Crystallographic, electronic and magnetic studies of Ce4Ni6Al23: a new ternary intermetallic compound in the cerium-nickel-aluminum phase diagram

The Al-rich portion of the ternary Ce-Ni-Al has been investigated and a new ternary phase of composition Ce₄Ni₆Al₂₃ has been found. This compound crystallizes in the monoclinic space group C2/m with the cell parameters a=16.042(8), b=4.140(4), c=18.380(8) Å and β=113.24(5)°. The structure has been determined by single crystal X-ray diffraction. The local environment of Ni and Ce is close to what is observed in the CeNi₂Al₅ and CeNiAl₄ structures. Band structure calculations, using the tight-binding-linear muffin-tin orbital-atomic-spheres approximation (TB-LMTO-ASA) method, have been performed to understand the electronic structure of Ce₄Ni₆Al₂₃ and the results are discussed in connection with those two other Ce-Ni-Al intermetallic compounds, which possess heavy-fermion behavior. Magnetic and heat capacity measurements have also been measured to analyze the low-temperature magnetic behavior of this new compound.     

Theoretical studies on cerium nickel aluminides: polar intermetallics with heavy fermion behavior

The electronic structures of Ce₄Ni₆Al₂₃, CeNiAl₄, CeNi₂Al₅, CeNiAl and CeNi₄Al have been calculated using the TB-LMTO-ASA (tight-binding, linear muffin-tin orbital, atomic-spheres approximation) approach to probe relationships between chemical bonding and physical properties in this series of intermetallic compounds. Analysis from crystal orbital Hamilton populations (COHP) reveal that the Al-rich compounds may be considered as “polar intermetallic” because the Fermi level coincides to the separation of bonding and antibonding states of the Ni-Al framework. On the other hand, although the densities of states (DOS) of CeNiAl suggest “polar intermetallic” behavior, the bonding is more complex. Finally, the Ni-rich example, CeNi₄Al, has significant Ni-3d character at the Fermi level. The results of these calculations are also discussed in connection with heavy fermion or possible valence fluctuation behavior observed for some of these intermetallic compounds: those showing exceptional properties also exhibit significant “lattice covalency” between Ce and the Ni-Al nets.     

Theoretical investigations of the elastic,electronic, optical and thermodynamics properties of SrSi

The crystal structural, electronic, optical and thermodynamic properties of SrSi are investigated by using the first-principles plane-wave pseudopotential density function theory within the generalized gradient approximation (GGA). We have calculated the ground states properties and they are in good agreement with the available experimental data and other theoretical results. We have obtained the electronic structure and density of states, and the results showed that both of Immm and Cmcm phases are metal material. The elastic properties such as elastic constants, shear modulus, Young's modulus and Poisson's ratio are obtained for the first time. Furthermore, the optical properties are reported for radiation up to 30 eV. Finally, the thermodynamic properties of Cmcm phase such as free energy, entropy, enthalpy, heat capacity and Debye temperature are given for reference.     

First principles calculations of electronic and optical properties of Ag2S

A theoretical study of structural, electronic and optical properties of Ag₂S is presented using the full potential linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). In this approach, the modified Becke Johnson (MBJ) potential coupled with Local Density Approximation (LDA) was used for the exchange-correlation potential calculation. Ground state properties are determined for the bulk material in monoclinic phase. Band structure reveals that this compound is a direct energy band gap semiconductor. MBJLDA results for the band gap of this compound are much better than those obtained using LDA, Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and Engel–Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gap and corresponding experimental values as compared to other calculations. Optical constants including the dielectric function, refractive index, extinction coefficient, electron energy loss function, reflectivity and absorption coefficient are obtained and discussed.     

