Electronic structure,magnetic properties,and mixed valence character of Ce2Ni3Si5 from first principles calculations

Cerium intermetallic compounds exhibit anomalous physical properties such as heavy fermion and Kondo behaviors. Here, an ab initio study of the electronic structure, magnetic properties, and mixed valence character of Ce₂Ni₃Si₅ using density functional theory (DFT) is presented. Two theoretical methods, including pure Perdew–Burke–Ernzerhof (PBE) and PBE + U , are used. In this study, Ce³⁺ and Ce⁴⁺ are considered as two different constituents in the unit cell. The formation energy calculations on the DFT level propose that Ce is in a stable mixed valence of 3.379 at 0 K. The calculated electronic structure shows that Ce₂Ni₃Si₅ is a metallic compound with a contribution at the Fermi level from Ce 4f and Ni 3d states. With the inclusion of the effective Hubbard parameter (U _eff), the five valence electrons of 5 Ce³⁺ ions are distributed only on Ce³⁺ 4f orbitals. Therefore, the occupied Ce³⁺ 4f band is located in the valence band (VB) while Ce⁴⁺ 4f orbitals are empty and Located at the Fermi level. The calculated magnetic moment in Ce₂Ni₃Si₅ is only due to cerium (Ce³⁺) in good agreement with the experimental results. The U _eff value of 5.4 eV provides a reasonable magnetic moment of 0.981 for the unpaired electron per Ce³⁺ ion. These results may serve as a guide for studying present mixed valence cerium‐based compounds. © 2017 Wiley Periodicals, Inc.     

Conformation of random polypeptide chains

The characteristic ratios C_of and C_∞ are calculated for infinitely long poly(L-amino acid) chains having a? CH₂? R′ substitution at the C^α atom, as functions of the valence angle τ at the C^α atom. The value of C_of is found to increase from 1.6 to 2.3 as τ varies from 105 to 115°. On the other hand, C_∞ decreases with increasing τ in the range 105–111°, passes through a minimum at 111°, and then increases slowly. Since, C_∞ is much less dependent on τ in the most commonly observed range (110–115°), any variation in τ in this range due to solvent would not appreciably affect C_∞. The calculated temperature coefficient of C_∞ is negative and increases significantly in absolute magnitude even for small deviations in τ from 110°.     

Analysis of chemical bonding in C2 using Dyson orbitals

Multireference configuration interaction wave functions with single and double excitations were calculated for the ¹Σ⁺_g ground state of the C₂ molecule and the excited states of C⁺₂ with symmetries ²Σ⁺_g, ²Σ^-_u, ²Π_u, and ²Π_g. The corresponding σ_g, σ_u, π_u, and π_g valence Dyson orbitals were calculated. Most of the density due to the valence electrons is accounted for by three σ_g, one σ_u, and one degenerate pair of π_u Dyson orbitals. Electron correlation plays an important role in the bond strength of C₂ by increasing the occupation of the σ_g valence orbitals and decreasing the occupation of the σ_u and π_u valence orbitals. © 1996 John Wiley & Sons, Inc.     

Configuration-Interaction study of lower excited states of O2: Valence and rydberg characters of the two lowest 3Σu − states

Configuration-interaction calculations, with an extended basis, are carried out on the ground and lower excited states of O₂ and O₂⁺ at and near the equilibrium internuclear distance (R = 2.3 a.u.) of the ground state of O₂. Particular attention has been paid to the two lowest ³Σ_u^? states, and the mixing of the valence and Rydberg characters in these states are studied. The lowest ³Σ_u^? state is a Rydberg-type state for R < 2.3 a.u., but becomes valence-type for R ? 2.3 a.u. The second ³Σ_u^? state, which is 1.6 eV above the lowest ³Σ_u^? at R = 2.3 a.u., changes its character from Rydberg to valence, valence to Rydberg, and then to valence again when R increases from 1.9 to 3.1 a.u. Satisfactory agreement between the calculated and experimental vertical excitation energies is obtained.     

