Analysis of matrix nuclei spin flip satellite lines in EPR spectra: Application to trapped hydrogen atoms

The relative intensities and energy separation of allowed EPR lines of a paramagnetic species and partially forbidden spin flip satellite lines due to dipole-dipole coupling between the paramagnetic species and the matrix nuclei have been analyzed in a new way. The analysis allows one to determine both the distance to and the number of matrix nuclei that contribute to a given satellite transition. For hydrogen atoms trapped in a gamma-irradiated sulphuric acid glass at 77 K the analysis indicates that interaction occurs with four matrix protons at a distance of 2.15 Å. For trapped hydrogen atoms in phosphoric acid glass the analysis indicates interaction with four matrix protons at 2.37 Å.     

Microgravimetric study of electrochemically controlled nucleophilic addition of sulfite to polyaniline

The sulfonation of polyaniline (PANI) films by nucleophilic addition of sulfite ion has been controlled through the polymer oxidation state under electrochemical control. The process was monitored by in situ electrochemical quartz crystal microbalance (EQCM), and the polymer oxidation was accomplished by electrode potential steps in sulfite aqueous solutions. The nucleophilic addition of sulfite to PANI only takes place on the oxidized polymer. From the ratio of added mass to the injected charge, the degree of sulfonation has been obtained with a yield as high as 50%. It has been observed that the ion-exchange mechanism during the oxidation-reduction process in the resulting sulfonated polymer is analogous to the polymer produced by electrophilic sulfonation of polyaniline or by copolymerization of aniline with aminosulfonic acids, unlike the ionic exchange observed for unmodified PANI.     

Validity of the Hammond postulate and constraints on general one-dimensional reaction barriers

The Hammond postulate is a useful, qualitative tool that interrelates structural similarities between reactants, transition structures, and products with the exo- or endothermicity of reactions. It applies to most chemical reactions, although several exceptions are known. In this study the following problem is addressed: is it possible to formulate conditions for the validity of the quantitative Hammond postulate in terms of simple physical quantities characteristic to the molecules involved? A detailed analysis is given for the conditions of validity of the postulate, in terms of bounds on the internal forces and force constants of nuclear arrangements encountered along a reaction path. We have determined a broad class of constraints on barrier shapes that must be satisfied in order to obtain a critical situation that violates the Hammond postulate: a reactant-like transition structure (“transition state”) for endothermic reactions, and a product-like one for exothermic reactions. The general constraints are formulated in terms of physically meaningful quantities: (i) energy differences, (ii) restrictions on slopes (e.g., an upper bound on internal forces), and (iii) restrictions on curvatures (e.g., upper bounds on force constants) along potential curves.     

Shape group theory of van der Waals surfaces

In this article we present a method for the study of shapes of general, asymmetric van der Waals surfaces. The procedure is simple to apply and it consists of two steps. First, the surface is decomposed into spherical domains, according to the interpenetration of the van der Waals atomic spheres. Each domain defines a topological object that is either a 2-manifold or some truncated 2-manifold. Second, we compute the homology groups for all the objects into which the surface is divided. These groups are topological and homotopical invariants of the domains, hence they remain invariant to conformational changes that preserve the essential features of these domains of decomposition. In particular, these homology groups do not depend explicitly on the molecular symmetry. Major rearrangements of the nuclear configurations, however, do alter the decomposition into spherical domains, and the corresponding variation of the homology groups can be followed easily under conformational rearrangements. We discuss a partitioning of the metric internal configuration spaceM into shape regions of van der Waals surfaces, which allows one to identify those rearrangements which introduce an essential change in shape and to distinguish them from those which do not alter the fundamental shape of the molecular surface. The dependence of the shape group partitioning ofM on the symmetry under permutation of nuclear changes is discussed briefly, considering a simple illustrative example.     

Electron-density-dependent fused-sphere surfaces derived from pseudopotential calculations

Fused-sphere surfaces can be used to mimic a molecular boundary associated with a constant value of the electron density. The simplest of such fused-sphere models are constructed by using the atomic radii for the spherical isodensity surfaces of individual atoms. In this work, we discuss the extension of this model to molecules containing atoms beyond the second row. In these many- electron systems, the computation of electron densities is usually simplified by adopting a pseudopotential (or effective-core potential) approach. Here, we discuss the performance of large- and small-core pseudo-potential calculations as a tool to derive atomic radii. Our results provide an optimum set of variable radii that can be used to build fused-sphere surfaces. This continuum of surfaces provides a simple approximation to the low-electron-density regions around molecules with heavy atoms.     

