Nonlocal Density Functional Calculations Research Articles

Quantitative assessment of basal-, edge- and defect-surfaces of carbonaceous materials and their influence on electric double-layer capacitance

Activated carbon is widely used as electrode materials in supercapacitors, and its capacitive performance is significantly influenced by the specific surface area. However, different types of wall surfaces, such as basal-, edge- and defect-surfaces, exist in carbonaceous materials due to the complex structure and atomic distribution of carbon atoms, rendering different influences on capacitive charge storage behavior. Therefore, it is of utmost significance to quantitatively determine their contribution to the capacitive performance. Herein, the commercial graphite powder is ball milled to introduce different types of edges and defects, resulting in different proportions of basal-, edge- and defect-surfaces. Moreover, a modified non-local density functional theory calculations method is adopted to quantitatively evaluate the basal-, edge- and defect-surface area. Then, a relationship is established between the electrode capacitance and wall surface area by the linear regression method. The results reveal that the basal-, edge- and defect-surfaces contributed an areal capacitance of 0.04 μF cm−2, 1.65 μF cm−2 and 7.95 μF cm−2, respectively. Hence, the capacitance is mainly provided by the defects of wall surfaces.

Formation of NV centers in diamond: A theoretical study based on calculated transitions and migration of nitrogen and vacancy related defects

Formation and excitation energies as well charge transition levels are determined for the substitutional nitrogen (N${}_{\mathrm{s}}$), the vacancy (V), and related point defects (NV, NVH, N${}_{2}$, N${}_{2}$V, and V${}_{2}$) by screened nonlocal hybrid density functional supercell plane wave calculations in bulk diamond. In addition, the activation energy for V and NV diffusion is calculated. We find good agreement between theory and experiment for the previously well-established data and predict missing ones. Based on the calculated properties of these defects, the formation of the negatively charged NV center is studied, because it is a prominent candidate for application in quantum information processing and for nanosensors. Our results indicate that NV defects are predominantly created directly by irradiation, while simultaneously produced vacancies will form V${}_{2}$ pairs during postirradiation annealing. Divacancies may pin the Fermi level, making the NV defects neutral.

Nature of visible-light responsive fluorinated titanium dioxides

The origin of the visible-light activity of fluorinated TiO2 has been systematically investigated via comprehensive theoretical studies. The 3-coordinated surface F atoms with higher 1s binding energy are identified to be the origin of the visible light activity of the fluorinated TiO2 by systematically analysing 1s core-level shifts of various types of F atoms in fluorinated TiO2. Further analysing the electronic structures of the fluorinated TiO2 using semi-local density functional theory and non-local hybrid density functional theory calculations demonstrates that the introduction of the 3-coordinated surface F atoms leads to the formation of Ti3+ ions in the sub-surface, which is the cause for the bandgap shrinking, increasing the visible-light activity. The results also reveal that the presence of 3-coordinated surface F atoms strengthens the adsorption of hydroxyl groups in their vicinity to enhance the photocatalytic activity. The findings of this work address a long-standing scientific question, providing useful guidance for the design and development of high performance VL photocatalysts.

A density functional investigation of the Bursten ligand additivity model for the d 6 octahedral complexes [Mn(CO) n(CNMe) (6−n)] + (M = Mn(I), Cr(0))

The Bursten ligand additivity model provides a description of orbital energies of complexes of the type [M(L) n (L′) 6− n ] x +, where L and L′ are ligands of different π-acceptor abilities. The model allows for the prediction molecular orbital energies, oxidation potentials and valence photoelectron spectra of octahedral complexes. The model was derived by Bursten from the results of Fenske–Hall approximate molecular orbital calculations. In this study non-local density functional calculations are used to determine optimized geometries, molecular orbital energies and charge distributions for the complexes [M(CO) n (CNMe) (6− n )] + (M = Mn(I), Cr(0)). The calculated DFT molecular orbital energies are used to derive ligand additivity relationships. Variations in C–O and M–C bond lengths and charge distributions are used to study the effect of ligand substitution on the degree of π-back-bonding in each complex. It is found that the results of DFT calculations show excellent agreement with the Bursten ligand additivity model, despite the considerable difference in computational methods and structural changes that occur on ligand substitution.