Single crystal structure and magnetic properties of MnSbS2Cl

The new compound MnSbS₂Cl was synthesised from a mixture of MnS, MnCl₂ and Sb₂S₃ at 500 °C. Single crystal study indicates orthorhombic symmetry, space group Pnma (No. 62), with , , , and Z=4. The refinement converged to R=0.0374 and wR=0.0716 for 742 unique reflections and 32 parameters. The crystal structure of MnSbS₂Cl is isotypic with stibnite Sb₂S₃ and consists of waved layers of corner-sharing MnS₄Cl₂ octahedra along the a axis and edge-sharing octahedra along the b axis, which are separated by antimony atoms. MnSbS₂Cl susceptibility shows an antiferromagnetic behaviour below 40 K with an increase at about 23 K.     

Matrix-isolation FT-IR study and theoretical calculations of the vibrational, tautomeric and H-bonding properties of hypoxanthine

The neutral compound hypoxanthine is investigated using the technique of matrix-isolation FT-IR spectroscopy combined with density functional theory (DFT) and ab initio methods. Two theoretical methods (RHF and DFT/B3-LYP) are compared for vibrational frequency prediction, and four methods (RHF//RHF, MP2//RHF, DFT//DFT and MP2//DFT) for prediction of the relative energies of the tautomers and the interaction energies of the complexes. All the possible tautomeric forms have been considered theoretically, and the results indicate that two oxo forms (O17 and O19) and one hydroxy form (H9-r1) are the three most stable forms. The experimental FT-IR spectra are consistent with this prediction, and nearly all the characteristic spectral features of these forms have been identified in the spectrum. A theoretical study of the H-bonded complexes of these three tautomers with water is also performed. Several structures have been found for each form and the results demonstrate that the closed complexes with two H-bonds are the most stable systems due to the H-bond cooperative effect.     

Characterization of electronic structure and physicochemical properties of antiparasitic nifurtimox analogues: A theoretical study

American trypanosomiasis, also known as Chagas' disease, is caused by Trypanosoma cruzi (T. cruzi). It is well known that trypanosomes, and particularly T. cruzi, are highly sensitive towards oxidative stress, i.e., to compounds than are able to produce free radicals. Generally, nifurtimox (NFX) and benznidazol are most effective in the acute phase of the disease; therefore, nitroheterocycles constitute good models to design other nitrocompounds with specific biological characteristics. Thus, we have performed an ab initio study at the Hartree‐Fock and Density Functional Theory levels of theory of several NFX analogues recently synthesized, to characterize them by obtaining their electronic, structural, and physicochemical properties, which might be linked to the observed antichagasic activity. The antitrypanosomal activity scale previously reported for the NFX analogues studied in this work is in good agreement with our theoretical results, from which we can conclude that the activity seems to be related to the reactivity along with the acidity observed for the most active molecules. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Influence of the ligand structure of hafnocene polymerization catalysts: A theoretical study on chain termination reactions in ethene polymerization

Ligand effects on chain termination reactions in hafnocene-catalyzed ethene polymerization process have been systematically studied by quantum chemical methods. β-hydrogen transfer to metal, β-hydrogen transfer to monomer and hydrogenolysis were studied for 27 hafnocenes, initiating the chain termination reactions after insertion of the second ethene monomer. The results of the calculations were studied as a function of the ligand structure, focusing on the effects of various ancillary ligands, ligand substituents and bridging units. The ligand effects on chain termination reactions are strongly affected by combined effects of various structural units, in particular, in the cases of β-hydrogen transfer to monomer and hydrogenolysis. The results are expected to aid in design and development of new hafnocene polymerization catalysts.     

Theoretical studies on the structure of M+BF−4 ion pairs M = Li+, NH+4: the role of electrostatics and electron correlation

A systematic investigation of the M⁺BF₄ ⁻ (M = Li or NH₄) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order M?ller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li⁺ is the cation involved and more than 90% for the case of NH₄ ⁺. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li⁺BF₄ ⁻ and NH₄ ⁺BF₄ ⁻ show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed. Received: 9 September 1997 / Accepted: 5 November 1997     

Theoretical study of the electronic excited states of tetracyanoethylene and its radical anion.

