Off-center Sites Research Articles

ChemInform Abstract: From Homoleptic to Heteroleptic Double Stranded Copper(I) Helicates: The Role of Self-Recognition in Self-Assembly Processes.

The ligands 2,9-bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl]-1,10-phenanthroline (6), 6' ',6' -bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl)]-2' ',2' -bipyridine (2), 5,5'-bis[(6-methyl-2,2'-bipyridin-6'yl)methyleneoxymethylenyl]-2, 2'-bithiophene (7), and 6,6'-bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl]-2,2'-biphenyl (8) and their respective homo- and heteroleptic double-stranded copper(I) complexes were prepared and characterized in order to estimate the importance of self-recognition in the self-assembly processes of double-stranded copper complexes. The homoleptic double-stranded copper complexes of 2, 6, 7, and 8 were characterized by NMR, FAB-MS, and electrochemistry. It was found that 6 and 2 each form a single double-stranded helicate having the structure of [(L)(2)Cu(3)](3+) (L = 2 or 6), 7 forms two double-stranded [(7)(2)Cu(3)](3+) complexes, and 8 results in a mixture of at least two [(8)(2)Cu(2)](2+) complexes. The potential shift, DeltaE degrees (,) of the Cu(+)/Cu(2+) redox process of these complexes reflects the binding affinity of the different binding sites to the copper cation. The electrochemical data show that the central units have a higher affinity to Cu(+) as compared to the off-center binding sites. NMR was used to determine the actual complex composition obtained from different mixtures of 2, 6, or 7 with Cu(+). Interestingly, we have found that, although 6, 2, and 7 each form homoleptic double-stranded complexes, no heteroleptic double-stranded copper complexes were formed from the mixtures of 7 with either 6 or 2. However, when mixtures of 6 and 2 are used, helicate distributions seem to follow simple statistics. These results are discussed in terms of the relative importance of self-recognition in the self-assembly of double-stranded helicates.

Aqueous alkali halide solutions: can osmotic coefficients be explained on the basis of the ionic sizes alone?

We use the AMSA, associative mean spherical theory of associative fluids, to study ion-ion interactions in explicit water. We model water molecules as hard spheres with four off-center square-well sites and ions as charged hard spheres with sticky sites that bind to water molecules or other ions. We consider alkali halide salts. The choice of model parameters is based on two premises: (i) The strength of the interaction between a monovalent ion and a water molecule is inversely proportional to the ionic (crystal) diameter sigma(i). Smaller ions bind to water more strongly than larger ions do, taking into account the asymmetry of the cation-water and anion-water interactions. (ii) The number of contacts an ion can make is proportional to sigma2(i). In short, small ions bind waters strongly, but only a few of them. Large ions bind waters weakly, but many of them. When both a monovalent cation and anion are large, it yields a small osmotic coefficient of the salt, since the water molecules avoid the space in between large ions. On the other hand, salts formed from one small and one large ion remain hydrated and their osmotic coefficient is high. The osmotic coefficients, calculated using this model in combination with the integral equation theory developed for associative fluids, follow the experimental trends, including the unusual behavior of caesium salts.

Modeling the Phase Behavior of H2S + n-Alkane Binary Mixtures Using the SAFT-VR+D Approach

A statistical associating fluid theory for potential of variable range has been recently developed to model dipolar fluids (SAFT-VR+D) [Zhao and McCabe, J. Chem. Phys. 2006, 125, 104504]. The SAFT-VR+D equation explicitly accounts for dipolar interactions and their effect on the thermodynamics and structure of a fluid by using the generalized mean spherical approximation (GMSA) to describe a reference fluid of dipolar square-well segments. In this work, we apply the SAFT-VR+D approach to real mixtures of dipolar fluids. In particular, we examine the high-pressure phase diagram of hydrogen sulfide+n-alkane binary mixtures. Hydrogen sulfide is modeled as an associating spherical molecule with four off-center sites to mimic hydrogen bonding and an embedded dipole moment (micro) to describe the polarity of H2S. The n-alkane molecules are modeled as spherical segments tangentially bonded together to form chains of length m, as in the original SAFT-VR approach. By using simple Lorentz-Berthelot combining rules, the theoretical predictions from the SAFT-VR+D equation are found to be in excellent overall agreement with experimental data. In particular, the theory is able to accurately describe the different types of phase behavior observed for these mixtures as the molecular weight of the alkane is varied: type III phase behavior, according to the scheme of classification by Scott and Konynenburg, for the H2S+methane system, type IIA (with the presence of azeotropy) for the H2S+ethane and+propane mixtures; and type I phase behavior for mixtures of H2S and longer n-alkanes up to n-decane. The theory is also able to predict in a qualitative manner the solubility of hydrogen sulfide in heavy n-alkanes.

