Ultrasonic and DFT study of intermolecular association through hydrogen bonding in aqueous solutions of d(−)-arabinose

The experimental measurements of density, viscosity and ultrasonic velocity of aqueous d-arabinose solutions were carried out as functions of concentration (0.1 ≤ m [mol kg^? 1] ≤ 1.0) and temperature (303.15 ≤ T [K] ≤ 323.15). The isentropic compressibility (β_s), acoustic impedance (Z), hydration number (H_n), intermolecular free length (L_f), classical sound absorption (α/f²)_class and shear relaxation time (τ) were calculated by using the measured data. These parameters have been interpreted in terms of solute–solvent interactions. The quantum chemical calculations were performed to study the hydrogen bonding in interacting complex formed between α-D-arabinopyranose in ¹C₄ conformation and water molecules. Computations have been done by using Density Functional Theory (DFT) method at B3LYP/6-31+g(d) level of theory to study the equilibrium structure of α-d-arabinose, α-D-arabinopyranose–water interacting complex and vibrational frequencies. The solution phase study was carried out using Onsager's reaction field model in water solvent. The computed and scaled vibrational frequencies are in good agreement with the main features of the experimental spectrum when seven water molecules are considered explicitly with α-D-arabinopyranose in ¹C₄ conformation. The interaction energy (E_total), hydrogen bond lengths and dipole moment (μ_m) of the interacting complex are also presented and discussed with in the light of solute–solvent interactions.     

DFT studies of CNT–functionalized uracil-acetate hybrids

Calculations based on density functional theory (DFT) have been performed to investigate the stabilities and properties of hybrid structures consisting of a molecular carbon nanotube (CNT) and uracil acetate (UA) counterparts. The investigated models have been relaxed to minimum energy structures and then various physical properties and nuclear magnetic resonance (NMR) properties have been evaluated. The results indicated the effects of functionalized CNT on the properties of hybrids through comparing the results of hybrids and individual structures. The oxygen atoms of uracil counterparts have been seen as the detection points of properties for the CNT–UA hybrids.     

Effect of pseudohalides in pentadentate-iron(III) complexes studied by DFT and Mössbauer spectroscopy

Mononuclear iron complexes in which the iron(III) ion is coordinated by a pentadentate Schiff base ligand L⁵ with two phenolate, two imino and one amino group can exhibit a spin crossover. In this contribution experimental results are presented for complexes with cyanate and thiocyanate as co-ligands. Furthermore, theoretical results of quantum chemical calculations of energies and entropies for the low-spin and high-spin state are shown and compared with Mössbauer results. We also demonstrate how the ligand field of the monodentate co-ligand influences the spin crossover energies and entropies in [Fe^IIIL⁵NCY] complexes.     

Graph-theoretical parameters for π-MO calculation of a series of aromatic amines in the light of DFT theory: comparison with experimental CT transition energies

Expressions for the energies of the highest occupied π-molecular orbitals of a series of aromatic amines have been derived in terms of the vertex weight h _N (for amine nitrogen) and edge weight k _C–N (for the C–N bond) by representing the amine molecule in terms of vertex- and edge-weighted graphs. Graph-theoretical methods have been used to evaluate the quantities involved in such expressions. The HOMO energies of the amines calculated by density functional theory using the 6-31++G** basis set have been correlated with these expressions to estimate the perturbational parameter h _N and the Coulomb integral α. The acceptability of the estimated values of α and h _N has been tested by their ability to explain the experimentally observed trends in the CT transition energies of a series of charge-transfer complexes of amines with tetracyanoethylene. An important structural feature, namely rotation of the –NH₂ group about the C–N bond due to steric repulsion with the nearest H-atom in the case of 1-amino compounds, has been observed by such a correlation. The result agrees well with the DFT-optimized geometries of the structures.     

Comparison of α CH and CF activation in alkyl transition metal complexes: a DFT and CASSCF study

DFT (B3PW91) and CASSCF calculations have been carried out to study the relative α migratory abilities of H and F in alkyl transition metal complexes. It is shown that the activation energy is considerably lower to migrate H than F, whereas the energies of reaction are similar for the two reactions. A study of the electron configurations and the orbitals describing these configurations shows that the high activation energy for F is due to a 4-electron repulsion between an F lone pair and the occupied Ru=C π orbital.     