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.     

Ab initio and DFT calculations for the structure and vibrational spectra of cyclopentene and its isotopomers

Ab initio calculations using the MP2/cc-pVTZ basis set do an excellent job of predicting the inversion barrier (247 vs. 232 cm⁻¹) and dihedral angle (26°) of cyclopentene. DFT calculations also do an excellent job of predicting the vibrational frequencies of the d₀, d₁, d₄, and d₈ isotopomers. They have also allowed the reassignments of several of the vibrational frequencies.     

Anharmonic vibrational spectroscopy calculations with electronic structure potentials: comparison of MP2 and DFT for organic molecules

Density functional theory (DFT) technique is the most commonly used approach when it comes to computation of vibrational spectra of molecular species. In this study, we compare anharmonic spectra of several organic molecules such as allene, propyne, glycine, and imidazole, computed from ab initio MP2 potentials and DFT potentials based on commonly used BLYP and B3LYP functionals. Anharmonic spectra are obtained using the direct vibrational self-consistent field (VSCF) method and its correlation-corrected extension (CC-VSCF). The results of computations are compared with available experimental data. It is shown that the most accurate vibrational frequencies are obtained with the MP2 method, followed by the DFT/B3LYP method, while DFT/BLYP results are often unsatisfactory. Contribution to the Mark S. Gordon 65th Birthday Festschrift Issue.     

Experimental and theoretical study of the atmospherically important O2-H2O complex

The theoretically predicted water-oxygen van der Waals adduct has been experimentally confirmed by vibrational characterization using matrix isolation spectroscopic studies at 10 K. Vibrational bands for asymmetric and symmetric OH-stretching for this adduct have been found at 3728 cm(-1) and 3639 cm(-1), respectively. Theoretical calculations performed with Gaussian 98 software at the MP2/6-311++G(2d,2p) level of theory support the alternative structure of the hydrated complex proposed by this study.     

Experimental and theoretical study of the electronic structure of methyl chloroformate and methyl cyanoformate

The HeI photoelectron spectra of methyl chloroformate (CH₃OC(O)Cl) and methyl cyanoformate (CH₃OC(O)CN) in the gas phase have been obtained for the first time. A complete theoretical study involving the calculation of the ionization energies using outer valence Green’s functional (OVGF) was performed, based on the calculated and previously reported energetically favorable cis-conformer (the carbonyl group eclipses the methyl group). Calculations of cationic-radical forms were carried out in order to interpret the main characters of the first six highest occupied molecular orbitals (HOMOs). The first vertical ionization potentials are 11.36 eV for CH₃OC(O)Cl and 11.65 eV for CH₃OC(O)CN, each attributed to {19a′(n_O(CO), n_Cl)}⁻¹ and {18a′(n_O(CO))}⁻¹, respectively.     

Synthesis of the anisobidentate compound bis(2-amino-cyclopent-1-ene-carbodithioate)diethyltin (IV). Experimental and theoretical study

The bis(2-amino-1-cyclopentenecarbodithioate)diethyltin (IV) compound (1) was synthesized by reacting diethyltin (IV) chloride with two equivalents of sodium 2-amino-1-cyclopentenecarbodithioate. The structure of (1) was determined by FT-IR and multinuclear magnetic resonance (¹H, ¹³C, ¹¹⁹Sn) spectroscopy, mass spectrometry and X-ray diffraction methods. The geometry obtained around Tin is a skewed trapezoidal bipyramid with both organic residues in mutual trans-positions. To study the coordination of Tin, theoretical calculations at Hartree-Fock (HF) level were carried out. Results for relativistic, quasi-relativistic and non-relativistic pseudopotentials are reported. The concept of local bond order is defined and applied to quantify the weak local interaction between the thiocarbonyl sulfur atom and Tin.     