Intermediate Valence Behavior of Yb2Cu9Al8

Intermediate valence behavior is frequently observed in materials containing Ce, Yb, Eu, Sm, or Tm. In the current work, we report synthesis and characterization of Yb₂Cu₉Al₈ (Th₂Zn₁₇ structure type). Its intermediate valence behavior can be described by an excitation energy E_ex/k_B = 319 K and a spin fluctuation temperature T_sf = 60 K. The valence state of Yb is estimated to be close to 2.04 for the low-temperature region. The valence gradually evolves to the value of 2.80 at T = 400 K. The specific heat coefficient of γ_exp = 59 mJ · mol_Yb^–1 · K^–2 indicates a moderate effective mass enhancement, together with finite density of states at the Fermi level. The latter is also confirmed by the band structure calculations.     

Synthesis and Electronic Structures and Linear Optics of Solid State Compound SrB2O4

The cluster (SrB₂O₄)₂ existing in crystalline states is employed to model the electronic structure and linear optical properties of solid state compound SrB₂O₄. This compound is synthesized by high temperature solution reaction, and it crystallizes in the orthorhombic space group Pbcn with cell dimensions a = 1.1995(3), b = 0.4337(1), c = 0.6575(1) nm, V = 0.34202 nm³, and Z = 4, μ = 15.14 cm^?1, D_caled = 3.36 g/cm³. The dynamic refractive indices are obtained in terms of INDO/SCI following combination with the Sum‐Over‐States method. A width of the calculated gap is 4.424 eV between the valence band and conduction band, and the calculated average refractive index is 1.980 at a wavelength of 1.065 μm. The charge transfers from O^2‐ anion orbitals to Sr²⁺cation orbitals make the significant contributions to linear polarizability in terms of analyses of atomic state density contributing to the valence and conduction bands.     

Theoretical study of spectroscopic and molecular properties of several low‐lying electronic states of CO molecule

The potential energy curves (PECs) of eight low‐lying electronic states (X¹Σ⁺, a³Π, a′³Σ⁺, d³Δ, e³Σ^?, A¹Π, I¹Σ^?, and D¹Δ) of the carbon monoxide molecule have been studied by an ab initio quantum chemical method. The calculations have been performed using the complete active space self‐consistent field method, which is followed by the valence internally contracted multireference configuration interaction (MRCI) approach in combination with the correlation‐consistent aug‐cc‐pV5Z basis set. The effects on the PECs by the core‐valence correlation and relativistic corrections are included. The way to consider the relativistic corrections is to use the third‐order Douglas–Kroll Hamiltonian approximation at the level of a cc‐pV5Z basis set. Core‐valence correlation corrections are performed using the cc‐pCVQZ basis set. To obtain more reliable results, the PECs determined by the MRCI calculations are corrected for size‐extensivity errors by means of the Davidson modification (MRCI+Q). The spectroscopic parameters (D_e, T_e, R_e, ω_e, ω_ex_e, ω_ey_e, B_e, α_e, and γ_e) of these electronic states are calculated using these PECs. The spectroscopic parameters are compared with those reported in the literature. Using the Breit–Pauli operator, the spin–orbit coupling effect on the spectroscopic parameters is discussed for the a³Π electronic state. With the PECs obtained by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations, the complete vibrational states of each electronic state have been determined. The vibrational manifolds have been calculated for each vibrational state of each electronic state. The vibrational level G(ν), inertial rotation constant B_ν, and centrifugal distortion constant D_ν of the first 20 vibrational states when the rotational quantum number J equals zero are reported and compared with the experimental data. Comparison with the measurements demonstrates that the present spectroscopic parameters and molecular constants determined by the MRCI+Q/aug‐cc‐pV5Z+CV+DK calculations are both reliable and accurate. © 2012 Wiley Periodicals, Inc.     

