Average Bond Angles Research Articles

N,N′-Dibenzyl-N′′-(2,6-difluorobenzoyl)-N,N′-dimethylphosphoric triamide

The phosphoryl and carbonyl groups in the title compound, C23H24F2N3O2P, are anti to each other. The P atom is in a tetra­hedral coordination environment and the environment of each N atom is essentially planar, the average bond angles at the two N atoms being 119.9 and 119.1°. The H atom of the C(=O)NHP(=O) group is involved in an inter­molecular –P=O⋯H–N– hydrogen bond, forming centrosymmetric dimers.

Structure transition of multiferroic hexagonal TmMnO3 compound under high pressure

The high-pressure-induced structure transition in multiferroic hexagonal TmMnO3 (h-TmMnO3) has been investigated using an in situ angle-dispersive synchrotron X-ray diffraction technique in a diamond anvil cell. The experimental results show that the phase transition from ambient hexagonal to orthorhombic structure with space group Pbnm begins around 10.2 GPa. The Rietveld refinement method was used to determine the lattice parameters and lattice compressibility of the h-TmMnO3 compound from 0.8 to 28.6 GPa. The pressure evolution of average bond distances and bond angles between the Mn and O atoms in the ab-plane was obtained. The magnetic properties under different pressures as well as their effect on multiferroic properties are discussed using extrapolations from the empirical relation of magnetic order versus rare-earth ionic radius.

In vitro assessment of cytotoxicity, anti-inflammatory, antifungal properties and crystal structures of metallacyclic palladium(II) complexes

Abstract Metallacyclic palladium(II) complexes [Pd(L)(R3P)Cl], L = TIQDTC (1,2,3,4-tetrahydroisoquinolinedithiocarbamate), 4MpipDTC (4-methylpipradinedithiocarbamate), MPizDTC (N-methylpiperazinedithiocarbamate), R3P = Ph3P, (o-tolyl)3P, Ph2ClP, were synthesized in a 1:1 molar metal–ligand ratio. These complexes were characterized by elemental analyses, FT-IR, multinuclear (1H, 13C and 31P) NMR. The X-ray crystal structures of [Pd(TIQDTC)(Ph3P)Cl] and [Pd(TIQDTC)((o-tolyl)3P)Cl] show a slightly distorted square planar environment around the Pd(II) ion with S–Pd–S and P–Pd–Cl average bond angles of 74.51 and 92.41, respectively. These complexes were screened for cytotoxic, antifungal, anti-inflammatory and antibacterial activity. Some complexes exhibit a significant activity against fungi.

Molecular and crystalline structures of three (S)-4-alkoxycarbonyl-2-azetidinones containing long alkyl side chains from synchrotron X-ray powder diffraction data

The (S)-4-alkoxo-2-azetidinecarboxylic acids are optically active beta-lactam derivatives of aspartic acid, which are used as precursors of carbapenem-type antibiotics and poly-beta-aspartates. The crystal structures of three (S)-4-alkoxo-2-azetidinecarboxylic acids with alkyl chains with 10, 12 and 16 C atoms were solved using parallel tempering and refined against the X-ray powder diffraction data using the Rietveld method. The azetidinone rings in the three compounds display a pattern of asymmetrical bond distances and an almost planar conformation; these characteristics are compared with periodic solid-state, gas-phase density-functional theory (DFT) calculations and MOGUL average bond distances and angles from the CSD. The compounds pack along [001] as corrugated sheets separated by approximately 4.40 A and connected by hydrogen bonds of the type N-H...O.

Structure of High Alumina Content Al2O3−SiO2 Composition Glasses

The structure of binary aluminosilicate glasses containing 60-67 mol % Al2O3 were investigated using high-resolution 27Al NMR and X-ray and neutron diffraction. The glasses were made by aerodynamic levitation of molten oxides. The 67% alumina composition required a cooling rate of approximately 1600 degrees C s(1-) to form glass from submillimeter sized samples. NMR results show that the glasses contain aluminum in 4-, 5-, and 6-fold coordination in the approximate ratio 4:5:1. The average Al coordination increases from 4.57 to 4.73 as the fraction of octahedral Al increases with alumina content. The diffraction results on the 67% composition are consistent with a disordered Al framework with Al ions in a range of coordination environments that are substantially different from those found in the equilibrium crystalline phases. Analysis of the neutron and X-ray structure factors yields an average bond angle of 125 +/- 4 degrees between an Al ion and the adjoining cation via a bridging oxygen. We propose that the structure of the glass is a "transition state" between the alumina-rich liquid and the equilibrium mullite phase that are dominated by 4- and 6-coordinated aluminum ions, respectively.

