Search results for "chemical bond"
showing 10 items of 123 documents
The static dielectric constant of solutions of water inn-alcohols at 15, 25, 35, and 45°C
1979
Dielectric constants (measured at 1 MHz) are reported for solutions of water (concentration range 0 to 0.2 mole fraction) in 1-propanol at 25°C, and in 1-butanol, 1-pentanol, and 1-hexanol at 15, 25, 35, and 45°C. These results, together with literature values for solutions of water in methanol, ethanol, 1-heptanol, and 1-octanol, show that water interacts with alcohols in at least two ways: (1) it can participate in the formation of dynamic hydrogen-bonded chains, thereby raising the polarizability; (2) it can form relatively stable structures such as H2O(ROH)4 which have zero net dipole moment and consequently diminish volume polarizability. For C n H 2n+1 OH alcohols,n≥4, most of the ini…
Dynamic Molecular Graphs: “Hopping” Structures
2013
This work aims to contribute to the discussion about the suitability of bond paths and bond-critical points as indicators of chemical bonding defined within the theoretical framework of the quantum theory of atoms in molecules. For this purpose, we consider the temporal evolution of the molecular structure of [Fe{C(CH2 )3 }(CO)3 ] throughout Born-Oppenheimer molecular dynamics (BOMD), which illustrates the changing behaviour of the molecular graph (MG) of an electronic system. Several MGs with significant lifespans are observed across the BOMD simulations. The bond paths between the trimethylenemethane and the metallic core are uninterruptedly formed and broken. This situation is reminiscen…
Optimizing density-functional simulations for two-dimensional metals
2022
Unlike covalent two-dimensional (2D) materials like graphene, 2D metals have non-layered structures due to their non-directional, metallic bonding. While experiments on 2D metals are still scarce and challenging, density-functional theory (DFT) provides an ideal approach to predict their basic properties and assist in their design. However, DFT methods have been rarely benchmarked against metallic bonding at low dimensions. Therefore, to identify optimal DFT attributes for a desired accuracy, we systematically benchmark exchange-correlation functionals from LDA to hybrids and basis sets from plane waves to local basis with different pseudopotentials. With 1D chain, 2D honeycomb, 2D square, …
<title>Computer modeling of point defects, polarons, excitons, and surfaces in perovskite ferroelectrics</title>
2003
We review results of our recent large-scale computer simulations of point defects, excitons and polarons in ABO3 perovskite crystals, focusing mostly on KNbO3 and KTaO3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption and defect-induced electron density redistribution. The majority of results are obtained using the quantum chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Frock formalism. The main findings are compared with results of ab initio Density Functional Theory (FP-LMTO) first-principles calculations. The results of the electronic structure calculations for different…
Cooperative mechanism for anchoring highly polar molecules at an ionic surface
2009
Structure formation of the highly polar molecule cytosine on the (111) cleavage plane of calcium fluoride is investigated in ultrahigh vacuum using noncontact atomic force microscopy at room temperature. Molecules form well-defined trimer structures, covering the surface as homogeneously distributed stable structures. Density-functional theory calculations yield a diffusion barrier of about 0.5 eV for individual molecules suggesting that they are mobile at room temperature. Furthermore, it is predicted that the molecules can form trimers in a configuration allowing all molecules to attain their optimum adsorption position on the substrate. As the trimer geometry facilitates hydrogen bonding…
<title>Multiphoton-absorption-induced structural changes in fused silica</title>
1991
The basic properties (light refractive index, density, mechanical strength, etc.) of fused silica are changed by the influence of high-intensity light from the glass transparency region capable of generating excitons by multiphoton absorption. The self-trapped exciton decay near the microcavity in the fused silica structure leads to the stable elementary intrinsic defect pair (nonbridging oxygen atom and three-fold-coordinated silicon atom) generation. At the large- enough light intensities near such a microcavity with a defect, the next exciton can be self- trapped. Then the next elementary defect can appear in the microcavity, and a chemical bond between it and the previously generated de…
Nonbonded Indirect Nuclear Spin–Spin Couplings (J Couplings “Through-Space”) for Structural Determination in Small Organic and Organometallic Species
2013
Abstract Spin–spin coupling constant J provides decisive data for organic compound characterization. This electron-mediated coupling is usually taught as transmitted between covalently bonded magnetic atoms. However, this physical interaction between nuclear spins is much more complex than that with regard to chemical bonding concept. Independent experimental and theoretical studies related to small organic and organometallic species (molecular mass below 2000 g mol − 1 ) have highlighted the existence of J couplings operating via clearly nonbonded interactions and known as “through-space” couplings. Interactions of this type are frequently reported and couplings involving 19 F, 13 C, 77 S…
Comparative ab initio calculations of SrTiO3 and CaTiO3 polar (111) surfaces
2012
We present the results of calculations of SrTiO3 and CaTiO3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO3 or CaO3) of the SrTiO3 and CaTiO3 (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO3-, and CaO3-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO3-terminated SrTiO3 (111) surface upper layer Sr atom. For both Ti-terminated SrTiO3 and CaTiO3 (111) surfaces our …
Ab initio calculations of SrTiO3, BaTiO3, PbTiO3, CaTiO3, SrZrO3, PbZrO3 and BaZrO3 (001), (011) and (111) surfaces as well as F centers, polarons, K…
2014
In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO 3 perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO 2-terminations of the nonpolar (001) surface and A , BO , and O terminations of the polar (011) surface, as well as B and AO 3-terminations of the polar (111) surface were considered. On the AO -terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO 2-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all …
First-principles calculations of the atomic and electronic structure of SrZrO3 and PbZrO3 (001) and (011) surfaces.
2010
We present the results of calculations of surface relaxations, rumplings, energetics, optical band gaps, and charge distribution for the SrZrO(3) and PbZrO(3) (001) and (011) surfaces using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We consider both SrO(PbO) and ZrO(2) terminations of the (001) surface and Sr(Pb), ZrO, and O terminations of the polar SrZrO(3) and PbZrO(3) (011) surfaces. On the (001) surfaces, we find that all upper and third layer atoms relax inward, while outward relaxations of all atoms in the second layer are found with the sole exception of the SrO-terminated SrZrO(3) (001) surface second layer O atom. Between all (001) and (011) s…