Relationship between the Auger parameter and the ground state valence charge of the core-ionized atom: The case of Cu(I) and Cu (II) compounds

We develop a simple semiempirical model that correlates the Auger parameter to the ground state valence charge of the core-ionized atom with closed shell electron configuration. Until now, the Auger parameter was employed to separate initial and final state effects that influence the core electron binding energy. The model is applied to Cu(I) and Cu (II) compounds with the Auger parameter defined as α' = E_b^FL (2p_3/2) + E_k^FL (L₃M₄₅M₄₅;¹G). The Auger parameter shift for Cu(I) ion in CuI, CuBr, CuFeS₂, Cu₂S, and Cu₂O compounds—with respect to the copper free atom—increases with the electronic polarizability of the nearest-neighbour ligands suggesting a nonlocal screening mechanism. This relaxation process is interpreted as due to an electron transfer from the nearest-neighbour ligands toward the spatially extended 4sp valence orbitals of the core-ionized Cu(I) ion. In agreement with our model, a linear relationship is found between the Auger parameter shift and the ground state Bader valence charge obtained by density functional theory calculations. The Auger parameter shift for the Cu (II) ion in CuF₂, CuCl₂, CuBr₂, CuSO₄, Cu (NO₃)₂•3H₂O, Cu₃(PO₄)₂, Cu (OH)₂, and CuO compounds is very close to the Auger parameter of metallic copper, and therefore, it is not related to the calculated ground state Bader valence charge. The relaxation process in the final state is dominated by the local screening mechanism, which involves an electron transfer from the nearest-neighbour ligands toward the spatially contracted 3d orbitals of the core-ionized Cu (II) ion.     

A valence bond study of the oxygen molecule

Ab initio valence bond calculations are performed for the three lowest states of the oxygen molecule (³Σ⁻_g, ¹Δ_g, and ¹Σ⁺_g). One objective of the present study was to make a contribution to previous valence bond discussions about the oxygen “double” bond. Further, we study the origin of a small barrier in the potential energy surface of the ground state. Two compact models are employed to maintain the clear picture that can be offered by the valence bond method. The first model has only the Rumer structures that are essential for bonding and a proper dissociation. The second model, in addition, has structures which represent excited atoms. These prove to be important for the dissociation energies. For both models, the orbitals are fully optimized. The spectroscopic data obtained are significantly better than are the (few) valence bond results on O₂ that have been published and have the quality of multiconfiguration self-consistent field calculations in which the same valence space is used. The “hump” in the potential energy surface of the ground state is shown to arise from a spin recoupling. The free atoms correspond to a spin coupling that is incapable of describing the formation of bonds. Only at short distances, an alternative spin coupling provides bonding and the repulsive curve is converted into an attractive one. Our results on this subject support a valence bond explanation previously given by McWeeny [R. McWeeny, Int. J. Quantum Chem. Symp. 24 , 733 (1990)]. © 1996 John Wiley & Sons, Inc.     

Ab initio valence bond calculations. VII. HF,HF+, and H2F+

Ab initio valence bond calculations for the ground and excited states of HF and HF⁺ are presented. Total energies, equilibrium geometries, dissociation energies, dipole moments, and spectroscopic constants for HF and HF⁺ have been calculated. The photoelectron spectrum of HF has been examined and interpreted by means of the valence bond formalism. The ground state of the protonated species H₂F⁺ has been investigated.     

Relativity and the chemistry of UF6: A molecular Dirac—Hartree—Fock—CI study

The electronic structure and bonding of UF₆ and UF₆⁻ are studied within a relativistic framework using the MOLFDIR program package. A stronger bonding but more ionic molecule is found if one compares the relativistic with the nonrelativistic results. The first peak in the photoelectron spectrum of Karlsson et al. is assigned to the 12γ_8u component of the 4t_1u orbital, in agreement with other theoretical and experimental results. Good agreement is found between the experimental and theoretical 5f spectrum UF₆⁻. Some properties, like the dissociation energy and electron affinity, are calculated and the necessity of a fully relativistic framework is shown. The Breit interaction has an effect on the core spinors and the spin-orbit splitting of these spinors but the influence on the valence spectrum is negligible. © 1996 John Wiley & Sons, Inc.     

