Structural,electronic and magnetic properties of cementite-type Fe3X (X = B,C, N) by first-principles calculations

Using first-principles technique, the crystal structure of cementite-type Fe₃N is predicted. The average magnetic moment (Ms) of cementite-type Fe₃N is also predicted as 1.4929 μ_B/atom. The Ms of Fe₃N is bigger than that of Fe₃C, but smaller than that of Fe₃B. Fe Ms between two different Fe sites in Fe₃N are different (2.0541 and 2.0139 μ_B), which indicates that Fe Ms are sensitive to the local short-range order in the cementite-type crystal. The Ms of B, C and N are ?0.3525, ?0.2474 and ?0.1102 μ_B/atom in Fe₃X (X = B, C, N), respectively. The chemical bonds of Fe₃X (X = B, C, N) take on metallicity, covalence, and ionicity. The ionicity of Fe₃X (X = B, C, N) strengthens and the covalence of Fe–X weakens, going from Fe₃B, Fe₃C to Fe₃N.     

Ab initio study of spin–orbit interaction in the ground electronic states of XH (X = C, Si, Ge and Sn) molecules

Electronic structure and spectroscopic properties for the ground electronic states of CH, SiH, GeH and SnH molecules were obtained using the multiconfigurational self-consistent field followed by spin–orbit multireference multistate perturbation theory. Spin–orbit splitting calculations for ground states of the four molecules were carried out with model core potential (MCP) and all-electron (AE) methods. MCP results are compared with corresponding AE values to estimate the accuracy of the saving cost MCP calculations. The potential energy curves, calculated for the Ω states CH(X₁²Π_1/2 and X₂²Π_3/2), SiH(X₁²Π_1/2 and X₂²Π_3/2), GeH(X₁²Π_1/2 and X₂²Π_3/2) and SnH(X₁²Π_1/2 and X₂²Π_3/2) using the MCP method, were fitted to analytical potential energy function using Murrell–Sorbie potential energy function. Based on the analytical potential energy function, force constants and spectroscopic constants for the Ω states were obtained.     

Self-assembly of two novel bisupporting Keggin-polyoxometalate derivatives: Hydrothermal synthesis and structure characterization of (X = P, m = 1; X = Si, m = 2)

Two novel organic–inorganic hybrid polyoxometalates, (X = P, m = 1 1; X = Si, m = 2 2; 2,2′-bpy = 2,2′-bpyridine), have been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. They are isostructural, possessing orthorhombic, and the parameters of unit cells for compound 1 are space group Pbca, a = 17.317(4) Å, b = 17.092(3) Å, c = 20.587(4) Å, V = 6445(2) Å³, Z = 4; for compound 2 are space group Pcab, a = 17.181(3) Å, b = 18.198(4) Å, c = 20.672(4) Å, V = 6463(2) Å³, Z = 4. The two compounds show a layer framework constructed from Keggin-polyoxoanion clusters and [Cu (2, 2′-bpy)₂]²⁺ coordination polymer fragments via weak covalent interactions, resulting in a 3D network via supramolecular interactions. Their electrochemical properties are studied in detail.     

Coupling of terminal alkynes by RuHXL2 (X = Cl or N(SiMe3)2, L = PiPr3)

The compounds RuL₂HX, where L = PⁱPr₃ and X = Cl or N(SiMe₃)₂, are catalyst precursors for dimerization of terminal alkynes to enynes and also to cumulenes at 23 °C; selectivity among these products is X-dependent, but not high. Conversion of Ru species onto the catalytic cycle was undetectably small, so alternative approaches to understanding the catalytic mechanism were employed: stoichiometric reactions, independent synthesis of candidate intermediates, and trapping with CO. These show the intermediacy of vinylidenes and vinyl compounds, and reveal conversion of cumulenes to the thermodynamically more stable enynes.     

Synthesis, structures and oxygen atom transfer catalysis of oxo-bridged molybdenum(V) complexes with heterocyclic bidentate ligands (N,X) X = S, Se

The dinuclear μ-oxomolybdenum(V) complexes [Mo₂O₃(PyS)₄] (1), [Mo₂O₃(PySe)₄] (2) and [Mo₂O₃(4-CF₃-PymS)₄] (3) were obtained by similar reactions of the [MoO₂Cl₂(DME)] precursor with the corresponding heterocyclic bidentate (N,X) ligands, X = S, Se, where PyS, PySe and 4-CF₃-PymS are the anions of pyridine-2-thione, pyridine-2-selenolato and 4-trifluoromethyl-2-pyrimidinthiol, respectively. All compounds were characterized by elemental analysis, IR, NMR, EI-MS spectroscopy and X-ray diffraction. The crystal structures of 1–3 all include the common [Mo₂O₃]⁴⁺ core. Compounds 1 and 2 are isostructural. The catalytic oxo-transfer properties of the molybdenum(V) compounds 1 and 2 were studied by the use of PPh₃ in DMSO with a considerably higher catalytic activity for the thionato containing complex 1 than for its selenolato containing analogue 2.     