Structural, electrical, magnetic and microstructural behavior of theLa0.7Sr0.3−xHgxMnO3 (0≤x≤0.3) system

We report a detailed investigation on the polycrystalline samples of the colossal magnetoresistancesystem La0.7Sr0.3−xHgxMnO3 (0≤x≤0.3). The emphasis is put on identifying the role of Hg in the perovskite matrix. We have usedthe neutron diffraction technique for structural analysis. The issues pertaining tocomposition-dependent variations in parameters like average ionic radii, tolerance factor,variance, bond length, bond angle, lattice parameters and cationic vacancies have beenaddressed. The low temperature resistivity measurement and subsequent curve fitting areused to explore the contributions from various disorders and interactions. The samples withx = 0.3 composition showed distinct electrical behavior and magnetoresistance up to 46% aroundT = 198 K. The low temperature neutron diffraction data revealed ferromagnetic character of thesamples. All the samples displayed similar microstructures. The variations in the electricaland magnetic properties are attributed to origination from cationic disorders.

Second-harmonic and reflectance-anisotropy spectroscopy of vicinalSi(001)∕SiO2interfaces: Experiment and simplified microscopic model

We directly compare experimental second-harmonic generation (SHG) spectra and reflectance-anisotropy spectra (RAS) of native-oxidized vicinal Si(001) interfaces with off-cut angles $\ensuremath{\zeta}=0\ifmmode^\circ\else\textdegree\fi{}$, 4\ifmmode^\circ\else\textdegree\fi{}, 6\ifmmode^\circ\else\textdegree\fi{}, 8\ifmmode^\circ\else\textdegree\fi{}, and 10\ifmmode^\circ\else\textdegree\fi{} from (001) toward [110]. We fit the measured azimuthal rotational-dependence of $p$-in/$p$-out SHG at two-photon energies $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}l\ensuremath{\hbar}\ensuremath{\omega}l3.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ including the ${E}_{1}$ critical point resonance using a simplified bond hyperpolarizability model. The model includes dipolar contributions from tetrahedrally coordinated interfacial bonds as well as quadrupolar contributions from the Si bulk. The fit yields complex axial hyperpolarizability spectra ${\ensuremath{\beta}}_{\ensuremath{\Vert},\ensuremath{\Vert},\ensuremath{\Vert}}(2\ensuremath{\omega})$ and averaged bond angles of the interfacial back-edge, terrace-edge and step-edge bonds, and bulk quadrupolar hyperpolarizability ${\ensuremath{\delta}}_{\ensuremath{\Vert},\ensuremath{\Vert},\ensuremath{\Vert}}(2\ensuremath{\omega})$. The fitted microscopic model accurately reproduces measured $s$-in/$p$-out SHG spectra. We then used a Miller's rule approximation to generate axial linear polarizability spectra ${\ensuremath{\alpha}}_{\ensuremath{\Vert}}(\ensuremath{\omega})$ from the fitted ${\ensuremath{\beta}}_{\ensuremath{\Vert},\ensuremath{\Vert},\ensuremath{\Vert}}(2\ensuremath{\omega})$. The measured RAS was satisfactorily reproduced within the same photon energy range. Including the bulk quadrupole contribution to SHG was crucial to accurately retrieving the RAS and to achieving fitted real and imaginary $\ensuremath{\beta}(2\ensuremath{\omega})$ spectra consistent with a nonlinear Kramers-Kronig relation. The results demonstrate the possibility of formulating a common microscopic model of SHG and RAS responses of complicated interfaces.

Synthesis and Redox Properties of Crowded Triarylphosphines Possessing Ferrocenyl Groups

Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6-diisopropylphenyl)(n)(2,4,6-triisopropylphenyl)(3-n)P (5a, n = 1; 5b, n = 2; 5c, n = 3)] were synthesized by the reaction of the corresponding arylcopper(I) reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by 1H, 13C, and 31P NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded 31P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are 110.8 degrees and 1.854 A, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers. The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at 293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the ferrocenyl groups in the periphery of the molecules are oxidized at first.