Construction and applications of symmetrized valence bond wave functions

A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O₃⁻, O₃, O₃⁺, and C₃⁻ are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O₃⁻, C₃H₅, and C₃⁻ are calculated with an optimized symmetrized valence bond wave function in the σ–π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S₃ are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X ¹A₁ of S₃, while the excited state 1 ¹B₂ is essentially composed of the dipole structures, and the 1 ³B₂ excited state is comprised from a triplet-biradical structure. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 1–7, 1998     

Conformation of tert‐butoxy­carbonyl­glycyl‐dehydro­alanyl‐glycine methyl ester in the crystalline state and calculated in the gas phase

tert‐Butoxy­carbonyl­glycyl‐dehydro­alanyl‐glycine methyl ester (systematic name: methyl {2‐[(tert‐butoxycarbonylamino)­acetamido]prop‐2‐enamido}acetate) (Boc⁰‐Gly¹‐ΔAla²‐Gly³‐OMe), C₁₃H₂₁N₃O₆, has been structurally characterized by single‐crystal X‐ray diffraction and by density functional theory (DFT) calculations at the B3LYP/6–311+G** level. The peptide chain in both the solid‐state and calculated structures adopts neither β nor γ turns. All amino acid residues in the tripeptide sequence are linked trans to each other. The bond lengths and valence angles of the amino acid units in the crystal structure and gas phase are comparable. However, the conformation of the third glycyl residue (Gly³) is different in the crystalline state and in the gas phase. It is stabilized in the calculated structure by an additional intra­molecular short contact between Gly³ NH and methyl ester COMe groups.     

Theoretical characterization of hydrogen pentoxide,H2O5

Following our investigations on hydrogen polyoxides, herein we employed coupled cluster theory in conjunction with Dunning's correlation consistent basis sets and density functional theory to study HOOOOOH (H₂O₅). The infrared spectra of H₂O₅ and its three deuterated isotopologues, as well as those of the five single‐substituted ¹⁸O isotopologues are discussed in detail. Internal valence coordinates were employed to classify the vibrational modes. The Raman activity is reported to help in the identification of hydrogen pentoxide. The suggested enthalpy of formation is ΔH_f,298° (HOOOOH) = 1.4 ± 1.5 kcal/mol. This value includes corrections for relativistic and core‐valence effects as well as anharmonic corrections to Zero‐point energy corrections. © 2013 Wiley Periodicals, Inc.     

Characterisation of Siliceous Extra‐Framework Species in DAY Zeolites by 29Si MAS NMR and IR Spectroscopic Measurements

Dealuminated Y zeolites (DAY) were obtained by steaming of NH₄NaY at temperatures between 450 °C and 700 °C. They were characterised by means of ²⁷Al and ²⁹Si MAS NMR, IR spectroscopic and XRD measurements. The Si/Al framework ratios of samples were calculated using the ²⁹Si MAS NMR signal intensities, the wave numbers of the double‐ring vibration band w_DR and the asymmetrical TOT valence vibration w_TOT of IR spectra as well as the XRD lattice constant a₀. In contrast to actual Si/Al ratio obtained from w_DR and a₀, the NMR spectroscopic and w_TOT values were determined to be too high because of the superposition of the signals coming from dealuminated zeolite framework and silica gel which forms in the zeolite as a result of steaming. The differently determined Si/Al ratios characterise the siliceous extra‐framework species.     

ESR spectra and electronic structure of the C120 ·+ radical cation and the paramagnetic C120O2+ dication and C120O2− dianion

Electronic structure of the C₁₂₀ ^·+ radical cation and the paramagnetic C₁₂₀O²⁺ dication and C₁₂₀O²⁻ dianion in the triplet state was calculated by the MNDO/PM3 method in the valence approximation. The density distributions of the unpaired electrons in these systems were found and the ESR spectra of the above species were interpreted. Translated fromIzvestiya Akademii Nauk Seriya Khimicheskaya, No. 7, pp. 1257–1260, July, 1999.     