DFT study on the structure and spectra of GasP5 cluster

In this paper, possible structures of GasP5 cluster were optimized by using density functional method with generalized gradient correction （B3LYP）. The electronic structure of the isomers with lower energy was studied. The most stable structure obtained for GasP5 is a distorted pentaprism. The Ga-P bond formed in the cluster is strongly ionic. Based on NBO analysis, an average value of 0.59 electron transfers from Gallium to Phosphorus. The bond length 2.33-2.43 is around the value in bulk GaP. The HOMO-LUMO gap is about 2.2 eV. The dipole moment and polarizability are calculated, and the IR and Raman spectra are also presented.     

Atomistic details of precipitates in lean Al–Mg–Si alloys with trace additions of Ag and Ge studied by HAADF-STEM and DFT

Abstract

Bonding energies and volume misfits for alloying elements and vacancies in multicomponent Al–Mg–Si alloys have been calculated using density functional theory (DFT). A detailed atomic scale analysis has been done for characteristic precipitate structures, using high-angle annular dark-field scanning transmission electron microscopy. Two new stacking configurations of the important strengthening phase β′′ were discovered in the Ge-added alloy. All three stacking variations were found to be energetically favourable to form from DFT calculations. The second stacking configuration, β₂′′, contains vacated columns in its unit cell, consequently requiring less solute to create the same volume fraction of precipitate needles. DFT suggests a lower formation enthalpy per atom for β₂′′ when Si is exchanged with Ge. In the alloy containing Ag additions, a new Q’/C-like local configuration containing Ag instead of Cu was discovered, also this phase was deemed energetically favourable from DFT.     

Assessment of DFT functionals with fluorine–fluorine coupling constants†

Density functional theory (DFT) calculations of nuclear magnetic resonance (NMR) spin–spin coupling constants (SSCCs) provide an important contribution for understanding experimentally observed values. It is known that calculated SSCCs using DFT methods correlate well with those experimentally measured. Unlike most of SSCCs, in fluorine compounds, fluorine–fluorine SSCC J_FF shows that the Fermi contact (FC) term is not dominant, particularly for J_FF in polyfluorinated organic molecules. In order to devise a DFT approach that would correctly reproduce the variation of SSCCs within a series of fluorine compounds, we test several DFT-based approaches, using different exchange and correlation functionals. Isotropic contributions to NMR fluorine–fluorine coupling constants (FC, spin-dipolar, SD, paramagnetic spin-orbit, PSO, and diamagnetic spin-orbit, DSO) have been calculated. Results show that DFT methods give appropriate values for ⁿJ_FF (n = 4 to 7), while for geminal and vicinal J_FF present large deviations from experimental values. For the latter SSCCs (²J_FF and ³J_FF), the four contributions (FC, SD, PSO and DSO) are analysed as a function of the local and nonlocal exchange in 1,1- and 1,2-difluoroethylene. Although FC term is not dominant for these SSCCs, the variation of this contribution with exchange is remarkable. On the other hand, SD and PSO contributions can be suitably computed without and with exact exchange, respectively.     

Functionalization of pentagon–pentagon edges of fullerenes by cyclic polysulfides: A DFT study

We have performed a computational study to investigate the cyclosulfurization of the pentagon–pentagon (p–p) junctions in the non-IPR fullerenes C₆₀(D₃) and C₇₀(C_2v), and also Stone-Wales defective C₆₀ fullerene. Our results indicate the exothermic character of cyclosulfurization processes which can be related to the increase of pyramidalization angle (spherical excesses) and p characters of natural hybrid orbitals of C atoms at the p–p junctions. In fact these lead to the structural strain relief and stability of the cyclosulfurization derivatives of the non-IPR fullerenes. Moreover, the cyclosulfurization reaction of p–p bonds on the C₇₀(C_2v) is more energetically favorable than that of C₆₀(D₃), due to the higher curvature of carbon sites and the larger values of the p characters of natural hybrid orbitals in the C₇₀(C_2v). On the other hand, localization of the excess electrons on the C atoms at the p–p junctions leads to the low tendency of the charged non-IPR fullerenes to cyclosulfurization process. The desulfurization pathway of the exohedral derivatives of C₇₀(C_2v) indicates that it is energetically unfavorable for the functionalized fullerenes to break into individual fullerene and sulfur molecules. HOMO–LUMO gaps almost are independent of the number of pentathiepin rings while sensitive to the type of parent fullerene.     

