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Local geometry and orientation of intercalated molecules of formamide (FA) and N-methylformamide (NMFA) in the clay mineral
dickite (D) was studied by means of Density Functional Theory (DFT) calculations. Ten configurations with different orientation
of the intercalated molecule were investigated for both D_FA and D_NMFA intercalates. Four groups of relaxed structures sorted
by the calculated total electronic energy were found in both cases. The experimental geometry of the D_FA intercalate was
denoted as the most stable structure from the investigated models. The differences in the total electronic energy of all D_FA
configurations are within the interval of ∼92 kJ/mol. On one hand FA forms intercalates specifically and a close relation
between the orientation of the FA molecules in the interlayer space and the stability of a particular configuration has been
observed. On the other hand, N-methylformamide forms intercalated structures non-specifically. Small differences in the total
energy, not larger than 18 kJ/mol, are observed for different orientations of the NMFA molecules The reorientation of the
intercalated molecules has only a small effect on the stabilization of the D_NMFA intercalate what is in contrast with the
D_FA intercalate. It was also observed that the experimental D_NMFA configuration is not the most stable. A small variation
of the total electronic energy of different configurations correlates with small changes of the orientation of the dipole
moment of the intercalated NMFA molecule. 相似文献
2.
E. Scholtzová L'. Smrčok D. Tunega L. Turi Nagy 《Physics and Chemistry of Minerals》2000,27(10):741-746
A 2-D periodic ab initio Hartree–Fock LCAO study was performed on Fe-substituted 1:1 sheet silicate, lizardite. The atomic
orbitals were described by large-core pseudopotentials (Fe atoms) and a sp basis set (the other atoms). Calculated atomic
charges and the results of bond population analysis indicate that the Fe-O bond was as ionic as the Mg-O. This hypothesis
was supported by calculated density of states and electron density maps. Increasing Fe for Mg substitution changed the shape
of projected density of states (PDOS) of O(p) orbitals.
Received: 27 January 2000 / Accepted: 21 May 2000 相似文献
3.
Adsorption of H2O, NH3 and C6H6 on H- and alkali metal-exchanged structures of mordenite and on corresponding cations on the smectite layer is investigated
by ab initio density-functional calculations. Proton or an alkali metal cation compensates one Al/Si framework substitution
and resides in the extra-framework position of zeolite or above flat smectite layer close to the Al/Si substitution. Pronounced
similarities between zeolite and smectite are observed in changes of the adsorption energies and location of the external
cation with changing character of the external cation. Calculated adsorption energies exhibit the following trend: E(NH3) > E(H2O) > E(C6H6). Because of looser contact with the framework, zeolitic cations are stronger adsorption centers and calculated adsorption
energies of zeolites are by ~20–30% larger than cations of smectites. The highest adsorption energy is calculated for H-exchanged
structures and down the group of alkali metal cations a decrease of the adsorption energy is observed. Deviations from the
smooth variation of the adsorption energy are caused by: (1) formation of strong hydrogen bonds in H-exchanged structures,
(2) adsorption induced migration of the external Li+ cation, and (3) steric hindrances of the flat C6H6 molecule adsorbed on the cation in the cage of zeolite. 相似文献
4.
Adelia J.A. Aquino Daniel Tunega Georg Haberhauer Hans Lischka 《Geochimica et cosmochimica acta》2008,72(15):3587-3602
Density functional theory is used to compute the effect of protonation, deprotonation, and dehydroxylation of different reactive sites of a goethite surface modeled as a cluster containing six iron atoms constructed from a slab model of the (1 1 0) goethite surface. Solvent effects were treated at two different levels: (i) by inclusion of up to six water molecules explicitly into the quantum chemical calculation and (ii) by using additionally a continuum solvation model for the long-range interactions. Systematic studies were made in order to test the limit of the fully hydrated cluster surfaces by a monomolecular water layer. The main finding is that from the three different types of surface hydroxyl groups (hydroxo, μ-hydroxo, and μ3-hydroxo), the hydroxo group is most active for protonation whereas μ- and μ3-hydroxo sites undergo deprotonation more easily. Proton affinity constants (pKa values) were computed from appropriate protonation/deprotonation reactions for all sites investigated and compared to results obtained from the multisite complexation model (MUSIC). The approach used was validated for the consecutive deprotonation reactions of the [Fe(H2O)6]3+ complex in solution and good agreement between calculated and experimental pKa values was found. The computed pKa for all sites of the modeled goethite surface were used in the prediction of the pristine point of zero charge, pHPPZN. The obtained value of 9.1 fits well with published experimental values of 7.0-9.5. 相似文献
5.