Crystal structure of bis(μ-thiocyanato-N,S)bis[aqua(3-methylbenzo- 15-crown-5)sodium]bis(thiocyanato-S)platinum(II), [Na(C15H22O5)(H2O)]2[Pt(SCN)4

Article Crystal structure of bis(μ-thiocyanato-N,S)bis[aqua(3-methylbenzo- 15-crown-5)sodium]bis(thiocyanato-S)platinum(II), [Na(C15H22O5)(H2O)]2[Pt(SCN)4] was published on April 1, 2005 in the journal Zeitschrift für Kristallographie - New Crystal Structures (volume 220, issue 1-4).

Anharmonic, Temperature, and Matrix Effects on the Molecular Structure and Vibrational Frequencies of Lanthanide Trihalides LnX3 (Ln = La, Lu; X = F, Cl)

MP2 and CCSD(T) ab initio calculations have been carried out to elucidate geometrical structure and vibrational frequencies of representative lanthanide trihalides LnX(3) (Ln = La, Lu; X = F, Cl) explicitly including temperature, anharmonic, inert-gas matrix, and spin-orbit effects. The results have been compared with gas-phase electron diffraction, gas-phase IR measurements, and IR spectra of molecules trapped in inert-gas matrices. On the Born-Oppenheimer surface LaCl(3), LuF(3), and LuCl(3) adopt trigonal planar (D(3)(h)()) geometry while LaF(3) assumes a slightly pyramidal (C(3)(v)()) structure. Because of normal-mode anharmonicities, the resulting thermal average bond angles are considerably lower than the equilibrium ones, while vibrationally averaged bond lengths are predicted to be longer. The inert-gas matrix effects, modeled by the coordination of two inert-gas molecules LnX(3).IG(2) (IG = Ne, Ar, Xe, and N(2)), are substantial and strongly depend on the polarizability of coordinating particles. Coordinating inert-gas units always favor the tendency of LnX(3) molecules to adopt planar structure and induce noticeable frequency shifts.

Electronic Structure and Chain‐Length Effects in Diplatinum Polyynediyl Complexes trans,trans‐[(X)(R3P)2Pt(CC)nPt(PR3)2(X)]: A Computational Investigation

Structure and bonding in the title complexes are studied using model compounds trans,trans-[(C6H5)(H3P)2Pt(C triple bond C)(n)Pt(PH3)2(C6H5)] (PtCxPt; x = 2n = 4-26) at the B3LYP/LACVP* level of density functional theory. Conformations in which the platinum square planes are parallel are very slightly more stable than those in which they are perpendicular (DeltaE = 0.12 kcal mol(-1) for PtC8Pt). As the carbon-chain length increases, progressively longer C triple bond C triple bonds and shorter triple bond C-C triple bond single bonds are found. Whereas the triple bonds in HCxH become longer (and the single bonds shorter) as the interior of the chain is approached, the PtC triple bond C triple bonds in PtCxPt are longer than the neighboring triple bond. Also, the Pt-C bonds are shorter at longer chain lengths, but not the H-C bonds. Accordingly, natural bond orbital charge distributions show that the platinum atoms become more positively charged, and the carbon chain more negatively charged, as the chain is lengthened. Furthermore, the negative charge is localized at the two terminal C triple bond C atoms, elongating this triple bond. Charge decomposition analyses show no significant d-pi* backbonding. The HOMOs of PtCxPt can be viewed as antibonding combinations of the highest occupied pi orbital of the sp-carbon chain and filled in-plane platinum d orbitals. The platinum character is roughly proportional to the Pt/Cx/Pt composition (e.g., x = 4, 31 %; x = 20, 6 %). The HOMO and LUMO energies monotonically decrease with chain length, the latter somewhat more rapidly so that the HOMO-LUMO gap also decreases. In contrast, the HOMO energies of HCxH increase with chain length; the origin of this dichotomy is analyzed. The electronic spectra of PtC4Pt to PtC10Pt are simulated. These consist of two pi-pi* bands that redshift with increasing chain length and are closely paralleled by real systems. A finite HOMO-LUMO gap is predicted for PtCinfinityPt. The structures of PtCxPt are not strictly linear (average bond angles 179.7 degrees -178.8 degrees ), and the carbon chains give low-frequency fundamental vibrations (x = 4, 146 cm(-1); x = 26, 4 cm(-1)). When the bond angles in PtC12Pt are constrained to 174 degrees in a bow conformation, similar to a crystal structure, the energy increase is only 2 kcal mol(-1). The above conclusions should extrapolate to (C triple bond C)(n) systems with other metal endgroups.

