Electronic structure calculations and dynamics of methane activation on nickel and cobalt

The dissociative chemisorption of CH4 on nickel and cobalt has been studied using different cluster models. D. functional theory is used to det. the structure and potential energy surface in the reactant-, transition state-, and product region. The transition state is explicitly detd. on a single atom, a one layer 7-atom cluster and a spherical 13-atom cluster. We find transition state barriers of 41 kJ/mol for a single nickel atom, 79 kJ/mol for a single cobalt atom, 214 kJ/mol for the Ni7-cluster, 216 kJ/mol for the Co7-cluster, 121 kJ/mol for the Ni13-cluster, and 110 kJ/mol for the Co13-cluster. The overall reaction energies are -34, 6, 142, 135, 30, and 8 kJ/mol, resp. The higher barrier for the single cobalt atom in comparison with the nickel atom can be attributed to the difference between both atoms in the occupation of the s-orbital in the lowest lying states. The higher and almost the same barrier for the 7-atom clusters can be attributed to the intrinsic lower reactivity of the central atom embedded in the cluster and the similar electronic nature of the atoms in the clusters; in both clusters the atoms have open s- and d-shells. The lower barrier for the 13-atom clusters compared with the 7-atom clusters is a result of each surface atom now having 5 bonds, which gives a more balanced description of the substrate model. [on SciFinder (R)

Lattice mechanical study of the structure of dodecasil-3C

A recently developed partial charge potential model for SiO2 polymorphs, derived from quantum chem. calcns., was applied to the calcn. of the lattice energy-minimized structure and phys. properties of the low-d. SiO2 crystal Dodecasil-3C. Calcns. were performed using Ewald summations and without symmetry constraints. Results are compared with calcns. using the shell model of Sanders et al. (1983). Best agreement between exptl. and theor. predicted elastic consts. is achieved assuming a triclinic structure. To establish lattice stability, the vibrational frequency spectrum was calcd. For structures with imaginary frequencies, the corresponding at. displacements are used to deform the quasi-stationary geometry. On lattice energy minimization, the deformed structure transforms to a stable energy min. The large anisotropic temp. factors obsd. exptl. are mainly due to static disorder in at. position

Fourier-transform infrared and inelastic neutron-scattering study of hy zeolites

A combination of FTi.r. and INS spectroscopy is used in a vibrational study of the bending and stretching vibrations of the acidic hydroxyl groups of Y zeolites. The influence of the number of acidic Bronsted sites and the Si/Al ratio is discussed. Out-of-plane hydroxyl bending modes are assigned to vibrations centered around 419 cm-1 and in-plane hydroxyl bending modes are assigned to vibrations centered around 1089 cm-1. Upon dealumination, these bands are shifted by approximately 30 cm-1 to lower values. The less intense bands at 319,470,565,765, and 1130 cm-1 are assigned to proton- coupled framework vibrations. Upon dealumination, the mode at 319 cm-1 is shifted to lower frequencies and the modes at 565 and 1130 cm-1 are shifted to higher frequencie

13C and 23Na Solid-state NMR study on zeolite Y loaded with Mo(CO)6

The Na-23 MAS and double rotation (DOR) NMR spectra of dehydrated zeolite NaY loaded with two molecules of Mo(CO)(6) per supercage show three components distinguished by the corresponding quadrupole coupling constants (QCC). Sodium cations in the hexagonal prisms (SI sites) are characterized by a narrow Gaussian line with a QCC value close to zero. A second component with quadrupole coupling constants between 4.4 and 4.8 MHz is assigned to Na+ cations located in the sodalite cages, and the third component with QCC ranging between 2.2 and 2.8 MHz is due to sodium cations in the supercages interacting with Mo(CO)(6). These sites are characterized by structural and/or dynamical disorder as observed by typical line shape properties. Adsorption of Mo(CO)(6) in Y zeolites, where all Na+ cations in the supercages of NaY have been exchanged with NH4+ Or H+, causes the sodium cations in the sodalite cages to migrate into the supercages in order to interact with the adsorbent. For the NH4+-exchanged sample, the anisotropic chemical shift parameters for C-13 in Mo(CO)(6) have been analyzed at ambient temperature. From the results a fast anisotropic local reorientation of Mo(CO)(6) follows. The rotation is about a 3-fold axis of the octahedral complex. The octahedron is slightly elongated along the rotation axis by about 4 degrees when it is located close to the sodium cation