A transferable coarse-grained model for hydrogen-bonding liquids

We present here a recent development of a generalized coarse-grained model for use in molecular simulations. In this model, interactions between coarse-grained particles consist of both van der Waals and explicit electrostatic components. As a result, the coarse-grained model offers the transferability that is lacked by most current effective-potential based approaches. The previous center-of-mass framework (P. A. Golubkov and P. Ren, J. Chem. Phys., 2006, 125, 64103) is generalized here to include arbitrary off-center interaction sites for both Gay-Berne and multipoles. The new model has been applied to molecular dynamic simulations of neat methanol liquid. By placing a single point multipole at the oxygen atom rather than at the center of mass of methanol, there is a significant improvement in the ability to capture hydrogen-bonding. The critical issue of transferability of the coarse-grained model is verified on methanol-water mixtures, using parameters derived from neat liquids without any modification. The mixture density and internal energy from coarse-grained molecular dynamics simulations show good agreement with experimental measurements, on a par with what has been obtained from more detailed atomic models. By mapping the dynamics trajectory from the coarse-grained simulation into the all-atom counterpart, we are able to investigate atomic-level structure and interaction. Atomic radial distribution functions of neat methanol, neat water and mixtures compare favorably to experimental measurements. Furthermore, hydrogen-bonded 6- and 7-molecule chains of water and methanol observed in the mixture are in agreement with previous atomic simulations.

Transitions between P2 1, [formula omitted], and P6 322 modifications of SrAl 2O 4 by in situ high-temperature X-ray and neutron diffraction

The results of in situ high-temperature X-ray and neutron powder diffraction experiments reconcile inconsistencies in previous reports on the symmetry of high-temperature phases of SrAl 2O 4. The material undergoes two reversible phase transitions P 2 1 ↔ P 6 3 ( 3 A ) and P 6 3 ( 3 A ) ↔ P 6 3 22 at ∼680 and ∼860 °C, respectively, and the latter one is experimentally observed and characterized for the first time. The higher symmetry above the P 6 3 ( 3 A ) ↔ P 6 3 22 transition is gained by disordering off-center split site of oxygen atoms around trigonal axis rather than by unbending Al–O–Al angle to the ideal value 180°. The analysis of the literature suggests that it is a common feature of the P6 322 phases of stuffed tridymites.

Geometric and electronic structures of non-IPR metallofullerene La@C 72

The geometric and electronic structures of non-IPR metallofullerenes La@C 72 were studied using density functional theory. The geometric optimization show that among four possible configurations for La@C 72, the most favorable endohedral position is off-center site along the C 2 axis in α plane pointing to the 5, 5 bond, the fusion of two pentagons. The calculations of electronic properties show that the spin densities are distributed onto all the carbons of C 72. And La ion has little distribution to molecular orbitals.

Modeling solution of flexible polyelectrolyte in explicit solvent

Mixture of flexible polyions with 120 monomer units, equivalent number of oppositely charged counterions and solvent molecules, represented by hard spheres with four square-well off-center sites, was studied by an extension of the product-reactant Ornstein–Zernike theory. Polyions were represented as freely jointed chains of charged hard spheres, and all the species were embedded in continuous dielectric. The calculations were performed varying the concentration of monomer units and strength of interaction between various species. The model is able to mimic rich experimental behavior of polyelectrolyte solutions.

