Interpretation of light-scattering spectra in terms of particle displacements

Quasielastic light-scattering spectroscopy is regularly used to examine the dynamics of dilute solutions of diffusing mesoscopic probe particles in fluids. For probes in a simple liquid, the light-scattering spectrum is a simple exponential; the field correlation function g(1)(q,tau) of the scattering particles is related to their mean-square displacements X2 identical with [(delta x(tau))2] during tau via g(1)(q,tau) = exp(-1/2 q2X2). However, demonstrations of this expression refer only to identical Brownian particles in simple liquids and show that if the form is correct then it is also true for all tau that g(1)(q,tau) = exp(-gamma tau), a pure exponential in tau. In general, g(1)(q,tau) is not a single exponential in time. A correct general form for g(1)(q,tau) in terms of the X(2n), replacing the incorrect exp(-1/2 q2X2), is obtained. A simple experimental diagnostic determining when the field correlation function gives the mean-square displacement is identified, namely, g(1)(q,tau) only reveals X2 if g(1)(q,tau) is a single exponential in tau. Contrariwise, if g(1)(q,tau) is not a single exponential, then g(1)(q,tau) depends not only on X2 but on all higher moments X(2n). Corrections to the crude approximation g(1)(q,tau) = exp(-1/2 q2X2) closely resemble the higher spectral cumulants from a cumulant expansion of g(1)(q,tau).     

Kinetics of RNA refolding in dynamic equilibrium by 1H-detected 15N exchange NMR spectroscopy

By implementing new NMR methods that were designed to map very slow exchange processes we have investigated and characterized the refolding kinetics of a thermodynamically stable 34mer RNA sequence in dynamic equilibrium. The RNA sequence was designed to undergo a topologically favored conformational exchange between different hairpin folds, serving as a model to estimate the minimal time required for more complex RNA folding processes. Chemically prepared RNA sequences with sequence-selective (15)N labels provided the required signal separation and allowed a straightforward signal assignment of the imino protons by HNN correlation experiments. The 2D version of the new (1)H-detected (15)N exchange spectroscopy (EXSY) pulse sequence provided cross-peaks for resonances belonging to different folds that interchange on the time scale of longitudinal relaxation of (15)N nuclei bound to imino protons. The 34mer RNA sequence exhibits two folds which exchange on the observable time scale (tau(obs) approximately T(1){(15)Nu} < 5 s) and a third fold which is static on this time scale. A 1D version of the (15)N exchange experiment allowed the measurement of the exchange rates between the two exchanging folds as a function of temperature and the determination of the corresponding activation energies E(a) and frequency factors A. We found that the refolding rates are strongly affected by an entropically favorable preorientation of the replacing strand. The activation energies are comparable to values obtained for the slow refolding of RNA sequences of similar thermodynamic stability but less favorable topology.     

Efficient algorithm of the transcorrelated method for periodic systems

The transcorrelated (TC) method is one of the promising wave-function-based approaches for the first-principles electronic structure calculations. In this method, the many-body wave function is approximated as the Jastrow-Slater type and one-electron orbitals in the Slater determinant are optimized with a one-body self-consistent-field equation such as that in the Hartree-Fock (HF) method. Although the TC method has yielded good results for both molecules and solids, its computational cost in solid-state calculations, being of order O(N(k)(3)N(b)(3)) with N(k) and N(b) the respective numbers of k-points and bands, has for some years hindered its wide application in condensed matter physics. Although an efficient algorithm was proposed for a Gaussian basis set, that algorithm is not applicable to a plane-wave basis that is suited to and widely used in solid-state calculations. In this paper, we present a new efficient algorithm of the TC method for the plane-wave basis or an arbitrary basis function set expanded in terms of plane waves, with which the computational cost of the TC method scales as O(N(k)(2)N(b) (2)). This is the same as that of the HF method. We applied the TC method with the new algorithm to obtain converged band structure and cell parameters of some semiconductors.     

Towards models of the oxygen-evolving complex (OEC) of photosystem II: a Mn4Ca cluster of relevance to low oxidation states of the OEC

Synthetic access has been achieved into high oxidation state Mn/Ca chemistry with the 4?:?1 Mn?:?Ca stoichiometry of the oxygen-evolving complex (OEC) of plants and cyanobacteria; the anion of (Et(3)NH)(2)[Mn(III)(4)Ca(O(2)CPh)(4)(shi)(4)] has a square pyramidal metal topology and an S = 0 ground state.     