A low-temperature redetermination of the monoclinic β-form of cyclooctaselenium

Monoclinic β-cyclo­octaselenium, Se8, crystallizes as discrete eight-membered ring mol­ecules. The low-temperature (150 K) crystal structure is composed of cyclic crown-shaped mol­ecules, similar to those of α- and γ-Se8. The average Se—Se bond length is 2.308 (8) A, the average bond angle is 105.7 (8)° and the average torsion angle is 101 (2)°. In contrast with α-Se8, the structure of β-Se8 does not show elongation of the ring at low temperatures.

Differences and limits in estimates of persistence length for semi-flexible macromolecules

Persistence length of (bio)macromolecules plays an increasingly important role in macromolecular science, especially in emerging fields like macromolecular biophysics. It is shown that using established approximate relations for the estimation of persistence length for semi-flexible macromolecules leads to differences between different methods or even to a modified behavior, especially at high chain stiffness. The approximate estimate obtained from the average bond angle ( B) performs better than the estimate from the decay of bond orientation correlation ( C) in chain but close to the coil-to-rod transition starts to fail. This approximate estimate is close to the estimate from the definition of persistence ( D) (with minimum approximations) not only in the theta state or the random coil but also in the good solvent. The value of persistence from the decay of bond orientation correlation along the macromolecule is underestimated when compared to the exact value. The persistence obtained from the worm-like chain model ( W) is close to the exact result in the coil regime (also in the good solvent) but near at and above the transition produces the largest values of all approximations. In systems with interactions this model starts to underestimate the persistence on decreasing temperature. We advocate using the estimate ( D) of persistence length generally since it avoids limitations mentioned above and at the same time does not pose additional requirements on the evaluation itself.

Radial distribution function analyses of amorphous carbon thin films containing various levels of silicon and hydrogen

Short-range order in amorphous carbon thin films containing silicon and hydrogen (Si-aC:H) was characterized using radial distribution functions (RDFs). RDFs describe the average number of neighbors located a distance r away from an arbitrary atom in the sample. Two methods in the transmission electron microscope (TEM) were used in this study to obtain RDFs: energy-filtered convergent-beam electron diffraction (EFCBED) and extended electron energy-loss fine structure (EXELFS) analyses. Four Si-aC:H film samples were prepared in cross section for TEM using a focused ion beam system. Both EFCBED and EXELFS techniques were used to study the same samples. RDF interpretation included estimation and comparison of near-neighbor distances, average bond angles, and coordination numbers as a function of sample composition. Key RDF characteristics were correlated to thin film material properties.

Hypothetical Uninodal Zeolite Structures: Comparison of AlPO4 and SiO2 Compositions Using Computer Simulation.

A total of 126 hypothetical zeolite structures have been modeled in the composition AlPO4. The framework topologies are those derived by tiling theory and comprise uninodal structures based on simple or quasi-simple tilings. Atomistic calculations have been used to optimize and evaluate each structure, and compare it to that of the SiO2 polymorph possessing the same topology. Characteristic properties including relative lattice energies, framework density, coordination sequences, accessible volumes, surface areas, average bond distances and angles, and the degree of distortion of the TO4 tetrahedra were calculated. The AlPOs are found to be more stable relative to berlinite than the zeolites are relative to quartz. The AlPO and SiO2 polymorphs show the same relationships between framework density and lattice energies, as well as between framework density and accessible volume, enabling us to predict whether individual hypothetical AlPO/silica polymorphs can be expected to be thermodynamically feasible. We...

Hypothetical Uninodal Zeolite Structures: Comparison of AlPO4 and SiO2 Compositions Using Computer Simulation

A total of 126 hypothetical zeolite structures have been modeled in the composition AlPO4. The framework topologies are those derived by tiling theory and comprise uninodal structures based on simple or quasi-simple tilings. Atomistic calculations have been used to optimize and evaluate each structure, and compare it to that of the SiO2 polymorph possessing the same topology. Characteristic properties including relative lattice energies, framework density, coordination sequences, accessible volumes, surface areas, average bond distances and angles, and the degree of distortion of the TO4 tetrahedra were calculated. The AlPOs are found to be more stable relative to berlinite than the zeolites are relative to quartz. The AlPO and SiO2 polymorphs show the same relationships between framework density and lattice energies, as well as between framework density and accessible volume, enabling us to predict whether individual hypothetical AlPO/silica polymorphs can be expected to be thermodynamically feasible. We...