The paper deals with data filtering on closed surfaces using linear and nonlinear diffusion equations. We define a surface finite-volume method to approximate numerically parabolic partial differential equations on closed surfaces, namely on a sphere, ellipsoid or the Earth’s surface. The closed surface as a computational domain is approximated by a polyhedral surface created by planar triangles and we construct a dual co-volume grid. On the co-volumes we define a weak formulation of the problem by applying Green’s theorem to the Laplace–Beltrami operator. Then the finite-volume method is applied to discretize the weak formulation. Weak forms of elliptic operators are expressed through surface gradients. In our numerical scheme we use a piece-wise linear approximation of a solution in space and the backward Euler time discretization. Furthermore, we extend a linear diffusion on surface to the regularized surface Perona–Malik model. It represents a nonlinear diffusion equation, which at the same time reduces noise and preserves main edges and other details important for a correct interpretation of the real data. We present four numerical experiments. The first one has an illustrative character showing how an additive noise is filtered out from an artificial function defined on a sphere. Other three examples deal with the real geodetic data on the Earth’s surface, namely (i) we reduce a stripping noise from the GOCE satellite only geopotential model up to degree 240, (ii) we filter noise from the real GOCE measurements (the component $T_{zz})$ , and (iii) we reduce a stripping noise from the satellite only mean dynamic topography at oceans. In all experiments we focus on a comparison of the results obtained by both the linear and nonlinear models presenting advantages of the nonlinear diffusion. 相似文献
6.
The electrostatic potential above a surface of a semi-infinite crystal of talc Mg3Si4O10(OH)2 in the idealized structure (layer group symmetry C2/m) was computed using the electron distribution obtained by a semiempirical INDO/2 calculation of the electron structure of one idealized layer. The potential of the semi-infinite crystal was obtained as the sum of contributions from an infinite number of individual layers. The adsorption enthalpy and the vibrational frequencies of a proton hypothetically adsorbed in vacuo were also computed. 相似文献
7.
The effect of the Si/Al distribution in the tetrahedral sheets of the vermiculite mineral has been investigated employing density functional theory. The structures of six models for vermiculite with the structural formula (Mg4)(Mg12)(Si8Al8)O40(OH)8·24(H2O) per unit cell were fully optimized. The models differ by the T···Mg2+···T coordination of the interlayer Mg2+ cations by two central cations from the adjacent tetrahedral sheets of the 2:1 vermiculite layers (T,T=Si,Al). We observed the formation of very strong hydrogen bonds between water molecules solvating the interlayer Mg2+ cations and the surface basal oxygen atoms of the 2:1 layers. The directionality of hydrogen bonds is the major factor determining the layer stacking in the vermiculite structure. Results showed that the most stable model is that where only silicon atoms in the tetrahedral sheets coordinate all interlayer Mg2+ cations. 相似文献
8.
Ľubomír Smrčok Daniel Tunega Anibal Javier Ramirez-Cuesta Eva Scholtzová 《Physics and Chemistry of Minerals》2010,37(8):571-579
The dynamics of the hydrogen atoms in the highly ordered kaolinite was studied by vibrational spectroscopy based on inelastic
neutron scattering method with the focus on the spectral region of 100–1,250 cm−1. The experimental spectrum was interpreted by means of the solid state density functional theory calculations covering both
normal mode analysis and molecular dynamics going beyond the harmonic approximation. The Al–O–H bending modes were found to
be spread over the large interval of 100–1,100 cm−1, with the dominant contributions located between 800 and 1,100 cm-1. The shapes of the individual hydrogen spectra depend on the strengths of the individual interlayer O–H···O hydrogen bonds
involving the inner surface hydroxyl groups. The modes assigned to the in-plane movements of the respective hydrogen atoms
are well-defined and always appear on the top of the intervals of energy transfer. In contrast, the modes generated by the
out-of-plane movements are spread over large intervals of energies spanning down to the region of external (lattice) modes. 相似文献
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