Model systems for flavoenzyme activity: site-isolated redox behavior in flavin-functionalized random polystyrene copolymers

[reaction: see text] A model system has been developed to study the redox behaviors of flavin derivatives appended onto random polystyrene copolymers through "click" chemistry strategies. The results demonstrate that flavin units attached onto polymers exhibit site-isolated redox behaviors, yielding new materials with electrochemically tunable associations (K(a)(ox) = 450 M(-)(1), K(a)(red) = 18,200 M(-)(1)) to complementary diamidopyridine (DAP) functionality.     

DFT-based studies on the Jahn-Teller effect in 3d hexacyanometalates with orbitally degenerate ground states

The topology of the ground-state potential energy surface of M(CN)(6) with orbitally degenerate (2)T(2g) (M = Ti(III) (t(2g)(1)), Fe(III) and Mn(II) (both low-spin t(2g)(5))) and (3)T(1g) ground states (M = V(III) (t(2g)(2)), Mn(III) and Cr(II) (both low-spin t(2g)(4))) has been studied with linear and quadratic Jahn-Teller coupling models in the five-dimensional space of the epsilon(g) and tau(2g) octahedral vibrations (Tg[symbol: see text](epsilon(g)+tau(2g)) Jahn-Teller coupling problem (T(g) = (2)T(2g), (3)T(1g))). A procedure is proposed to give access to all vibronic coupling parameters from geometry optimization with density functional theory (DFT) and the energies of a restricted number of Slater determinants, derived from electron replacements within the t(2g)(1,5) or t(2g)(2,4) ground-state electronic configurations. The results show that coupling to the tau(2g) bending mode is dominant and leads to a stabilization of D(3d) structures (absolute minima on the ground-state potential energy surface) for all complexes considered, except for [Ti(CN)(6)](3-), where the minimum is of D(4h) symmetry. The Jahn-Teller stabilization energies for the D3d minima are found to increase in the order of increasing CN-M pi back-donation (Ti(III) < V(III) < Mn(III) < Fe(III) < Mn(II) < Cr(II)). With the angular overlap model and bonding parameters derived from angular distortions, which correspond to the stable D(3d) minima, the effect of configuration interaction and spin-orbit coupling on the ground-state potential energy surface is explored. This approach is used to correlate Jahn-Teller distortion parameters with structures from X-ray diffraction data. Jahn-Teller coupling to trigonal modes is also used to reinterpret the anisotropy of magnetic susceptibilities and g tensors of [Fe(CN)(6)](3-), and the (3)T(1g) ground-state splitting of [Mn(CN)(6)](3-), deduced from near-IR spectra. The implications of the pseudo Jahn-Teller coupling due to t(2g)-e(g) orbital mixing via the trigonal modes (tau(2g)) and the effect of the dynamic Jahn-Teller coupling on the magnetic susceptibilities and g tensors of [Fe(CN)(6)](3-) are also addressed.     

Kinetics of liposome adhesion on a mercury electrode

The adhesion of liposomes on a mercury electrode leads to capacitive signals due to the formation of islands of lecithin monolayers. Integration of the current-time transients gives charge-time transients that can be fitted by the empirical equation Q(t) = Q(0) + Q(1)(1 - exp(-t/tau(1))) + Q(2)(1 - exp(-t/tau(2))), where the first term on the right side is caused by the docking of the liposome on the mercury surface, the second term is caused by the opening of the liposome, and the third term is caused by the spreading of the lecithin island on the mercury surface. The temperature dependence of the two time constants tau(1) and tau(2) and the temperature dependence of the overall adhesion rate allow determination of the activation energies of the opening, the spreading, and the overall adhesion process both for gel-phase 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and for liquid-crystalline-phase DMPC liposomes. In all cases, the spreading is the rate-determining process. Negative apparent activation energies for the spreading and overall adhesion process of liquid-crystalline-phase DMPC liposomes can be explained by taking into account the weak adsorption equilibria of the intact liposomes and the opened but not yet spread liposomes. A formal kinetic analysis of the reaction scheme supports the empirical equation used for fitting the charge-time transients. The developed kinetic model of liposome adhesion on mercury is similar to kinetic models published earlier to describe the fusion of liposomes. The new approach can be used to probe the stability of liposome membranes.     

A high-nuclearity 3d/4f metal oxime cluster: an unusual Ni(8)Dy(8) "core-shell" complex from the use of 2-pyridinealdoxime

The initial employment of 2-pyridinealdoxime in 3d/4f chemistry has led to a Ni(II)(8)Dy(III)(8) cluster with an unprecedented metal topology; the compound has an unusual structure, is the highest-nuclearity metal oxime cluster to date, and exhibits slow magnetization relaxation.     