Coarse-Grained Molecular Dynamics Modeling of Associating Fluids: Thermodynamics, Liquid Structure, and Dynamics in the Limit of Zero Association Strength

A continuous coarse-grained potential model for associating fluids, consisting of an off-center specific site bonded with a harmonic potential to a center particle, has been developed and used in canonical molecular dynamics simulations. The thermodynamic, structural, and dynamic properties of the limiting nonassociating reference coarse-grained fluid are investigated as functions of the mass distribution and bond strength between center and site particles. It is theoretically shown and confirmed by simulation that in this limit variations in these potential parameters do not alter the equation of state of the reference coarse-grained fluid but that they have profound influences on both the translational and the rotational dynamics. From the simulation results we arrive at some guidelines that should be kept in mind in the selection of appropriate values for the model parameters. This work provides the precursory knowledge for the study of coarse-grained associating fluids using the conventional molecular dynamics method.

Loading dynamics of optical trap and parametric excitation resonances of trapped atoms

We study the loading dynamics of an optical dipole trap and an optical lattice both experimentally and theoretically. A simple power dependence for the number of trapped atoms (N∝P3∕2) is revealed in both cases. We then study the parametric excitation of the trapped atoms. High order parametric resonance is observed close to but lower than 4ν in a CO2 laser optical lattice. The existence of the high harmonic and its shift toward lower frequency are attributed to the anharmonicity of the optical trapping potential. We investigate the discrepancy between experimentally measured optical lattice frequencies and the theoretical calculation for several published experiments and present corrected formula for trap frequency calculations. Using this corrected formula and accounting for the off-center lattice sites, we present a modified formula for calculating the optical lattice frequency and obtain better agreement between experimental measurement and theoretical calculation.

Rattling of Pr in Heavy-Fermion Superconductor PrOs4Sb12Revealed by Neutron Powder Diffraction

Neutron powder diffraction experiments were carried out on the heavy-fermion superconductor PrOs 4 Sb 12 down to 7.7 K in order to reveal the thermal displacement of Pr and investigate the possible existence of off-center potential minima for Pr in the Sb cage. Rietveld analysis with the structure in which Pr is located at the center of the Sb cage well reproduces the observed patterns. A large thermal displacement was obtained for Pr of U Pr =0.0344 Å 2 at 295.1 K, roughly corresponding to a mean thermal displacement of 0.18 Å. This value is 5 times larger than those of other constituents, U Os and U Sb . U Pr remains large, U Pr =0.0062 Å 2 , even at 7.7 K, which is twice as large as U Os and U Sb . Neither structural transition nor significant change of the Sb cage accompanied this change in U Pr . The large U Pr and its strong temperature dependence indicate the rattling of Pr under a shallow potential in the Sb cage. An equal distribution of Pr at 12 e sites ( x 0 0) was introduced to approximate the off-center potential minima. The R factors were not improved by the introduction of the off-center site.

Dynamic Disorder in Perovskites: Local Symmetry Breaking at the Sr Site in SrTiO3

Perovskites such as SrTiO3 and BaTiO3 have been considered to be classical displacive systems where the phase transitions are connected with a soft mode instability. Only recently a disorder component has been detected in addition to the soft mode. It has been shown [1–3] by quadrupole perturbed Ti (I = 5/2) and Ti (I = 7/2) NMR that in the high temperature cubic phases of BaTiO3 and SrTiO3 the Ti ions are dynamically disordered between 8 off-center sites so that the ideal Oh symmetry is locally broken. The electric field gradient (EFG) tensor is non-zero at the Ti sites whereas it should be zero by symmetry in the ideal perovskite lattice with the Ti ion in the center of the oxygen octahedron. The Ti off-center sites are located at the cubic body diagonals in agreement with the extended X-ray absorption fine structure (EXAFS) data [4, 5], the early X-ray work of Comes et al. [6], and the theoretical model of Chaves et al. [7] (Fig. 1). The off-centering of the Ti ions is accompanied by a local tetragonal distribution of the unit cell, so that fluctuating tetragonal nanodomains (+a, − a, + b, − b, + c, − c) are formed in the cubic phase (Fig. 2). The macroscopic cubic symmetry is the result of space and time averaging. Below the cubic-tetragonal transition long range correlations appear and the nanodomains transform into macroscopic 90◦ domains. ∗corresponding author

Tunneling states in [formula omitted

The clathrate Eu 8 Ga 16 Ge 30 is a cage compound with magnetic guest atom Eu. The RKKY interaction between Eu 2 + moments leads to the ferromagnetic ordering at low temperatures. The most interesting properties of the compound are associated with the off-center fourfold split sites of Eu atoms. The appropriate four-well potential leads to the four-level tunneling states (FLS). The influence of FLS on the thermal conductivity is studied and the experimental situation considered. Direct evidence for the FLS is obtained from the anomalous elastic properties.

