray, Electron, and Neutron Structure Determination and Magnetism

New quaternary intermetallic phases REMGa(3)Ge (1) (RE = Y, Sm, Tb, Gd, Er, Tm; M = Ni, Co) and RE(3)Ni(3)Ga(8)Ge(3) (2) (RE = Sm, Gd) were obtained from exploratory reactions involving rare-earth elements (RE), transition metal (M), Ge, and excess liquid Ga the reactive solvent. The crystal structures were solved with single-crystal X-ray and electron diffraction. The crystals of 1 and 2 are tetragonal. Single-crystal X-ray data: YNiGa(3)Ge, a = 4.1748(10) A, c = 23.710(8) A, V = 413.24(2) A(3), I4/mmm, Z = 4; Gd(3)Ni(3)Ga(8)Ge(3), a = 4.1809(18) A, c = 17.035(11) A, V = 297.8(3) A(3), P4/mmm, Z = 1. Both compounds feature square nets of Ga atoms. The distribution of Ga and Ge atoms in the REMGa(3)Ge was determined with neutron diffraction. The neutron experiments revealed that in 1 the Ge atoms are specifically located at the 4e crystallographic site, while Ga atoms are at 4d and 8g. The crystal structures of these compounds are related and could be derived from the consecutive stacking of disordered [MGa](2) puckered layers, monatomic RE-Ge planes and [MGa(4)Ge(2)] slabs. Complex superstructures with modulations occurring in the ab-plane and believed to be associated with the square nets of Ga atoms were found by electron diffraction. The magnetic measurements show antiferromagnetic ordering of the moments located on the RE atoms at low temperature, and Curie-Weiss behavior at higher temperatures with the values of mu(eff) close to those expected for RE(3+) free ions.

Isolation of the new cubic phases RE4FeGa(12-x)Ge(x) (RE = Sm, Tb; x = 2.5) from molten gallium: single-crystal neutron diffraction study of the Ga/Ge distribution

The compounds RE4FeGa(12-x)Ge(x) (RE = Sm, Tb) were discovered in reactions employing molten Ga as a solvent at 850 degrees C. However, the isostructural Y4FeGa(12-x)Ge(x) was prepared from a direct combination reaction. The crystal structure is cubic with space group Imm, Z = 2, and a = 8.657(4) A and 8.5620(9) A for the Sm and Tb analogues, respectively. Structure refinement based on full-matrix least squares on F(o)2 resulted in R1 = 1.47% and wR2 = 4.13% [I > 2(I)] for RE = Sm and R1 = 2.29% and wR2 = 7.12% [I > 2(I)] for RE = Tb. The compounds crystallize in the U4Re7Si6 structure type, where the RE atoms are located on 8c (1/4, 1/4, 1/4) sites and the Fe atoms on 2a (0, 0, 0) sites. The distribution of Ga and Ge in the structure, investigated with single-crystal neutron diffraction on the Tb analogue, revealed that these atoms are disordered over the 12d (1/4, 0, 1/2) and 12e (x, 0, 0) sites. The amount of Ga/Ge occupying the 12d and 12e sites refined to 89(4)/11 and 70(4)/30%, respectively. Transport property measurements indicate that these compounds are metallic conductors. Magnetic susceptibility measurements and Mössbauer spectroscopy performed on the Tb analogue show a nonmagnetic state for Fe, while the Tb atoms carry a magnetic moment corresponding to a mu(eff) of 9.25 mu(B).

Diastereoselective cycloreductions and cycloadditions catalyzed by Co(dpm)(2)-silane (dpm = 2,2,6,6-tetramethylheptane-3,5-dionate): mechanism and partitioning of hydrometallative versus anion radical pathways

In the presence of phenylsilane and 5 mol % cobalt(II) bis(2,2,6,6-tetramethylheptane-3,5-dionate), aryl-substituted monoenone monoaldehydes and bis(enones) undergo reductive cyclization to afford syn-aldol and anti-Michael products, respectively. For both aldol and Michael cycloreductions, five- and six-membered ring formation occurs in good yield with high levels of diastereoselectivity. Cycloreduction of monoenone monoaldehyde 1a in the presence of d(3)-phenylsilane reveals incorporation of a single deuterium at the enone beta-position as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization. The deuterated product was characterized by single-crystal neutron diffraction analysis. For bis(enone) substrates, modulation of the silane source enables partitioning of the competitive Michael cycloreduction and [2 + 2] cycloaddition manifolds. A study of para-substituted acetophenone-derived bis(enones) reveals that substrate electronic features also direct partitioning of cycloreduction and cycloaddition manifolds. Further mechanistic insight is obtained through examination of the effects of enone geometry on product stereochemistry and electrochemical studies involving cathodic reduction of bis(enone) substrates. The collective experiments reveal competitive enone reduction pathways. Enone hydrometalation produces metallo-enolates en route to aldol and Michael cycloreduction products, that is, products derived from coupling at the alpha-position of the enone. Electron-transfer-mediated enone reduction produces metallo-oxy-pi-allyls en route to [2 + 2] cycloadducts and, under Ni catalysis, homoaldol cycloreduction products, that is, products derived from coupling at the beta-position of the enone. The convergent outcome of the metal-catalyzed and electrochemically induced transformations suggests the proposed oxy-pi-allyl intermediates embody character consistent with the mesomeric metal-complexed anion radicals.

How to make a major shift in a redox potential: ligand control of the oxidation state of dimolybdenum units

A compound reported earlier (Polyhedron 1989, 8, 2339) as (Bu(n)()(4)N)(2)H(2)[Mo(2)[Mo(CO)(4)(PhPO(2))(2)](2)] has been reexamined. We find that the hydrogen atoms in this formula are not present. Therefore, the complex must be considered as having a central triply bonded Mo(2)(6+) unit, instead of a quadruply bonded Mo(2)(4+) unit. Our conclusion is based on a variety of experimental evidence, including X-ray crystal structures of four crystal forms, as well as the neutron crystal structure of one. This explains the relatively long Mo-Mo bond lengths found in the range 2.1874(7)-2.2225(7) A and the absence of a delta --> delta transition in the visible spectrum. From electrochemistry we also find that the diphosphonate ligand has such an exceptional ability to stabilize higher oxidation states that even common solvents such as CH(2)Cl(2) and C(2)H(5)OH readily oxidize the Mo(2)(4+) unit that is introduced from the Mo(2)(O(2)CCH(3))(4) or [Mo(2)(O(2)CCH(3))(2)(NCCH(3))(6)](BF(4))(2) employed in the preparation. The only chemically reversible wave at E(1/2) = -1.54 V vs Ag/AgCl corresponds to the reduction process Mo(2)(6+) --> Mo(2)(5+).