CaSO4 and its pressure-induced phase transitions. A density functional theory study

Theoretical investigations concerning possible calcium sulfate, CaSO(4), high-pressure polymorphs have been carried out. Total-energy calculations and geometry optimizations have been performed by using density functional theory at the B3LYP level for all crystal structures considered. The following sequence of pressure-driven structural transitions has been found: anhydrite, Cmcm (in parentheses the transition pressure) → monazite-type, P2(1)/n (5 GPa) → barite-type, Pnma (8 GPa), and scheelite-type, I4(1)/a (8 GPa). The equation of state of the different polymorphs is determined, while their corresponding vibrational properties have been calculated and compared with previous theoretical results and experimental data.     

Method of local-scaling transformations and density functional theory in quantum chemistry. III. The energy density functional: Spin-restricted approach

The rigorous derivation of the energy density functional is proposed within the framework of the spinfree, or spin-restricted formulation of the energy density functional theory. It is shown particularly that the kinetic energy density functional is given by a sum of the Weizsacker term and the so-called “modified” Thomas–Fermi one. The variational principle is formulated for the energy density functional theory in terms of the Euler–Lagrange equation, and the virial theorem is proposed.     

Gas-phase acidity of D-glucose. A density functional theory study

The gas-phase acidity of D-glucopyranose was studied by means of B3LYP calculations combined with 6-31G(d,p) or 6-31+G(d,p) standard basis sets. For each anomer, deprotonation of the various primary and secondary hydroxyl groups was considered. As in solution, the anomeric hydroxyl is found to be the most acidic for both anomers, but only when the 6-31+G(d,p) basis set is used for geometry optimization. Deprotonation of the anomeric hydroxyl induces an important C(1)--O endocyclic bond elongation and subsequently promotes an energetically favored ring-opening process as attested by the very small calculated activation barriers. The results also suggest that interconversion between the various deprotonated alpha- and beta-anomers may easily occur under slightly energetic conditions. B3LYP/6-311+G(2df,2p) calculations led to the an absolute gas-phase acidity of deltaacidGo(298)(alpha-D-glucose) = 1398 kJ mol(-1). This estimate matches well the only experimental value available to date. Finally, this study again confirms that the use of diffuse functions on heavy atoms is necessary to describe anionic systems properly and to achieve good relative and absolute gas-phase acidities.     

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 according to PCM. Also, the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM simulation (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.     

A density functional theory study of the benzene-water complex

The intermolecular interaction of the benzene-water complex is calculated using real-space pseudopotential density functional theory utilizing a van der Waals density functional. Our results for the intermolecular potential energy surface clearly show a stable configuration with the water molecule standing above or below the benzene with one or both of the H atoms pointing toward the benzene plane, as predicted by previous studies. However, when the water molecule is pulled outside the perimeter of the ring, the configuration of the complex becomes unstable, with the water molecule attaching in a saddle point configuration to the rim of the benzene with its O atom adjacent to a benzene H. We find that this structural change is connected to a change in interaction from H (water)/pi cloud (benzene) to O (water)/H (benzene). We compare our results for the ground-state structure with results from experiments and quantum-chemical calculations.     

Silabutadienes. Internal rotations and pi-conjugation. A density functional theory study

The potential energy surfaces (PESs) for internal rotation around the central single bond of nine silabutadienes, which include all possible mono-, di-, tri-, and tetrasilabutadienes, are investigated computationally by using DFT with the B3LYP functional and the 6-311+G(d,p) basis set. For 1-silabutadiene (3), 2-silabutadiene (4), 1,4-disilabutadiene (5), 2,3-disilabutadiene (6), and 1,3-disilabutadiene (7), the s-trans rotamer is the most stable. For 1,2-disilabutadiene (8), 1,2,3-trisilabutadiene (9), and 1,2,4-trisilabutadiene (10), all having a trans-bent SiSi double bond, the most stable conformers are those having an antiperiplanar (ap) structure. For tetrasilabutadiene (11), the global minimum is the gauche rotamer. The internal rotation barriers (RB) (relative to the global minimum) follow the order (kcal/mol) 5 (10.0) > 3 (7.4) > 1,3-butadiene (12, (6.6)) > 10 (4.9) > or = 7 (4.4) > or = 4 (4.0) approximately = 8 (3.9) > 9 (2.7) approximately = 6 (2.6) > 11 (2.4). The barriers are slightly smaller at CCSD(T)/cc-PVTZ, but the trend remains the same. The size of the rotation barrier is mainly dictated by the length of the central single bond; that is, it is the largest for dienes with the shorter C-C central bond (5, 3, and 12), and it is smaller for dienes with the longer Si-C and Si-Si central bonds. The strength of pi-conjugation in the s-trans conformers of silabutadienes was estimated by resonance stabilization energies (RE) calculated by using the Natural Bond Orbital (NBO) and Block Localized Wave function (BLW) methods and bond separation equations. A linear correlation is found between the barrier heights for internal rotation and pi-conjugation energies. The calculated RBs are significantly smaller than the corresponding REs, indicating that pi-resonance energies are not the only factor that dictate the RB, and therefore, RBs, although suitable for estimating trends in pi-conjugation in a series of compounds, cannot be used for estimating absolute resonance energies.     