Off-centerTimodel of barium titanate

It has recently been shown by NMR techniques that in the high-temperature cubic phase of ${\mathrm{BaTiO}}_{3}$ the $\mathrm{Ti}$ ions are not confined to the high-symmetry cubic sites, but rather occupy one of the eight off-center positions along the $[111]$ directions. The off-center $\mathrm{Ti}$ picture is in apparent contrast with most soft-mode-type theoretical descriptions of this classical perovskite ferroelectric. Here we apply the Girshberg-Yacoby off-center cation model of perovskite ferroelectrics assuming that the symmetrized occupation operators or ``pseudospins'' for the $\mathrm{Ti}$ off-center sites are linearly coupled to the normal coordinates for $\mathrm{TO}$ lattice vibrations. In the adiabatic limit, the coupling is eliminated by transforming to displaced phonon coordinates, and after excluding the self-interaction terms an effective $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{Ti}$ interaction is obtained. Using the Langevin equations of motion and the soft-spin formalism for the $\mathrm{Ti}$ pseudospin degrees of freedom with ${T}_{1u}$ symmetry, the dynamic response of the coupled system is derived. The results are shown to be in qualitative agreement with the experimental data of Vogt et al. [Phys. Rev. B, 26, 5904 (1982)] obtained by hyper-Raman scattering. The nature of the phase transition, which is of a mixed displacive and order-disorder type, is discussed.

Possible mechanism for glass-like thermal conductivities in crystals with off-center atoms

In the filled Ga/Ge clathrate, Eu and Sr are off-center in site 2 but Ba is on-center. All three filler atoms (Ba,Eu,Sr) have low temperature Einstein modes; yet only for the Eu and Sr systems is there a large dip in the thermal conductivity, attributed to the Einstein modes. No dip is observed for Ba. Here we argue that it is the off-center displacement that is crucial for understanding this unexplained difference in behavior. It enhances the coupling between the "rattler" motion and the lattice phonons for the Eu and Sr systems, and turns on/off another scattering mechanism (for 1K < T < 20K) produced by the presence/absence of off-center sites. The random occupation of different off-center sites produces a high density of symmetry-breaking defects which scatters phonons. It may also be important for improving our understanding of other glassy systems.

Ferroelectricity in SrTiO3 Ceramics Induced by Pr Doping

ABSTRACTFerroelectricity was induced in SrTiO3 by the Sr exchange for Pr ion in the A site of the perovskite ABO3-type structure. X-ray diffraction patterns show single phase crystalline structure in the SrxPr1-xTiO3 compound for x=0, 0.025, 0.050, 0.075 and 0.1 compositions. Rietveld refinement shows that the unit cell volume decreases with the increasing of Pr content as a consequence of the difference between Sr/Pr ionic radii. Furthermore, high-temperature differential thermoanalysis (DTA) displays a small anomaly at about 118 °C which is probably due to the Pr ion producing a distortion of the perovskite structure via an off-center site. This deformation in the lattice induces a measurable polar behavior of the solid solution. Dielectric permittivity (ε vs T) measurements display a well defined peak at about 238 °C. Furthermore, a well defined hysteresis loop at 30 °C with a remnant polarization that tends to decrease with increasing Pr concentration is observed. Both experimental results confirm the ferroelectric state induced by the Pr ion.

Order and Disorder in Ferroelectrics

Recent developments in the understanding of order-disorder and displacive type transitions are presented. It is shown that classical order-disorder type transitions like those in squaric acid and KDP contain in addition to the order disorder also a displacive component in the phase transition mechanism. Similarly classical displacive ferroelectrics like BaTiO3 which have been traditionally described by the condensation of the soft mode contain an order-disorder component as the Ti ion is dynamically disordered between different off-center sites already in the cubic phase. The comparison of the experimental and theoretical electric field gradient tensors at the Ti sites in the various phases of BaTiO3 allows for a quantitative determination of the mixed character of this phase transition.