Comparative properties of siloxane vs phosphonate monolayers on a key titanium alloy

A direct comparison of surface loading, interface shear strength, and interface hydrolytic stability was made between a phosphonate and two siloxane monolayers formed on the native oxide surface of Ti-6Al-4V. Surface loading for the phosphonate was ca. four times greater (on a nanomole/area basis) than for the siloxanes; mechanical strengths per surface-bound molecule were comparable, but the hydrolytic stability (pH 7.5) of the siloxanes was poor. These results suggest that phosphonate monolayer interfaces are more desirable than comparable siloxane ones for applications where such interfaces contact even slightly alkaline water.     

Nonequilibrium synthesis and assembly of hybrid inorganic-protein nanostructures using an engineered DNA binding protein

We show that a protein with no intrinsic inorganic synthesis activity can be endowed with the ability to control the formation of inorganic nanostructures under thermodynamically unfavorable (nonequilibrium) conditions, reproducing a key feature of biological hard-tissue growth and assembly. The nonequilibrium synthesis of Cu(2)O nanoparticles is accomplished using an engineered derivative of the DNA-binding protein TraI in a room-temperature precursor electrolyte. The functional TraI derivative (TraIi1753::CN225) is engineered to possess a cysteine-constrained 12-residue Cu(2)O binding sequence, designated CN225, that is inserted into a permissive site in TraI. When TraIi1753::CN225 is included in the precursor electrolyte, stable Cu(2)O nanoparticles form, even though the concentrations of [Cu(+)] and [OH(-)] are at 5% of the solubility product (K(sp,Cu2O)). Negative control experiments verify that Cu(2)O formation is controlled by inclusion of the CN225 binding sequence. Transmission electron microscopy and electron diffraction reveal a core-shell structure for the nonequilibrium nanoparticles: a 2 nm Cu(2)O core is surrounded by an adsorbed protein shell. Quantitative protein adsorption studies show that the unexpected stability of Cu(2)O is imparted by the nanomolar surface binding affinity of TraIi1753::CN225 for Cu(2)O (K(d) = 1.2 x 10(-)(8) M), which provides favorable interfacial energetics (-45 kJ/mol) for the core-shell configuration. The protein shell retains the DNA-binding traits of TraI, as evidenced by the spontaneous organization of nanoparticles onto circular double-stranded DNA.     

Pattern of separatrices and intrinsic reaction coordinates for degenerate thermal rearrangements

A structurally stable model of the standard adiabatic gradient field of the potential energy surface for certain pericyclic reactions is derived.These reactions are not subjected to the principles of orbital isomerism or to the Woodward-Hoffmann rules.Use is made of a principle established by Ariel Fernández and Oktay Sinanolu which precludes direct meta-IRC connections between transition states.It is shown that Jahn-Teller isomers of the singlet biradicals involved in the process are not interconvertible since the biradical configuration is not a transition state but a critical point with Hessian matrix with two negative eigenvalues.The topological features of the PES obtained by combinatorial methods are in full agreement with earlier results obtained from MINDO calculations.     

Selective, controllable, and reversible aggregation of polystyrene latex microspheres via DNA hybridization

The directed three-dimensional self-assembly of microstructures and nanostructures through the selective hybridization of DNA is the focus of great interest toward the fabrication of new materials. Single-stranded DNA is covalently attached to polystyrene latex microspheres. Single-stranded DNA can function as a sequence-selective Velcro by only bonding to another strand of DNA that has a complementary sequence. The attachment of the DNA increases the charge stabilization of the microspheres and allows controllable aggregation of microspheres by hybridization of complementary DNA sequences. In a mixture of microspheres derivatized with different sequences of DNA, microspheres with complementary DNA form aggregates, while microspheres with noncomplementary sequences remain suspended. The process is reversible by heating, with a characteristic "aggregate dissociation temperature" that is predictably dependent on salt concentration, and the evolution of aggregate dissociation with temperature is observed with optical microscopy.     

Kinetics of octadecyltrimethylammonium bromide self-assembled monolayer growth at mica from an aqueous solution

We have studied the growth kinetics of self-assembled monolayers (SAMs) ofoctadecyltrimethylammonium bromide (C18TAB) on mica below the critical micelle concentration at 22, 30, 40, and 50 degrees C. A combination of atomic force microscopy, contact angle goniometry, and transmission infrared spectroscopy was used to follow the growth processes to determine the rates involved in the growth of a C18TAB SAM on mica. The growth of a SAM consisted of four distinct processes: deposition of adsorbate molecules, growth of a disordered 2D liquid phase, nucleation of islands of an ordered 2D solid phase, and subsequent growth of the solid phase. The rates of these various processes are determined, and the activation energies for several processes were calculated including those for the adsorption onto a bare substrate (20 kJ/mol), adsorption into the saturated liquid phase (100 kJ/mol), and nucleation of islands (0.3 kJ/mol). Despite the small activation barrier to island nucleation, the nucleation rate is qualitatively slow, suggesting that entropic effects dominate the nucleation rate.