A density functional theory study of sulfur poisoning

Density functional theory calculations have been used to investigate the chemisorption of H, S, SH, and H(2)S as well as the hydrogenation reactions S+H and SH+H on a Rh surface with steps, Rh(211), aiming to explain sulfur poisoning effect. In the S hydrogenation from S to H(2)S, the transition state of the first step S+H-->SH is reached when the S moves to the step-bridge and H is on the off-top site. In the second step, SH+H-->H(2)S, the transition state is reached when SH moves to the top site and H is close to another top site nearby. Our results show that it is difficult to hydrogenate S and they poison defects such as steps. In order to address why S is poisoning, hydrogenation of C, N, and O on Rh(211) has also been calculated and has been found that the reverse and forward reactions possess similar barriers in contrast to the S hydrogenation. The physical origin of these differences has been analyzed and discussed.     

Electronic structures and reactivities of iodinating agents in the gas phase and in solutions: A density functional study

The electronic and spatial structures of a broad spectrum of neutral compounds with X-Hal (X = N, O, Cl; Hal = Cl, Br, I) bonds and their protonated forms and of different electronic states of triiodide cation, I₃ ⁺, were determined from density functional B3LYP/6 311G* quantum chemical calculations. The effects of the structure of these compounds on the parameters of electrophilic reactivity were revealed and the thermochemical characteristics of homolytic and heterolytic X-Hal bond dissociation and of iodine transfer in hydroxyl-containing solvents were calculated. Due to low homolytic bond dissociation energies of X-I, the formation of molecular iodine and triiodide cation I₃ ⁺ becomes thermodynamically favorable and the cation should act as iodinating agent alternative to acylhypoiodites and N-iodoimides. The solvation effects of MeOH and CH₂Cl₂ on the X-Hal bond homolysis and heterolysis were determined using the PCM model. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1280–1288, August, 2006.     

Exact exchange-only density functional theory by means of local scaling transformations

The density-constrained variation of the kinetic energy of a non-interacting system carried out within the framework of local-scaling transformations is employed in the present work for the purpose of determining exchange-only Kohri Sham orbitals for the beryllium atom as well as the Kohn-Sham exchange-only correlation potential. The starting basis functions are those of Clementi-Roetti and Raffenetti-types. We have also performed an optimization via a densitydriven method. The resulting exchange-only potentials for the examples studied are almost indistinguishable from the Talman-Shadwick potential for the beryllium atom.     

Prediction of high-pressure adsorption equilibrium of supercritical gases using density functional theory

In this paper, we present the results of the prediction of the high-pressure adsorption equilibrium of supercritical gases (Ar, N2, CH4, and CO2) on various activated carbons (BPL, PCB, and Norit R1 extra) at various temperatures using a density-functional-theory-based finite wall thickness (FWT) model. Pore size distribution results of the carbons are taken from our recent previous work,(1,2) using this approach for characterization. To validate the model, isotherms calculated from the density functional theory (DFT) approach are comprehensively verified against those determined by grand canonical Monte Carlo (GCMC) simulation, before the theoretical adsorption isotherms of these investigated carbons calculated by the model are compared with the experimental adsorption measurements of the carbons. We illustrate the accuracy and consistency of the FWT model for the prediction of adsorption isotherms of the all investigated gases. The pore network connectivity problem occurring in the examined carbons is also discussed, and on the basis of the success of the predictions assuming a similar pore size distribution for accessible and inaccessible regions, it is suggested that this is largely related to the disordered nature of the carbon.     

