Local order in fully deuterated liquid N-methylacetamide (C3D7NO) as studied by neutron diffraction and density-functional theory calculations

A structural investigation of fully deuterated liquid N-methylacetmide (NMAd7) is performed at 308 K and atmospheric pressure by using neutron diffraction together with density-functional theory (DFT). The analysis of experimental data yields the total structure factor SM(Q), the molecular form factor F1(Q), and the distinct pair correlation function gL(r). The DFT calculations are performed to study the relative stability of the two possible isomers (trans and cis) and to examine some possible clusters recently published that may describe the intermolecular arrangement in the liquid state. Neutron measurements can be interpreted in terms of trans linear trimer (T1) and cis cyclic trimer (T2) where the total number of hydrogen bonds is respectively equal to two and three. The theoretical structure factors obtained on the basis of intermolecular arrangements agree fairly well with the experimental one beyond Q = 2 A-1. All through the study, a comparison is made with complementary X-ray results.     

Time-dependent density-functional theory calculations of triplet-triplet absorption

We present density-functional theory calculations of triplet-triplet absorption by three different approaches based on time-dependent density-functional theory (DFT): unrestricted DFT linear response, open-shell restricted DFT linear response applied to the triplet state, and quadratic response with triplet excitations applied to the ground state. Comparison is also made with corresponding results obtained by Hartree-Fock and multiconfiguration self-consistent-field response theory. Two main conclusions concerning triplet-triplet transitions are drawn in this study: First, the very good agreement between unrestricted and restricted DFT results indicates that spin contamination of the triplet state is not a serious problem when computing triplet-triplet spectra of common organic molecules. Second, DFT response calculations of triplet-triplet transitions can be affected by triplet instability problems, especially for the combination of DFT quadratic response with functionals containing fractional exact Hartree-Fock exchange.     

Intermolecular association in liquid N-methylacetamide as studied by x-ray scattering

A structural investigation of liquid N methylacetamide was performed at 308 K using x-ray scattering. To extract the molecular form factor F1(q), the geometry of the conformer which has been found in the crystal is considered. The intermolecular structure function DM(q) is interpreted in terms of H-bonding interactions. The crystal N...O distance is taken into accounted and the number of H bond(s) is assumed to be, respectively, equal to one and two. The liquid structure can be described by a linear dimer or chainlike trimer similar to the ones existing in the crystal. The structure factors SM(q) extracted from these clusters fairly agree with the experimental one beyond q=2.5 A(-1).     

Photoabsorption in sodium clusters on the basis of time-dependent density-functional theory

The photoabsorption spectra of a continuous series of Na(n) clusters (n相似文献   

Structure and stability of Al-doped boron clusters by the density-functional theory

The geometries, stabilities, and electronic properties of Bn and AlBn clusters, up to n=12, have been systematically investigated by using the density-functional approach. The results of Bn clusters are in good agreement with previous conclusions. When the Al atom is doped in Bn clusters, the lowest-energy structures of the AlBn clusters favor two-dimensional and can be obtained by adding one Al atom on the peripheral site of the stable Bn when n相似文献   

First-principles density-functional theory calculations of electron-transfer rates in azurin dimers

We have conceived and implemented a new method to calculate transfer integrals between molecular sites, which exploits few quantities derived from density-functional theory electronic structure computations and does not require the knowledge of the exact transition state coordinate. The method uses a complete multielectron scheme, thus including electronic relaxation effects. Moreover, it makes no use of empirical parameters. The computed electronic couplings can then be combined with estimates of the reorganization energy to evaluate electron-transfer rates that are measured in kinetic experiments: the latter are the basis to interpret electron-transfer mechanisms. We have applied our approach to the study of the electron self-exchange reaction of azurin, an electron-transfer protein belonging to the family of cupredoxins. The transfer integral estimates provided by the proposed method have been compared with those resulting from other computational techniques, from empirical models, and with available experimental data.     