Phase diagram for Ca_{1-x}Y_xMnO_3 type crystals

We present a simple model to study the electron doped manganese perovskites. The model considers the competition between double exchange mechanism for itinerant electrons and antiferromagnetic superexchange interaction for localized electrons. It represents each Mn^{4+} ion by a spin 1/2, on which an electron can be added to produce Mn^{3+}; we include a hopping energy t, a strong intratomic interaction exchange J (in the limit J/t>>1), and an interatomic antiferromagnetic interaction K between the local spins. Using the Renormalized Perturbation Expansion and a Mean Field Approximation on the hopping terms and on the superexchange interaction we calculate the free energy. From it, the stability of the antiferromagnetic, canted, ferromagnetic, and novel spin glass phases can be determined as functions of the parameters characterizing the system. The model results can be expressed in terms of t and K for each value of the doping x in phase diagrams. The magnetization m and canting angle can also be calculated as fuctions of temperature for fixed values of doping and model parameters.Comment: 4 figure

The reduction of nitric-oxide by hydrogen over Pt, Rh and Pt-Rh single crystal surfaces

R.A. van Santen: question, p. 35

ATPγS stalls splicing after B complex formation but prior to spliceosome activation.

The ATP analog ATPγS inhibits pre-mRNA splicing in vitro, but there have been conflicting reports as to which step of splicing is inhibited by this small molecule and its inhibitory mechanism remains unclear. Here we have dissected the effect of ATPγS on pre-mRNA splicing in vitro. Addition of ATPγS to splicing extracts depleted of ATP inhibited both catalytic steps of splicing. At ATPγS concentrations ≥0.5 mM, precatalytic B complexes accumulate, demonstrating a block prior to or during the spliceosome activation stage. Affinity purification of the ATPγS-stalled B complexes (B(ATPγS)) and subsequent characterization of their abundant protein components by 2D gel electrophoresis revealed that B(ATPγS) complexes are compositionally more homogeneous than B complexes previously isolated in the presence of ATP. In particular, they contain little or no Prp19/CDC5L complex proteins, indicating that these proteins are recruited after assembly of the precatalytic spliceosome. Under the electron microscope, B(ATPγS) complexes exhibit a morphology highly similar to B complexes, indicating that the ATPγS-induced block in the transformation of the B to B(act) complex is not due to a major structural defect. Likely mechanisms whereby ATPγS blocks spliceosome assembly at the activation stage, including inhibition of the RNA helicase Brr2, are discussed. Given their more homogeneous composition, B complexes stalled by ATPγS may prove highly useful for both functional and structural analyses of the precatalytic spliceosome and its conversion into an activated B(act) spliceosomal complex

The partially asymmetric zero range process with quenched disorder

We consider the one-dimensional partially asymmetric zero range process where the hopping rates as well as the easy direction of hopping are random variables. For this type of disorder there is a condensation phenomena in the thermodynamic limit: the particles typically occupy one single site and the fraction of particles outside the condensate is vanishing. We use extreme value statistics and an asymptotically exact strong disorder renormalization group method to explore the properties of the steady state. In a finite system of $L$ sites the current vanishes as $J \sim L^{-z}$, where the dynamical exponent, $z$, is exactly calculated. For $0<z<1$ the transport is realized by $N_a \sim L^{1-z}$ active particles, which move with a constant velocity, whereas for $z>1$ the transport is due to the anomalous diffusion of a single Brownian particle. Inactive particles are localized at a second special site and their number in rare realizations is macroscopic. The average density profile of inactive particles has a width of, $\xi \sim \delta^{-2}$, in terms of the asymmetry parameter, $\delta$. In addition to this, we have investigated the approach to the steady state of the system through a coarsening process and found that the size of the condensate grows as $n_L \sim t^{1/(1+z)}$ for large times. For the unbiased model $z$ is formally infinite and the coarsening is logarithmically slow.Comment: 12 pages, 9 figure