Structural investigations of the Se–Ag–I system

Glasses from the Se–Ag–I system have been investigated. Structural information is gathered based on results collected by a combination of several types of diffraction measurements. First coordination sphere at r=2.3–2.5 Å determined by radial distribution function calculations can be assumed to be composed by Se–Ag and Ag–I correlations in the network units and Se–Se correlation in the Se cluster units. The interatomic distances and average bond angles decrease with the introduction of additives to selenium. The Raman spectra reveal that the introduction of silver and iodine, in particular, brings about a shift of the Se chain stretching mode towards lower wave numbers (251–236 cm−1). Apparently, the newly formed structure after the introduction of additives into selenium is pretty compact and is becoming more covalent with a stronger interchain interaction. Depending on the concentration of additives the breathing modes of Ag–Se and Ag–I bonds also can be seen.

Internal Reorganization Energy Contributed by Torsional Motion in Electron Transfer Reaction between Biphenyl and Biphenyl Anion Radical

AbstractConcerning the theoretical estimation of internal reorganization energy contributed by the tortional motion between biphenyl and biphenyl anion radical, direct calculation of self‐exchange electron transfer reaction was investigated. With the introduction of a proper average bond length and angle parameters (bond Bp), a multiple step relaxation Nelson method was developed to deal with the torsional reorganization energy. Based on the above model, an estimation of pure torsional reorganization energy λt.p with an approximation of λt.1 was achieved. The results of 0.140 and 0.125 eV of torsional reorganization energy for a cross‐reaction at the levels of 4‐31G and HP/DZP, respectively, are in good agreement with the value of 0.13 eV obtained by Miller et al. from the rate measurements. This implies the efficiency and validity of our method to estimate the reorganization energy contributed by pure torsional motion of Bp.

Elasticity through Nanoscale Distortions in Periodic Surfactant-Templated Porous Silica under High Pressure

High-pressure infrared absorption spectroscopy is used to examine changes in local bonding upon hydrostatic compression in both ordered surfactant-templated mesoporous silica and sintered sol−gel silica with a goal of connecting atomic scale structural changes with variations in nanoscale periodicity. High-pressure IR absorption spectra are analyzed on the basis of a noncentral force model. It is found that the intertetrahedral bond angle and its distribution width in both the dense and the mesoporous silica decrease at elevated pressure up to 4 GPa. With increasing pressure above this value, decreases in the average bond angle and distribution width cease in the mesoporous silica, while they continue in the bulk material. The results suggest that in the mesoporous silica the nanometer length scale of the silica framework makes it energetically unfavorable to form high-density atomic scale structures at higher pressures (>4 GPa). Instead, further compression of the mesoporous silica takes place by distortion of the periodic pore structures on the nanometer scale (deformation of pores). Surprisingly, upon the release of pressure, structural changes on both the nanometer and atomic length scales are reversible. The results suggest that reversible nanometer scale distortions in periodic porous materials can replace the irreversible atomic scale distortions observed in bulk amorphous silica.

The 31P NMR Chemical Shielding Tensors in Quaternary Metal Thiophosphates, NaNb2PS10, AgNb2PS10, Au0.5Nb2PS10, and NaSmP2S6

For quaternary metal thiophosphates, NaNb2PS10, AgNb2PS10, Au0.5Nb2PS10, and NaSmP2S6, the principal elements of the 31P NMR chemical shielding tensor (δ11, δ22, and δ33) are acquired from the nonlinear least-squares fitting the static powder patterns and MAS spectra. Isotropic chemical shift (δiso) and chemical shielding anisotropy (Δδ) have been employed to correlate with the X-ray crystallographic data, the average (P−S) bond length and the average (S−P−S) bond angle deviation from 109.5°, respectively, at the tetrahedrally coordinated phosphorus atoms. On the basis of these successful correlations, the charge transfer from Na, Ag, and Au metals to the (P−S) ligand is discussed and complemented by the band-structure calculation.