Use of 6-methylpyridine-2-carbaldehydeoxime in nickel(II) carboxylate chemistry: synthetic, structural and magnetic properties of penta and hexanuclear complexes

The use of 6-methylpyridine-2-carbaldehydeoxime ligand (6-mepaoH), in nickel(II) chemistry has been investigated and three new clusters isolated in mild conditions. Depending on the nature of the metal starting salts and the reaction conditions, the Ni(II)/6-mepaoH system has provided access to the complexes [Ni(6)(O(2)CPh)(6)(6-mepao)(6)] (1), [Ni(6)(O(2)CMe)(6)(6-mepao)(6)] (2) and [Ni(5)(3-Cl-BzO)(4)(6-mepao)(4)(6-mepaoH)(2)(N(3))(2)] (3), where 3-Cl-BzO(-) is the 3-chlorobenzoate anion. Compounds 1 and 2 are two new members of the [Ni(6)(O(2)C-R)(oximato)(6)] family of hexanuclear complexes whereas 3 exhibits an unusual irregular bowtie topology including end-on azido bridges. The structures of the three compounds have been determined by single-crystal X-ray crystallography. Variable-temperature dc magnetic susceptibility studies were carried out for 1-3. The data indicate antiferromagnetic exchange for complexes 1 and 2 and ferrimagnetic interaction for complex 3.     

Bridge function for the dipolar fluid from simulation

The exact bridge function of the Lennard-Jones dipolar (Stockmayer) fluid is extracted from Monte Carlo simulation data. The projections g(mnl)(r) onto rotational invariants of the non-spherically symmetric pair distribution function g(r, Ω) are accumulated during simulation. Making intensive use of anisotropic integral equation techniques, the molecular Ornstein-Zernike equation is then inverted in order to derive the direct correlation function c(mnl)(r), the cavity function y(mnl)(r), the negative excess potential of mean force lny|(mnl)(r), and the bridge function b(mnl)(r) projections. b(r, Ω) presents strong, non-universal anisotropies at high dipolar coupling. This simulation data analysis may serve as reference and guide for approximated bridge function theories of dipolar fluids and is a valuable step towards the case of more refined, nonlinear water-like geometries.     

Fluorine-fluorine interactions in the solid state: an experimental and theoretical study

The solid state structures of three compounds that contain a perfluorinated chain, CF(3)(CF(2))(5)CH(2)CH(CH(3))CO(2)H, CF(3)(CF(2))(5)(CH(2))(4)(CF(2))(5)CF(3) and {CF(3)(CF(2))(5)CH(2)CH(2)}(3)P═O have been compared and a number of C-F···F-C and C-F···H-C interactions that are closer than the sum of the van der Waals radii have been identified. These interactions have been probed by a comprehensive computational chemistry investigation and the stabilizing energy between dimeric fragments was found to be 0.26-29.64 kcal/mol, depending on the type of interaction. An Atoms-in-Molecules (AIM) study has confirmed that specific C-F···F-C interactions are indeed present, and are not due simply to crystal packing. The weakly stabilizing nature of these interactions has been utilized in the physisorption of a selected number of compounds containing long chain perfluorinated ponytails onto a perfluorinated self-assembled monolayer, which has been characterized by IRRAS (Infrared Reflection Absorption Spectroscopy).     

Direct measurement of relative and collective diffusion in a dilute binary colloidal suspension

Experimental characterization of the dynamics of multicomponent fluids is a problem of general importance to the field of complex fluids. We demonstrate a new experimental approach, termed two-color Fourier imaging correlation spectroscopy, which allows direct measurement of the partial dynamic structure factors, S(11)(k,tau), S(22)(k,tau), and S(12)(k,tau), where 1, 2 label the component species of a binary colloidal suspension. Linear combinations of the partial dynamic structure factors yield the characteristic time-correlation functions of the binary fluid. These are the correlation functions of concentration fluctuations S(CC)(k,tau), number density fluctuations S(NN)(k,tau), and cross-correlations between number density and concentration fluctuations S(NC)(k,tau). Test measurements are performed on a dilute symmetric mixture of fluorescently labeled 0.5 and 1.0 microm polystyrene spheres. From these data, we determine generalized collective and relative diffusion coefficients, and compare them to the predictions for an ideal mixture of noninteracting particles.     

A highly porous interpenetrated metal-organic framework from the use of a novel nanosized organic linker

The initial use of a novel elongated tricarboxylic acid H(3)hmpib in metal-organic framework (MOF) chemistry resulted in a [Zn(4)O(hmpib)(2)] MOF (UCY-1) with pyrite topology. The compound displays a remarkably high internal surface area despite its double-interpenetrated structure as well as high CO(2) uptake and selective adsorption for it over CH(4).