Quantum Mechanical Control of Artificial Minimal Living Cells

Artificial minimal living cells and their substructures are quantum mechanically self-assembling due to competition of weak electrostatic forces and weak attraction Van der Waals dispersion forces, and hydrogen bonds originated due to electron correlation interactions among biological and water molecules. The best available method to simulate in minimal cells these weak electrostatic and Van der Waals dispersion forces, and hydrogen bonds is to perform quantum mechanical non-local density functional potential calculations of artificial minimal living cells consisting of around 400 atoms. The cell systems studied are based on peptide nucleic acid and are 3.0 – 4.5 nm in diameter. The electron tunneling and associated light absorption of most intense transitions as calculated by the time dependent density functional theory method (TD DFT) differs from spectroscopic experiments by only 0.3 nm, which are within the value of experiment errors. This agreement implies that the quantum mechanically self-assembled structure of artificial minimal living cells very closely approximate the realistic ones. Analysis of TD DFT method calculated absorption spectrum and images of electron transfer trajectories in the different excited states allow to separate two different logically controlled functions of molecular device consisting of guaninecytosine-PNA-1,4-dihydroquinoxaline-1,4-bis(N,N-dimethylamino)naphthalene supermolecule and Van der Waals bonded precursor of fatty acid molecule. These two different logically controlled functions of artificial minimal living cells are: 1) initiation of metabolic fatty acid production in five excited states or 2) initiation of gene dehybridization in one excited state. This designed supermolecule works in the artificial minimal cell as molecular electronics classic OR logic function (Boolean logics OR gate). TD DFT were used to investigate various photoactive systems of artificial minimal cells based on Ru(bipyridine)32+ sensitizer. The electron correlation interactions associated with the weak Van der Waals and hydrogen bonds increase due to the addition of a polar solvent (water or methanol) shells. The addition of precursor fatty acid or cytosine molecules plays a role in the quantum mechanical interaction based self-assembly of the photosynthetic center and the functioning of the photosynthetic processes of the artificial minimal cells. As a result the associated nonlinear quantum interactions compress the overall system resulting in an even smaller gap between the HOMO and LUMO electron energy levels and photoexcited electron tunneling occurs from the sensitizer (covalently bonded 8-oxo-guanine molecule to the [Ru(bpy)2(4-Bu-4'-Me-2,2'-bpy)]2+) to pFA molecule (notation used: Me = methyl; Bu = butyl; bpy = bipyridine). The electron tunneling and associated light absorption for the most intense transitions as calculated by the TD DFT method are within the values of experiment errors.

Question 9: Quantum Self-Assembly and Photoinduced Electron Tunneling in Photosynthetic Systems of Artificial Minimal Living Cells

Natural and artificial living cells and their substructures are self-assembling, due to electron correlation interactions among biological and water molecules, which lead to attractive dispersion forces and hydrogen bonds. Dispersion forces are weak intermolecular forces that arise from the attractive force between quantum multipoles. A hydrogen bond is a special type of quantum attractive interaction that exists between an electronegative atom and a hydrogen atom bonded to another electronegative atom; and this hydrogen atom exist in two quantum states. The best method to simulate these dispersion forces and hydrogen bonds is to perform quantum mechanical non-local density functional potential calculations of artificial minimal living cells consisting of around 1,000 atoms. The cell systems studied are based on peptide nucleic acid and are 3.0-4.2 nm in diameter. The electron tunneling and associated light absorption of the most intense transitions, as calculated by the time dependent density functional theory method, differs from spectroscopic experiments by only 0.2-0.3 nm, which is within the value of experiment errors. This agreement implies that the quantum mechanically self-assembled structures of artificial minimal living cells very closely approximate realistic ones.

Binding energies in benzene dimers: Nonlocal density functional calculations

The interaction energy and minimum energy structure for different geometries of the benzene dimer have been calculated using the recently developed nonlocal correlation energy functional for calculating dispersion interactions. The comparison of this straightforward and relatively quick density functional based method with recent calculations provides a promising first step to elucidate how the former, quicker method might be exploited in larger more complicated biological, organic, aromatic, and even infinite systems such as molecules physisorbed on surfaces and van der Waals crystals.