Photocatalytic properties of graphene-supported titania clusters from density-functional theory

Density-functional theory calculations of (TiO₂)_n clusters (n = 1–5) in the gas phase and adsorbed on pristine graphene as well as graphene quantum dots are presented. The cluster adsorption is found to be dominated by van der Waals forces. The electronic structure and in particular the excitation energies of the bare clusters and the TiO₂/graphene composites are found to vary largely in dependence on the size of the respective constituents. This holds in particular for the energy and the spatial localization of the highest occupied and lowest unoccupied molecular orbitals. In addition to a substantial gap narrowing, a pronounced separation of photoexcited electrons and holes is predicted in some instances. This is expected to prolong the lifetime of photoexcited carriers. Altogether, TiO₂/graphene composites are predicted to be promising photocatalysts with improved electronic and photocatalytic properties compared to bulk TiO₂.     

High-pressure study of lithium azide from density-functional calculations

The structural, electronic, optical, and vibrational properties of LiN(3) under high pressure have been studied using plane wave pseudopotentials within the generalized gradient approximation for the exchange and correlation functional. The calculated lattice parameters agree quite well with experiments. The calculated bulk modulus value is found to be 23.23 GPa, which is in good agreement with the experimental value of 20.5 GPa. Our calculations reproduce well the trends in high-pressure behavior of the structural parameters. The present results show that the compressibility of LiN(3) crystal is anisotropic and the crystallographic b-axis is more compressible when compared to a- and c-axes, which is also consistent with experiment. Elastic constants are predicted, which still awaits experimental confirmation. The computed elastic constants clearly show that LiN(3) is a mechanically stable system and the calculated elastic constants follow the order C(33) > C(11) > C(22), implying that the LiN(3) lattice is stiffer along the c-axis and relatively weaker along the b-axis. Under the application of pressure the magnitude of the electronic band gap value decreases, indicating that the system has the tendency to become semiconductor at high pressures. The optical properties such as refractive index, absorption spectra, and photoconductivity along the three crystallographic directions have been calculated at ambient as well as at high pressures. The calculated refractive index shows that the system is optically anisotropic and the anisotropy increases with an increase in pressure. The observed peaks in the absorption and photoconductivity spectra are found to shift toward the higher energy region as pressure increases, which implies that in LiN(3) decomposition is favored under pressure with the action of light. The vibrational frequencies for the internal and lattice modes of LiN(3) at ambient conditions as well as at high pressures are calculated from which we predict that the response of the lattice modes toward pressure is relatively high when compared to the internal modes of the azide ion.     

A computational investigation of copper-doped germanium and germanium clusters by the density-functional theory

The geometries, stabilities, and electronic properties of Ge(n) and CuGe(n) (n = 2-13) clusters have been systematically investigated by using density-functional approach. According to optimized CuGe(n) geometries, growth patterns of Cu-capped Ge(n) or Cu-substituted Ge(n+1) clusters for the small- or middle-sized CuGe(n) clusters as well as growth patterns of Cu-concaved Ge(n) or Ge-capped CuGe(n-1) clusters for the large-sized CuGe(n) clusters are apparently dominant. The average atomic binding energies and fragmentation energies are calculated and discussed; particularly, the relative stabilities of CuGe10 and Ge10 are the strongest among all different sized CuGe(n) and Ge(n) clusters, respectively. These findings are in good agreement with the available experimental results on CoGe10- and Ge10 clusters. Consequently, unlike some transition metal (TM)Si12, the hexagonal prism CuGe12 is only low-lying structure; however, the basket-like structure is located as the lowest-energy structure. Different from some TM-doped silicon clusters, charge always transfers from copper to germanium atoms in all different sized clusters. Furthermore, the calculated highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) gaps are obviously decreased when Cu is doped into the Ge(n) clusters, together with the decrease of HOMO-LUMO gaps, as the size of clusters increases. Additionally, the contribution of the doped Cu atom to bond properties and polarizabilities of the Ge(n) clusters is also discussed.     