High-pressure characteristics of α-Fe2O3 using DFT+U

We have calculated the structural, magnetic and electronic properties of corundum-type α-Fe₂O₃ from first principles using density-functional theory (DFT) and the DFT?+?U method to account for correlation effects in this material. Although the correct magnetic ground state is obtained by pure DFT, the magnetic moments and the band gap are too small, and the predicted structural phase transition coupled with a transition from the insulating high-spin to a metallic low-spin phase at a pressure of 14?GPa is not observed experimentally. We find that considering the Coulomb interaction directly by including a Hubbard-like term U in the density functional greatly improves the results with respect to band gap and magnetic moments. The phase transition is shifted to higher pressures with increasing values of U and disappears for U?>?3?eV. The best overall agreement of structural, magnetic and electronic properties with experimental data is obtained for U?=?4?eV.     

Effect of substituent on UV–visible absorption and photostability of liquid crystals: DFT study

The electronic transitions in the ultraviolet–visible (UV–Vis) range of two nematogens, namely 4′-cyanophenyl-4-n-pentylbenzoate and 4′-cyanophenyl-4-n-pentoxybenzoate, have been studied. The UV–Vis and circular dichroism spectra of these molecules have been simulated using the TDDFT/B3LYP/6-31+G(d) method. Mulliken atomic charges for each molecule have been compared with Loewdin atomic charges to analyze the molecular charge distribution and phase stability. The highest occupied molecular orbital and lowest unoccupied molecular orbital energies corresponding to the electronic transitions in the UV–Vis range have been reported. The excited states have been calculated via configuration interaction singles with semi-empirical Hamiltonian ZINDO (Zerner's intermediate neglect of differential overlap method). Further, the effect of substituent on ultraviolet absorption and photostability of the molecules has been discussed. The photostability of the molecules has been investigated in order to understand the application and operation with ultraviolet and visible light regions.     

On the performance of DFT/MRCI-R and MR-MP2 in spin–orbit coupling calculations on diatomics and polyatomic organic molecules

ABSTRACT

We have investigated the performance of different multi-reference quantum chemical methods with regard to electronic excitation energies and spin–orbit matrix elements (SOMES). Among these methods are two variants of the combined density functional theory and multi-reference configuration interaction method (DFT/MRCI and DFT/MRCI-R) and a multi-reference second-order Møller–Plesset perturbation theory (MR-MP2) approach. Two variants of MR-MP2 have been tested based on either Hartree–Fock orbitals or Kohn–Sham orbitals of the BH-LYP density functional. In connection with the MR-MP2 approaches, the first-order perturbed wave functions have been employed in the evaluation of spin–orbit coupling. To validate our results, we assembled experimental excitation energies and SOMES of eight diatomic and fifteen polyatomic molecules. For some of the smaller molecules, we carried out calculations at the complete active space self-consistent field (CASSCF) level to obtain SOMEs to compare with. Excitation energies of the experimentally unknown states were assessed with respect to second-order perturbation theory corrected (CASPT2) values where available. Overall, we find a very satisfactory agreement between the excitation energies and the SOMEs obtained with the four approaches. For a few states, outliers with regard to the excitation energies and/or SOMEs are observed. These outliers are carefully analysed and traced back to the wave function composition.     

The keto–enol equilibrium and thermal conversion kinetics of 2- and 4-hydroxyacetophenone in the gas phase: a DFT study

Keto–enol tautomeric equilibrium and the mechanism of thermal conversion of 2- and 4-hydroxyacetophenone in gas phase have been studied by means of electronic structure calculations using density functional theory (DFT). A topological analysis of electron density evidence that the structure of keto and enol forms of 2-hydroxyacetophenone are stabilised by a relatively strong intramolecular hydrogen bond. 2- and 4-hydroxyacetophenone undergo deacetylation reactions yielding phenol and ketene. Two possible mechanisms are considered for these eliminations: the process takes place from the keto form (mechanism A), or occurs from the enolic form of the substrate (mechanism B). Quantum chemical calculations support the mechanism B, being found a good agreement with the experimental activation parameters. These results suggest that the rate-limiting step is the reaction of the enol through a concerted, non-synchronous, semi-polar, four-membered cyclic transition state (TS). The most advanced reaction coordinate in the TS is the rupture of O₁···H₁ bond, with an evolution in the order of 79.7%–80.9%. Theoretical results also suggest a three-step mechanism for the phenyl acetate formation from 2-hydroxyacetophenone.     