Darstellung,Kristallstrukturen, Schwingungsspektren und Normalkoordinatenanalysen von trans-(PNP)[TcCl4(Py)2] und trans-(PNP)[TcBr4(Py)2]

Preparation, Crystal Structures, Vibrational Spectra, and Normal Coordinate Analysis of trans-(PNP)[TcCl₄(Py)₂] and trans-(PNP)[TcBr₄(Py)₂] By reaction of (PNP)₂[TcX₆] with pyridine in the presence of [BH₄]^? (PNP)[TcX₄(Py)₂], X = Cl, Br, are formed. X-ray structure determinations on single crystals of these isotypic Tc^III complexes (monoclinic, space group P2₁/n, Z = 2, for X = Cl: a = 13.676(4), b = 9.102(3), c = 17.144(2) Å, β = 91.159(1)°; for X = Br: a = 13.972(2), b = 9.146(3), c = 17.285(4) Å, β = 90.789(2)°) result in the averaged bond distances Tc? Cl: 2.386, Tc? Br: 2.519, Tc? N: 2.132(3) (X = Cl) and 2.143(4) Å (X = Br). The two pyridine rings are coplanar and vertical to the X? Tc? X-axes, forming angles of 42.28° (X = Cl) and 43.11° (X = Br). Using the molecular parameters of the X-ray structure determination and assuming D_2h point symmetry, the IR and Raman spectra are assigned by normal coordinate analysis based on a modified valence force field. Good agreement between observed and calculated frequencies is obtained with the valence force constants f_d(TcCl) = 1.45, f_d(TcBr) = 1.035, f_d(TcN) = 1.37 (X = Cl) and 1.45 mdyn/ Å (X = Br), respectively.     

Structure of associates in tetramethylurea aqueous solutions from the data of Raman scattering spectroscopy

The results from investigating interactions between tetramethylurea (TMU) and water molecules by means of Raman light scattering spectroscopy (RS) are presented. It is established that spectroscopic manifestations of association of TMU and H₂O molecules are observed for the TMU molecule valence vibration band ν(CO) 1638 cm⁻¹. By means of quantum-chemical calculations, it is shown that the band sensitivity to molecular environment is defined by the number of 5–6 H₂O molecules. It is found that in a pure TMU medium, chain dimers are formed due to the interactions of molecular dipoles arranged in parallel and hydrogen bonds of the C-H…O type. Ground state geometries of TMU, TMU-TMU, H₂O molecules and TMU · (H₂O) _n complexes (n = 1−14) are optimized by the density functional B3LYP procedure, using the 6–31++G(d, p) basis set. The main vibration frequencies are calculated in the harmonic approximation. Associate formation energies are calculated with allowance for the basis set superposition error (BSSE).     

Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc,Y, La) and the 18‐Electron Rule

We report the gas‐phase synthesis of stable 20‐electron carbonyl anion complexes of group 3 transition metals, TM(CO)₈^? (TM=Sc, Y, La), which are studied by mass‐selected infrared (IR) photodissociation spectroscopy. The experimentally observed species, which are the first octacarbonyl anionic complexes of a TM, are identified by comparison of the measured and calculated IR spectra. Quantum chemical calculations show that the molecules have a cubic (O_h) equilibrium geometry and a singlet (¹A_1g) electronic ground state. The 20‐electron systems TM(CO)₈^? are energetically stable toward loss of one CO ligand, yielding the 18‐electron complexes TM(CO)₇^? in the ¹A₁ electronic ground state; these exhibit a capped octahedral structure with C_3v symmetry. Analysis of the electronic structure of TM(CO)₈^? reveals that there is one occupied valence molecular orbital with a_2u symmetry, which is formed only by ligand orbitals without a contribution from the metal atomic orbitals. The adducts of TM(CO)₈^? fulfill the 18‐electron rule when only those valence electrons that occupy metal–ligand bonding orbitals are considered.