Hydrogen-bonding interactions in acetic acid monohydrates and dihydrates by density-functional theory calculations

Equilibrium structures and the respective binding energies of acetic acid monohydrates and dihydrates have been determined by density-functional theory calculations with different basis sets, including 6-31+G(3d,p), 6-311++G(d,p), and 6-311++G(3df,3pd). Given that the C=O and OH groups in acetic acid provide the predominant hydrogen-bonding interactions with water, six stable conformer structures have been found each for the monohydrate and syn-dihydrate. Of the three syn- and three anti-conformers of acetic acid with water, the most stable monohydrate structure is found to be that of the syn-conformer bonding with water in a cyclic double H-bonded geometry. Similarly, the syn-conformer bonding with two water molecules in a cyclic double H-bonded geometry has also been determined to be the most stable among the six plausible structures for the syn-dihydrate. Frequency analysis of the stable conformers has been performed and the vibrational spectra of the most stable monohydrate and dihydrate structures are compared with the experimental gas-phase and matrix data. Furthermore, the calculated binding energies between an acetic acid and a water molecule for both monohydrate and dihydrate are larger than that between two water molecules, which supports our recent experimental observation of coevaporation of acetic acid with water upon annealing acetic acid on ice.     

Interaction energies in hydrogen-bonded systems: a testing ground for subsystem formulation of density-functional theory

The formalism based on the total energy bifunctional (E[rhoI,rhoII]) is used to derive interaction energies for several hydrogen-bonded complexes (water dimer, HCN-HF, H2CO-H2O, and MeOH-H2O). Benchmark ab initio data taken from the literature were used as a reference in the assessment of the performance of gradient-free [local density approximation (LDA)] and gradient-dependent [generalized gradient approximation (GGA)] approximations to the exchange-correlation and nonadditive kinetic-energy components of E[rhoI,rhoII]. On average, LDA performs better than GGA. The average absolute error of calculated LDA interaction energies amounts to 1.0 kJ/mol. For H2CO-H2O and H2O-H2O complexes, the potential-energy curves corresponding to the stretching of the intermolecular distance are also calculated. The positions of the minima are in a good agreement (less than 0.2 A) with the reference ab initio data. Both variational and nonvariational calculations are performed to assess the energetic effects associated with complexation-induced deformations of molecular electron densities.     

Geometric and magnetic properties of Pt clusters supported on graphene: relativistic density-functional calculations

The geometric and magnetic structures of small Pt(n) clusters (n = 1 - 5) supported on a graphene layer have been investigated using ab initio density functional calculations including spin-orbit coupling. Pt-Pt interactions were found to be much stronger than the Pt-C interactions promoting the binding to the support. As a consequence, the equilibrium structure of the gas-phase clusters is preserved if they are deposited on graphene. However, the clusters bind to graphene only via at most two Pt-C bonds: A Pt(2) dumbbell prefers an upright position, the larger clusters are bound to graphene only via one edge of the planar cluster (Pt(3) and Pt(5)) or via two terminal Pt atoms of a bent Pt(4) rhombus. Evidently, the strong buckling of the graphene layer induced by the Pt-C bonds prevents the formation of a larger number of cluster-support bonds. As the local spin and orbital magnetic moments are quenched on the Pt atoms forming Pt-C bonds, the magnetic structure of the supported clusters is much more inhomogeneous as in the gas-phase. This leads to noncollinear magnetic structures and a strongly reduced magnetic anisotropy energy.     