Anhydrous and Water-Assisted Proton Mobility in Phosphotungstic Acid

Nonlocal gradient-corrected density functional theoretical calculations were used to determine the energetics associated with proton migration in phosphotungstic acid. The activation energy for anhydrous proton hopping between two oxygen atoms on the exterior of the molecular Keggin unit was calculated to be 103.3 kJ mol(-1). The quantum-tunneling effect on the rate of proton movement was determined using semiclassical transition-state theory and was found to be a major contributor to the overall rate of proton movement at temperatures below approximately 350 K. The adsorption of water on an acidic proton decreases the activation barrier for hopping to 11.2 kJ mol(-1) by facilitating proton transfer along hydrogen bonds. The overall rate constant for proton hopping was determined as a function of temperature and water partial pressure. Small amounts of water greatly enhance the overall rate of proton movement.

Hydrogenation of Two-Electron Mixed-Valence Iridium Alkyl Complexes

Two-electron mixed-valence complexes of the general formula (tfepma)(3)Ir(2)(0,II)RBr [tfepma = bis(bis(trifluoroethoxy)phosphino)methylamine, MeN[P(OCH(2)CF(3))(2)](2), and R = CH(3) (2), CH(2)C(CH(3))(3) (3)] have been synthesized and structurally characterized and their reactivity with H(2) investigated. Hydrogenation of 2 and 3 proceeds in a cascade reaction to produce alkane upon initial H(2) addition, followed by the formation of the Ir(2)(I,III) binuclear trihydride-bromide complex (tfepma)(3)Ir(2)(I,III)H(3)Br (4) upon the incorporation of a second molecule of H(2). Hydrogenation of two-electron mixed-valence di-iridium alkyl complexes is examined with nonlocal density-functional calculations. H(2) attacks the Ir(II) metal center prior to alkyl protonation to produce an eta(2)-H(2) complex. Transition states link all intermediates to a complex that has the same regiochemistry as the crystallographically determined final product. Calculated atomic charges suggest that the second H(2) molecule is homolytically cleaved within the di-iridium coordination sphere and that a hydrogen atom migrates across the intact Ir-Ir metal bond. These results are consistent with the emerging trend that two-electron mixed-valence cores manage the two-electron chemistry of substrates with facility when hydrogen is the atom that migrates between metal centers.

Cooperative bimetallic reactivity: hydrogen activation in two-electron mixed-valence compounds.

Reversible dihydrogen uptake by a two-electron mixed-valence di-iridium complex is examined with nonlocal density-functional calculations. Optimized metrics compare favorably with crystal structures of isolated species, and the calculated activation enthalpy of acetonitrile exchange is accurate within experimental error. Dihydrogen attacks the Ir(2) core at Ir(II); the Ir(0) center is electronically saturated and of incorrect orbital parity to interact with H(2). Isomeric eta(2)-H(2) complexes have been located, and harmonic frequency calculations confirm these to be potential energy minima. A transition state links one such complex with the final dihydride; calculated atomic charges suggest a heterolytic H(2) bond scission within the di-iridium coordination sphere. This investigation also establishes a ligand-design criterion for attaining cooperative bimetallic reactivity, namely, that the supporting ligand framework has sufficient mechanical flexibility so that the target complex can accommodate the nuclear reorganizations that accompany substrate activation.

N 2 sorption scanning behavior of SBA-15 porous substrates

The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in principle, a very adequate experimental system to analyze the N 2 sorption scanning behavior according to the theorems of the Independent Domain Theory of Sorption Hysteresis. In this work, SBA-15 materials of two general types have been synthesized: (i) substrates made by hexagonal arrays of cylinders of about the same sizes, and (ii) solids including a relatively wide distribution of cylindrical pores of varying diameters arranged in a fairly distorted hexagonal packing. Boundary and primary N 2 sorption scanning curves have been experimentally determined on all these materials in order to observe the possible irruption of cooperative processes during the filling and emptying of tubular SBA-15 pores. Pore domain complexion diagrams (i.e. graphs indicating the amount of pore sizes that are occupied by condensate or vapor) have been constructed by means of Non-Local Density Functional Theory calculations of the pore-size distributions that are incumbent to both boundary and primary scanning curves. It has been inferred that two phenomena branded as advanced adsorption and single pore-blocking can take place in reason of the undulating cross-section nature of some SBA-15 tube materials; the extents of the above irreversible processes depending on the degree of sinuosity of the cross sections of the tubes.