A density functional study on the electronic structures of TiN solid

The electronic structures of TiN bulk have been studied by using different theoretical formalisms, and the DFT method, especially the BLYP method can produce reasonable results. The band structure of TiN (001) surface is also investigated and two a type surface states are presented in our results. The state located at 2.9 eV below EF in angle resolved photoemis-sion in (ARPES) is well reproduced in this work, which consists essentially of 2pz orbital of surface N atom. Another surface state is associated with the bands originated from 3d orbital of surface Ti atom. Furthermore, the elastic constants of TiN are also calculated by using BLYP method.     

Comparative density functional theory study of the structures and properties of metallophthalocyanines of group IV B

Density functional theory (DFT) calculations were carried out to comparatively describe the molecular structures, molecular orbital energy gaps, atomic charges, infrared (IR) and Raman spectra of lead phthalocyaninate (PbPc), tin phthalocyaninate (SnPc), germanium phthalocyaninate (GePc), tin (IV) dichlorophthalocyaninate (PcSnCl₂), and germanium (IV) dichlorophthalocyaninate (PcGeCl₂). The calculated structural data and the simulated IR spectrum of PbPc correspond well with the experimental result. The important effects of axial ligands and ionic radius of metal center to the molecular structures, molecular orbital and atomic charges are described, and the metal-sensitive peaks in the IR and Raman spectra are identified by comparative study of the five complexes with different central metals and axial ligands.     

A density functional theory study on acetylene-functionalized BN nanotubes

Covalent functionalization of a zigzag boron nitride nanotube (BNNT) with acetylene has been investigated by density functional theory in terms of energetic, geometric, and electronic properties. It has been found that the most stable functionalized BNNT is the one in which the acetylene is diffused into the tube wall so that two heptagonal and two pentagonal rings are formed, releasing energy of 1.54 eV. In addition, the effect of substituting the hydrogen atoms of C₂H₂ by different functional groups including –F, –CH₂F, –CN, and –OCH₃ on the geometric and electronic properties of the BNNT has been investigated. The reaction energies are found to be in the range of ?1.03 to ?3.13 eV so that their relative magnitude order is as follows: C₂F₂ > (OCH₃)₂C₂ > C₂H₂ > (CH₂F)₂C₂ > (CN)₂C₂, suggesting that the functionalization energy is increased by increasing the electron donating character of the functional groups. Overall, chemical modification of BNNT by the studied groups results in little changes in electronic properties of the tube and may be an effective way for the purification of BNNTs.     

Mechanism of the cyclopropenone decarbonylation reaction. A density functional theory and transient spectroscopy study

The density functional theory analysis predicts that the thermal decarbonylation of cyclopropenones proceeds by the sequential and regioselective cleavage of both single bonds in a three-membered ring. The initial ring-opening process results in the formation of a reactive zwitterionic intermediate 6, which is separated from the free alkyne and carbon monoxide by a very low energy barrier. Femtosecond pump-probe transient absorption spectroscopy experiments showed that light-induced decarbonylation is also a stepwise process but apparently proceeds on the excited-state surface. The lifetime of the intermediate in the photodecarbonylation reaction is very short and is dependent on substitution and solvent polarity. Thus, bis-p-anisyl-substituted species decays with tau = 0.6 ps, bis-alpha-naphthyl-substituted intermediate has a lifetime of tau = 11 ps, while the bis(2-methoxy-1-naphthyl)-substituted analogue survives for 83 ps in chloroform and for 168 ps in argon-saturated methanol. The loss of carbon monoxide from these intermediates results in the formation of corresponding acetylenes in an electronically ground state. The addition of triplet quenchers does not affect the dynamics or outcome of the reaction.     