Interaction potentials from periodic density-functional theory calculations: molecular-dynamics simulations of Au clusters deposited on the TiN (001) surface

Molecular-dynamics simulations of gold particles deposited on a TiN (001) surface have been accounted for through classical pair potentials describing the atom force field. The interaction between Ti-N, Ti-Ti, N-N, Au-Au, Au-Ti, and Au-N pairs was estimated by following a procedure in which the interaction energy between two sets of atoms is estimated from density-functional calculations performed with periodic boundary conditions using plane waves as basis set. The pair potentials were expressed as the sum of two contributions: long range in a Coulomb form and a short-range term, which included the rest of the energy contributions. Simulations of the TiN (001) isolated surface reproduced the already described surface relaxation, with a rippling parameter in agreement with that found from a purely first-principles approach. Simulations of gold deposition on such surfaces showed the formation of metal clusters with well-defined fcc structure and epitaxially grown.     

Reaction dynamics study of O- + C2H2 with crossed beams and density-functional theory calculations

The reactions between O(-) and C(2)H(2) have been studied using the crossed-beam technique and density-functional theory (DFT) calculations in the collision energy range from 0.35 to 1.5 eV (34-145 kJmol). Both proton transfer and C-O bond formation are observed. The proton transfer channel forming C(2)H(-) is the dominant pathway. The center-of-mass flux distributions of the C(2)H(-) product ions are highly asymmetric, with maxima close to the velocity and direction of the precursor acetylene beam, characteristic of direct reactions. The reaction quantitatively transforms the entire reaction exothermicity into internal excitation of the products, consistent with mixed energy release in which the proton is transferred in a configuration in which both the breaking and the forming bonds are extended. The C-O bond formation channel producing HC(2)O(-) displays a distinctive kinematic picture in which the product distribution switches from predominantly forward scattering with a weak backward peak to sideways scattering as the collision energy increases. At low collision energies, the reaction occurs through an intermediate that lives a significant fraction of a rotational period. The asymmetry in the distribution leads to a lifetime estimate of 600 fs, in reasonable agreement with DFT calculations showing that hydrogen-atom migration is rate limiting. At higher collision energies, the sideways-scattered products arise from repulsive energy release from a bent transition state.     

Theoretical study on cooperative and extra-additive behavior of hydrogen-bonded clusters

The stabilization energy △ E(n) and four typical properties of hydrogen bond F-H…F in chain-like and cyclic (HF)n clusters (n = 1-5) have been calculated using MP2 and three DF levels of theory with the Gaussian 98 program, and 6-31++G** bases set. The results demonstrate that the extra-additive or cooperative behavior in (HF)n clusters is very obvious. In addition, we studied much larger chain-like (HF)n (n = 6, 9, 12,18, 24) clusters using one of these DF methods.     

X-ray scattering study of molecular order in a liquid crystal-side group polysiloxane

The molecular structure of a polysiloxane with phenyl benzoate mesogenic side groups was investigated in an x-ray scattering study in the partially crystalline, smectic and nematic phase, and in the melt. In the crystalline phase polymer molecules have the form of straight ribbons with a double-comb-conformation. A bilayer structure is built up by regular stacking. Layers are the dominating structure element not only in the crystalline and smectic phase, but also in the nematic phase, and even in the isotropic melt. Layers are planar in the smectic phase and curved in the nematic phase, with an asymmetric distribution of the normal vectors about the director. In the isotropic melt there is evidence for the occurrence of clusters with layer-like short-range order.     

Quantum chemical density-functional theory calculations of the structures of defect C60 with four vacancies

Quantum chemical density-functional theory (DFT) calculations have been carried out for the six isomers obtained by removing four adjacent atoms from C60. The most stable isomer consists of twelve 5-member and eighteen 6-member rings, indicating that the removal of some atoms from C60, which contains twelve 5-member rings and twenty 6-member rings, does not always generate larger holes. Each of the other five isomers contains at least one 4-member ring and one larger ring (7-, 8-, 9-, or 10-member ring) besides the 5- and 6-member rings. All isomers have similar structures for singlet and triplet spin multiplicities but with different stabilities. The ground states for two of the isomers are triplets, whereas the ground states for the other isomers are singlets. Furthermore, a comparison between the various isomers allowed one to examine the effect of the structure on the stability of fullerene cages.