N2 sorption scanning behavior of SBA-15 porous substrates

The independent non-intersecting cylindrical pore nature of SBA-15 materials represents, in principle, a very adequate experimental system to analyze the N2 sorption scanning behavior according to the theorems of the Independent Domain Theory of Sorption Hysteresis. In this work, SBA-15 materials of two general types have been synthesized: (i) substrates made by hexagonal arrays of cylinders of about the same sizes, and (ii) solids including a relatively wide distribution of cylindrical pores of varying diameters arranged in a fairly distorted hexagonal packing. Boundary and primary N2 sorption scanning curves have been experimentally determined on all these materials in order to observe the possible irruption of cooperative processes during the filling and emptying of tubular SBA-15 pores. Pore domain complexion diagrams (i.e. graphs indicating the amount of pore sizes that are occupied by condensate or vapor) have been constructed by means of Non-Local Density Functional Theory calculations of the pore-size distributions that are incumbent to both boundary and primary scanning curves. It has been inferred that two phenomena branded as advanced adsorption and single pore-blocking can take place in reason of the undulating cross-section nature of some SBA-15 tube materials; the extents of the above irreversible processes depending on the degree of sinuosity of the cross sections of the tubes.

Excited-State Distortion of Rhenium(III) Sulfide and Selenide Clusters

The deactivation process for the excited-state decay of hexanuclear rhenium(III) sulfide and selenide clusters, [Re6S8]2+ and [Re6Se8]2+, has been identified from temperature-dependent photophysical measurements and electronic structure calculations. Plots of ln knr vs T-1, where knr is the nonradiative decay rate constant, yield activation energies for nonradiative energy dissipation ranging from 1221 to 1933 cm-1, after allowing for low-temperature tunneling. Nonlocal density-functional calculations, with varying proportions of Hartree−Fock exchange, well approximate the experimental 9 K emission maximum of the representative compound (Bu4N)4[Re6S8Cl6]. The adiabatic potential energy surface of the luminescent, lowest energy triplet excited state is substantially fixed by symmetry, and the calculations provide quantitative characterization of its features. Notably, the experimental activation energies for a D4h(3B1g) → Oh(1A1g) excited-state decay process have the same magnitude as zero-point corrected energies of well-defined PES local maxima. The results herein further contribute to rhenium chalcogenide clusters' stature as the most thoroughly characterized luminescent metal−metal bonded clusters known.

Rapid analysis of phase behavior with density functional theory. II. Capillary condensation in disordered porous media

For some time, there has been interest in understanding adsorption and capillary condensation in disordered porous media from a molecular perspective. It has been documented that the free energy landscape in these systems is complex with many metastable states. In this paper we explore the complexity of adsorption and capillary condensation in several simple models of disordered porous media constructed with parallel cylindrical fibers. We present nonlocal density functional theory calculations on a Lennard-Jones model fluid adsorbing in these porous materials coupled with the arclength continuation and phase transition tracking algorithms we presented in Paper I of this series. The arclength continuation algorithm allows us to trace out all the possible states between vapor-filled and liquid-filled pores. We find that capillary condensation is likely to occur in stages at high temperatures and strong wall–fluid interactions while the condensation occurs as a single transition at low temperatures and weak wall–fluid interactions. This paper also compares the extent of hysteresis on adsorption and desorption, discusses the validity of the Gibbs adsorption equation, and considers application of simple pore models in predicting the complexity of phase diagrams in disordered porous media.