Stability of Copper Atoms Embedded in Sodium-Chloride Crystals

Because it has been pointed out by Nagasaka et al. that the optical absorption spectrum of copper atoms embedded in alkali-chloride crystals may depend on the position of the copper atoms, we calculate the relation between the optimal position and the total energy of copper atom embedded in NaCl crystal by means of the first-principles pseudopotential plane-wave-expansion method. These results shows that most stable position of embedded Cu+ ion in these alkali halide system are not the substitutional on-center site but off-center site along ‹111› axis.

NMR evidence for the coexistence of order-disorder and displacive components in barium titanate.

A quadrupole coupling induced 47Ti and 49Ti satellite background which transforms into well-defined satellite lines below T(c) in the ferroelectric phase has been observed in the cubic phase of an ultrapure BaTiO3 single crystal. The results demonstrate the coexistence of a displacive and order-disorder component in the phase transition mechanism and tetragonal breaking of the cubic symmetry due to biased Ti motion between off-center sites in the paraelectric phase above T(c).

Crystal chemistry of protonated 1,2-bis(di-R-aminomethyl)benzenes. Part VI: 1,2-bis(diethylaminomethyl)-4,5-dichlorobenzene perchlorate

The crystals of a 1:1 complex of 1,2-bis(diethylaminomethyl)-4,5-dichlorobenzene perchlorate with HClO 4 were studied using X-ray diffraction and FT-IR spectroscopy. There are two symmetry-independent ionic pairs in the crystal, monoclinic, space group P2 1/ n. In both symmetry-independent cations, the diethylaminomethyl substituents are in a similar Z conformation relative to the phenyl moieties, however significant conformational differences in the diethylaminomethyls have been noted. The diethylaminomethyls are linked by intracationic +N–H⋯N hydrogen bonds of 2.758(9) and 2.721(7) Å in two symmetry-independent cations. The protons in these H-bonds are disordered in two off-center sites, which is most likely to be connected with the considerable disorder of the perchlorate anions. The IR spectrum of the crystals shows a broad intense band and a relatively intense continuous absorption indicating relatively large proton polarizability in the intramolecular hydrogen bond of a medium length.

From homoleptic to heteroleptic double stranded Copper(I) helicates: the role of self-recognition in self-assembly processes

The ligands 2,9-bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl]-1,10-phenanthroline (6), 6' ',6' "-bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl)]-2' ',2' "-bipyridine (2), 5,5'-bis[(6-methyl-2,2'-bipyridin-6'yl)methyleneoxymethylenyl]-2, 2'-bithiophene (7), and 6,6'-bis[(6-methyl-2, 2'-bipyridin-6'-yl)methyleneoxymethylenyl]-2,2'-biphenyl (8) and their respective homo- and heteroleptic double-stranded copper(I) complexes were prepared and characterized in order to estimate the importance of self-recognition in the self-assembly processes of double-stranded copper complexes. The homoleptic double-stranded copper complexes of 2, 6, 7, and 8 were characterized by NMR, FAB-MS, and electrochemistry. It was found that 6 and 2 each form a single double-stranded helicate having the structure of [(L)(2)Cu(3)](3+) (L = 2 or 6), 7 forms two double-stranded [(7)(2)Cu(3)](3+) complexes, and 8 results in a mixture of at least two [(8)(2)Cu(2)](2+) complexes. The potential shift, DeltaE degrees (,) of the Cu(+)/Cu(2+) redox process of these complexes reflects the binding affinity of the different binding sites to the copper cation. The electrochemical data show that the central units have a higher affinity to Cu(+) as compared to the off-center binding sites. NMR was used to determine the actual complex composition obtained from different mixtures of 2, 6, or 7 with Cu(+). Interestingly, we have found that, although 6, 2, and 7 each form homoleptic double-stranded complexes, no heteroleptic double-stranded copper complexes were formed from the mixtures of 7 with either 6 or 2. However, when mixtures of 6 and 2 are used, helicate distributions seem to follow simple statistics. These results are discussed in terms of the relative importance of self-recognition in the self-assembly of double-stranded helicates.