左旋葡聚糖热解机理的密度泛函理论研究

采用密度泛函理论B3LYP/6-31++G(d,p)方法,对纤维素热解的主要产物左旋葡聚糖的热解反应机理进行了理论计算分析,设计了四种可能的热解反应途径, 对各种反应的反应物、产物和过渡态的结构进行了能量梯度全优化。计算结果表明,左旋葡聚糖开环成链状中间体时,首先,左旋葡聚糖中的两个半缩醛键C(1)-O(7)和C(6)-O(8)断裂,经过渡态TS₁形成中间体IM₁,同时,C(6)-O(7)结合成键使C(5)-C(6)-O(7)形成环状结构,该反应的能垒较高,为296.53 kJ/mol,然后IM₁经过渡态TS₂转变为中间体IM₂,该反应的能垒为234.09 kJ/mol;对IM₂设计了四条可能的反应路径,反应路径2和3能垒较低,是IM₂最可能的热解反应途径;在反应路径1和4中都包含了脱羰基反应,其反应能垒较高,不易发生。     

Characterization of the high-pressure structural transition and thermodynamic properties in sodium chloride: a computational investigation on the basis of the density functional theory

Using first-principles calculations, the elastic constants, the thermodynamic properties, and the structural phase transition between the B1 (rocksalt) and the B2 (cesium chloride) phases of NaCl are investigated by means of the pseudopotential plane-waves method. The calculations are performed within the generalized gradient approximation to density functional theory with the Perdew-Burke-Ernzerhof exchange-correlation functional. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure Pt between the B1 phase and the B2 phase of NaCl is determined. The calculated values are generally speaking in good agreement with experiments and with similar theoretical calculations. From the theoretical calculations, the shear modulus, Young's modulus, rigidity modulus, and Poisson's ratio of NaCl are derived. According to the quasi-harmonic Debye model, we estimated the Debye temperature of NaCl from the average sound velocity. Moreover, the pressure derivatives of elastic constants, partial differentialC11/partial differentialP, partial differentialC12/partial differentialP, partial differentialC44/partial differentialP, partial differentialS11/partial differential P, partial differentialS12/partial differentialP, and partial differentialS44/partial differentialP, for NaCl crystal are investigated for the first time. This is a quantitative theoretical prediction of the elastic and thermodynamic properties of NaCl, and it still awaits experimental confirmation.     

A density functional theory study of hydrogen adsorption in MOF-5

Ab initio molecular dynamics in the generalized gradient approximation to density functional theory and ground-state relaxations are used to study the interaction between molecular hydrogen and the metal-organic framework with formula unit Zn4O(O2C-C6H4-CO2)3. Five symmetrically unique adsorption sites are identified, and calculations indicate that the sites with the strongest interaction with hydrogen are located near the Zn4O clusters. Twenty total adsorption sites are found around each Zn4O cluster, but after 16 of these are populated, the interaction energy at the remaining four sites falls off significantly. The adsorption of hydrogen on the pore walls creates an attractive potential well for hydrogen in the center of the pore. The effect of the framework on the physical structure and electronic structure of the organic linker is calculated, suggesting ways by which the interaction between the framework and hydrogen could be modified.     

A density functional theory study of copper-catalyzed aziridination of olefins

Density functional theory calculations are reported for the reaction mechanism of selected XCuNHX(X = Cl, Br, I) with olefins to form three-membered ring products. The copper reagents react with olefins via an asynchronous attack on one CH₂ group of ethylene with a relatively low barrier (<78 kJ/mol). These computational results are in good agreement with experimental results, and this suggests that the nitrene transfer process is favored. The BrCuNHBr is found to be the most reactive reagent in the XCuNHX (X = Cl, Br, I) series of reagents. These results are qualitatively consistent with the agreement between copper-catalyzed species character and experimental conditions needed for efficient reaction.     

Density functional theory study of high-pressure behavior of crystalline hexanitrostilbene

A detailed study of the structural, electronic, and absorption properties of crystalline hexanitrostilbene (HNS) under hydrostatic pressure of 0–80 GPa was performed with density functional theory. The results show that the structure is much stiffer in the b and c direction than along the a axis, showing that the compressibility of HNS crystal is anisotropic. As the pressure increases, the band gap gradually decreases. An analysis of density of states shows that the electronic delocalization in HNS gradually increases under the influence of pressure. An understanding of the stabilities of HNS under compression based on the electronic structure shows that an applied pressure increases the impact sensitivity of HNS to detonation initiation. As the pressure increases, HNS has relatively high optical activity. The absorption spectra of HNS at high-pressure display a few, strong bands in the fundamental absorption region.