Electronic properties of novel bismuthene nanosheets with adsorption studies of G-series nerve agent molecules – a DFT outlook

In order to confirm the ability of bismuthene nanosheet in detecting the G-series nerve agents, electronic and adsorption properties are investigated. The alteration in the band gap is observed (from the energy band structure) upon adsorption of G-series nerve agents, which results in the modification in the conductivity of BiNS. In addition, to establish the potential application of BiNS as a nerve agent sensor, the adsorption energy, average energy gap variation and Bader charge transfer (vital adsorption properties) are evaluated for the nerve agents, which is physisorbed on BiNS. Furthermore, the supreme statement made in the existing research on adsorption of G-series nerve agents on BiNS is also claimed by comparing the electron density of isolated and nerve agents adsorbed BiNS.     

Comparison of O–H and C–H activation of methanol on Ni-based cluster: a DFT investigation

Using the density functional theory, the initial dehydrogenation of methanol on Ni_xM_y (M?=?Ni, Co, Fe, Mn, Cr, x?+?y?=?4, y?=?1, 2) clusters is investigated. Two adsorption and dehydrogenation mechanisms of methanol are studied: one proceeds along the C–H scission and another begins with the breaking of the O-H bond. The adsorption sites of methanol on the Ni or M sites of the Ni_xM_y clusters are considered. The adsorption of methanol on Ni₄ cluster is stronger than those on bimetallic clusters, while the initial dehydrogenation barriers on Ni_xM_y clusters are lower than that on Ni₄ cluster. The comparable energy barriers of two pathways (O–H or C–H dissociation) on Ni-based clusters indicate that these two paths are quite competitive. In addition, the Ni₂M₂ clusters show superior activation performance compared with the Ni₃M clusters, especially for Ni₂Mn₂ and Ni₂Cr₂ clusters. The effects of alloyed metal on the catalytic activity of Ni for methanol initial dehydrogenation, including the adsorption energy, O–H or C–H bond scission barrier and frontier molecular orbital levels, are discussed. It can be concluded that the addition of Co, Fe, Mn and Cr to Ni catalyst is able to enhance the activity of the methanol dehydrogenation reaction.     

ab-initio investigations of electronic and magnetic properties of the tetragonal chalcopyrite BeTiTe2 compound: DFT + U study

The spin-polarised structural, electronic, and magnetic properties of the chalcopyrite BeTiTe₂ compound in tetragonal structure (Be_0.50Ti_0.50Te) have been studied by employing first-principles full-potential linearised augmented plane wave plus local orbitals (FP-L/APW?+?lo) method within the density functional theory (DFT) and implemented in WIEN2k code. The exchange and correlation energy are described in two frameworks of GGA (generalised gradient approximation) and GGA?+?U (U is the Hubbard term). The structural analysis confirms that the ferromagnetic phase of the tetragonal BeTiTe₂ compound (Be_0.50Ti_0.50Te) is energetically more favourable; also different equilibrium lattice parameters, such as lattice constants (a₀ and c₀), bulk modulus (B₀), and its first-pressure derivative (B?) have been evaluated in both paramagnetic and ferromagnetic phases. The electronic results of the tetragonal BeTiTe₂ compound show a complete half-metallic behaviour. Moreover, the computed total magnetic moment of this compound is close to 4 μ_B, confirming its half-metallic ferromagnetic nature.     