Theoretical study of the structural,elastic, electronic and optical properties of XCaF3 (X = K and Rb)

The PLANE WAVE pseudo-potential method within density functional theory (DFT) has been used to investigate the structural, elastic, electronic and optical properties of XCaF₃ (X = K and Rb) insulating. The studied compounds show a weak resistance to shear deformation compared to the resistance to the unidirectional compression. KCaF₃ and RbCaF₃ are considered ductile. The elastic constants and related parameters were predicted. The stiffness is more important in KCaF₃, whereas, the lateral expansion is more important in RbCaF₃. KCaF₃ and RbCaF₃ have R- Г indirect band gap. The main peaks in the imaginary part of the dielectric function correspond to the transition from the occupied state F₋p to the unoccupied states Ca: s or K, Rb: p. At lower energies, KCaF₃ and RbCaF₃ show the same optical properties. Under pressure effect, the peaks of imaginary part of dielectric function were shifted toward high energy.     

Exocyclic push–pull conjugated compounds. Part 3. An experimental NMR and theoretical MO ab initio study of the structure, the electronic properties and barriers to rotation about the exocyclic partial double bond in 2-exo-methylene- and 2-cyanoimino-quinazolines and -benzodiazepines

The structure of a number of 2-exo-methylene substituted quinazolines and benzodiazepines, respectively, 1, 3a,b, 4 (X=–CN,–COOEt) and their 2-cyanoimino substituted analogues 2, 3c,d (X=–CN,–SO₂C₆H₄–Me(p) was completely assigned by the whole arsenal of 1D and 2D NMR spectroscopic methods. The E/Z isomerism at the exo-cyclic double bond was determined by both NMR spectroscopy and confirmed by ab initio quantum chemical calculations; the Z isomer is the preferred one, its amount proved dependent on steric hindrance. Due to the push–pull effect in this part of the molecules the restricted rotation about the partial C²,C¹¹ and C²,N¹¹ double bonds, could also be studied and the barrier to rotation measured by dynamic NMR spectroscopy. The free energies of activation of this dynamic process proved very similar along the compounds studied but being dependent on the polarity of the solvent. Quantum chemical calculations at the ab initio level were employed to prove the stereochemistry at the exo-cyclic partial double bonds of 1–4, to calculate the barriers to rotation but also to discuss in detail both the ground and the transition state of the latter dynamic process in order to better understand electronic, inter- and intramolecular effects on the barrier to rotation which could be determined experimentally. In the cyanoimino substituted compounds 2, 3c,d, the MO ab initio calculations evidence the isomer interconversion to be better described by the internal rotation process than by the lateral shift mechanism.     

A combined experimental and theoretical study on the complexation of Li<Superscript>+</Superscript> with valinomycin

Abstract  From extraction experiments in the two-phase water–nitrobenzene system and γ-activity measurements, the stability constant of the valinomycin–lithium complex in nitrobenzene saturated with water was determined. Further, the structure of the resulting complex was indicated by means of the density functional level of theory (DFT) calculations. Graphical abstract        

Synthesis and molecular structure of the novel monohydrated 3-p-nitrophenylpyrazole derived from 1,3-diketone malonate

The synthesis of dimethyl {2-[3-(4-nitrophenyl)-1H-pyrazol-5-yl]ethyl}malonate monohydrate 1, C₁₆H₁₇N₃O₆·H₂O was performed and the molecular structure has been studied by using NMR, single crystal X-ray diffraction and ab initio calculations. The title compound presents a pyrazole ring (N1 to C5), a phenyl ring (C1″ to C6″) attached to C3 and the ethylene dimethyl malonate frame (C1′ to C7′) attached to C5. The torsion angle defined by N2C3C1″C2″ (−12.26°) showed that pyrazole and phenyl rings are not in the same plane. Monohydration in (1) is present in the structure by a NHOH₂ hydrogen bonding, with a bond length of 1.782 Å. Experimental and theoretical evidences indicated the preference of the 3-tautomer over the corresponding 5-tautomer in the titled pyrazole.