A First Principles Study of Carbon−Carbon Coupling over the {0001} Surfaces of Co and Ru

The coupling of CH (methylidyne) and CH2 (methylene), to form CHCH2 (vinyl), over Co and Ru surfaces has been studied with the nonlocal gradient-corrected density functional theory slab calculations. The results show that this reaction is slightly exothermic on Co while endothermic on Ru within a (2 × 2) surface unit cell. Transition states were isolated on both surfaces, and the reaction barriers were found to be 55.9 and 116.5 kJ/mol on Co and Ru, respectively. The structures of the transition state on the two metal surfaces are similar; both involve the formation of a multicentered bond.

A Periodic Density Functional Theory Study of the Dehydrogenation of Methanol over Pt(111)

Nonlocal gradient-corrected periodic density functional theory calculations were used to examine the dehydrogenation of methanol to CO over the Pt(111) surface. Two decomposition routes were examinedone involving the activation of the O−H bond of methanol to form the methoxide intermediate and the other involving C−H bond activation to form the hydroxymethyl intermediate. These intermediates can subsequently react to form formaldehyde, formyl, and finally CO on the surface. Although these pathways are interesting because of their potential relevance for methanol fuel cells, under UHV conditions, we find that methanol will more likely desorb than react on Pt(111). The barriers for C−H and O−H bond activation were found to be much higher than the measured heat of desorption. This is consistent with experimental evidence. Our results indicate that methanol adsorbs weakly at the atop site on Pt. At 25% surface coverage, the hydroxymethyl (CH2OH), methoxide (CH3O), formaldehyde (HCHO), and formyl (HCO) species...

Model potential nonlocal density functional calculations of small cobalt clusters, Co n, n⩽5

The results of self-consistent nonlocal generalized gradient approximation (GGA) density functional calculations are reported for small cobalt clusters Co n (2⩽n⩽5) . An emphasis is made on a proper treatment of exchange and correlation effects. The enhancement of the magnetic moments as well as the bonding properties of these clusters are discussed in terms of the cluster size and symmetry. We compare some results from deMon–KS (D) module release 3.2 and the computational scheme of Sambe–Felton and Dunlap (SFD).

Main group monocarbonyls

Non-local density functional calculations are used to investigate the properties and bonding of the monocarbonyl complexes of the s- and p-block main group elements of the first five periods. There is good agreement between the calculated properties and those of experimentally observed monocarbonyls. Predictions of the vibrational frequencies and stabilities of, as yet, unknown complexes are given. A bonding model involving synergistic M←CO σ-donation and M→CO π-backbonding analogous to that used for transition metal carbonyls is described. It is used to describe the variations in the properties of these systems within the groups and periods of the main group elements. The limitations of this description when applied to electron-poor and electron-rich main group MCO molecules are outlined. The bonding in the s-block monocarbonyls is dominated by the repulsive σ-interaction between the metal s-electrons and the CO lone pair. In the monocarbonyls of electronegative elements, there is significant involvement of the filled π-level on CO.

A theoretical study on the aromaticity of thiadiazoles and related compounds

Three different approaches have been considered to determinate the aromatic character of isomeric thiadiazoles. Several indices have been obtained from calculations made in the context of nonlocal (B3LYP) density functional calculations. The results were compared with the existing experimental evidence for analog heterocycles. It is concluded that the thiadiazoles are aromatic compounds, with a larger aromatic character than pyrrole, thiophene and furan. The monoxide and dioxide derivatives are shown to be non-aromatic or antiaromatic molecules.

Hartree–Fock (HF) and local and nonlocal density functional (DFT) calculations of the molecular structure of isomeric thiadiazolidines

A detailed study of isomeric thiadiazolidine molecules has been done using HF and local and nonlocal density functionals and optimizing the molecular geometries by means of the gradient technique. A charge sensitivity analysis of the studied molecules has been performed by resorting to density functional theory, obtaining several sensitivity coefficients such as the molecular energy, net atomic charges, global and local hardness, global and local softness and Fukui functions. Molecular structure and electronic properties calculated using the hybrid B3LYP proved to be the most accurate. It is concluded that the 1,3,4-thiadiazolidine would be the most reactive of the four isomers.