Efficient implementation of the analytic second derivatives of Hartree–Fock and hybrid DFT energies: a detailed analysis of different approximations

In this paper, various implementations of the analytic Hartree–Fock and hybrid density functional energy second derivatives are studied. An approximation-free four-centre implementation is presented, and its accuracy is rigorously analysed in terms of self-consistent field (SCF), coupled-perturbed SCF (CP-SCF) convergence and prescreening criteria. The CP-SCF residual norm convergence threshold turns out to be the most important of these. Final choices of convergence thresholds are made such that an accuracy of the vibrational frequencies of better than 5 cm^?1 compared to the numerical noise-free results is obtained, even for the highly sensitive low frequencies (<100–200 cm^?1). The effects of the choice of numerical grid for density functional exchange–correlation integrations are studied and various weight derivative schemes are analysed in detail. In the second step of the work, approximations are introduced in order to speed up the computation without compromising its accuracy. To this end, the accuracy and efficiency of the resolution of identity approximation for the Coulomb terms and the semi-numerical chain of spheres approximation to the exchange terms are carefully analysed. It is shown that the largest performance improvements are realised if either Hartree–Fock exchange is absent (pure density functionals) and otherwise, if the exchange terms in the CP-SCF step of the calculation are approximated by the COSX method in conjunction with a small integration grid. Default values for all the involved truncation parameters are suggested. For vancomycine (176 atoms and 3593 basis functions), the RIJCOSX Hessian calculation with the B3LYP functional and the def2-TZVP basis set takes ～3 days using 16 Intel® Xeon® 2.60GHz processors with the COSX algorithm having a net parallelisation scaling of 11.9 which is at least ～20 times faster than the calculation without the RIJCOSX approximation.     

Catalytic mechanisms of potassium salts on pyrolysis of β-O-4 type lignin model polymer based on DFT study

In this study, the possible catalytic mechanisms of potassium salts on lignin pyrolysis was investigated with two β-O-4 type lignin model dimers with different oxygen functional groups on C_α using density functional theory (DFT) calculations. It was found that potassium salts (such as KOH and K₂CO₃) can react with phenol hydroxyl in lignin to form organic-K and further catalyze pyrolysis progress. Through this catalytic mechanism, potassium salts promoted the removal of C_α on specific intermediates and the demethylation of methoxy groups during β-O-4 type lignin pyrolysis, promoted the decomposition of intermediates, and facilitated the formation of phenol and CO₂.     

Assessment of a composite CC2/DFT procedure for calculating 0–0 excitation energies of organic molecules

The task to assess the performance of quantum chemical methods in describing electronically excited states has in recent years started to shift from calculation of vertical (ΔE_ve) to calculation of 0–0 excitation energies (ΔE₀₀). Here, based on a set of 66 excited states of organic molecules for which high-resolution experimental ΔE₀₀ energies are available and for which the approximate coupled-cluster singles and doubles (CC2) method performs particularly well, we explore the possibility to simplify the calculation of CC2-quality ΔE₀₀ energies using composite procedures that partly replace CC2 with more economical methods. Specifically, we consider procedures that employ CC2 only for the ΔE_ve part and density functional theory methods for the cumbersome excited-state geometry optimisations and frequency calculations required to obtain ΔE₀₀ energies from ΔE_ve ones. The results demonstrate that it is indeed possible to both closely (to within 0.06–0.08 eV) and consistently approximate ‘true’ CC2 ΔE₀₀ energies in this way, especially when CC2 is combined with hybrid density functionals. Overall, the study highlights the unexploited potential of composite procedures, which hitherto have found widespread use mostly in ground-state chemistry, to also play an important role in facilitating accurate studies of excited states.     

Calculation of the Tautomeric Properties of 3‐Formyl‐Tetrinic Acid by the ab initio and DFT Methods

We have carried out a nonempirical quantumchemical calculation with full optimization of the geometry of all theoretically possible tautomeric forms of 3formyltetrinic acid (FTRA) according to the Møller–Plesset secondorder perturbation theory with the use of a 6–31G(d) double zeta basis set. The correlation corrections to the total energy of molecules for optimized geometric configurations were calculated in the 631G(d,p) basis. All the possible tautomeric forms of FTRA and the harmonic vibrational frequencies were also calculated within the density functional theory (DFT) with the use of the Perdew–Burke–Ernzerhof (PBE) functional in a threeexponent basis. It is shown that FTRA in vapors (in the perfect gas approximation) exists in the form of a mixture of three enol forms, among which two exoforms predominate. The frequencies and forms of normal vibrations for each cisenol tautomer in the region of vibrations of ketogroups and double bonds differ widely, which permits identification of the tautomers present in the mixture. The possible mechanisms of enolenol transformations of 3acyltetrinic acids are discussed.