Binding energies and bonding nature of MX(CO)(PH3)2(C60) (M = Rh or Ir; X = H or Cl): Theoretical study

IrH(CO)(PH₃)₂(C₆₀), IrCl(CO)(PH₃)₂(C₆₀), and RhH(CO)(PH₃)₂(C₆₀) were theoretically investigated with DFT and MP2 to MP4(SDQ) methods.  Because the DFT method considerably underestimates the binding energy compared to the MP2 method, their binding energies were evaluated by the ONIOM(MP4(SDQ):UFF) method.  The binding energy decreases in the order IrH(CO)(PH₃)₂(C₆₀) (59.4) > RhH(CO)(PH₃)₂(C₆₀) (48.2) > Pt(PH₃)₂(C₆₀) (47.2) > IrCl(CO)(PH₃)₂(C₆₀) (43.0), where in parentheses are the binding energy (in kcal/mol) calculated with the ONIOM(MP4(SDQ):UFF) method and that of Pt(PH₃)₂(C₆₀) was calculated with the same method and the same basis sets in our previous work.  This decreasing order is interpreted in terms of the d_π orbital energy, the d orbital expansion, the presence of the empty d_σ orbital, and the distortion energy of the metal fragment induced by the complexation; for instance, the d_π orbital is at higher energy and more expands in IrH(CO)(PH₃)₂ than in the Rh analogue, which leads to the larger binding energy of IrH(CO)(PH₃)₂(C₆₀) than that of the Rh analogue. IrCl(CO)(PH₃)₂ is less favorable than IrH(CO)(PH₃)₂ because of the lower energy of d_π orbital.  Although the π-back donation is stronger in IrCl(CO)(PH₃)₂(C₆₀) than in RhH(CO)(PH₃)₂(C₆₀), the binding energy of IrCl(CO)(PH₃)₂(C₆₀) is smaller than that of RhH(CO)(PH₃)₂(C₆₀) due to the larger distortion energy of the IrCl-(CO)(PH₃)₂ moiety.  Although the d_π orbital of Pt(PH₃)₂ is at higher energy than that of IrH-(CO)(PH₃)₂, the binding energy of IrH(CO)(PH₃)₂(C₆₀) is larger than that of Pt(PH₃)₂(C₆₀) because the distortion energy is large and the d_σ orbital is doubly occupied in Pt(PH₃)₂(C₆₀).  It is also noted that these binding energies are much larger than those of the ethylene analogues like those of the Pt(0) complexes, which is reasonably interpreted in terms that the LUMO of C₆₀ is at much lower energy than those of ethylene.     

The structures,stability, and photoelectron spectroscopy of GaPX (X = C,Si, Ge; O,S; P and Ga) clusters

The equilibrium geometries and vibrational frequencies of GaPX⁻ and GaPX (X = C, Si, Ge; O, S; P and Ga) have been studied by hybrid B3LYP functional at cc-PVTZ and aug-cc-PVTZ levels. The results predict that the most stable structure of GaPC⁻ is linear while the others are trigonal. As for GaPX (X = C, Si, Ge; O, S; P and Ga), the ground structures of GaPC and GaPO are linear while the others are trigonal. The adiabatic electron affinities (AEAs) and vertical detachment energies (VDEs) of GaPX⁻ are calculated at B3LYP/aug-cc-PVTZ level. And the order of the AEAs and VDEs of GaPX⁻ are C < O < Ge ≈ Si < P < S < Ga and C < Ge ≈ Si < P < O < S < Ga, respectively. GaPC exhibits the lowest adiabatic electron affinities of all the clusters studied, indicating a particularly stable neutral species.     

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.     

Chemical bonding and elastic properties of Ti3AC2 phases (A = Si,Ge, and Sn): A first-principle study

The chemical bonding and elastic properties as well as the effect of atomic radii for A element in the Ti₃AC₂ phases (A = Si, Ge, and Sn) were studied by ab initio total energy calculations using plane-wave pseudopotential method based on DFT. The atomic radii of A element has a weak effect on the electronic structure. However, the bond stiffness was quantitatively examined, which shows that the bond stiffness is affected by the atomic radii of A element. The calculated results including lattice constants, internal coordinate, elastic modulus, sound velocity, and Debye temperature agree with experimental values very well. With the increase of atomic radii of A element from Si, Ge to Sn, the cohesive energy and elastic moduli as well as Debye temperature decrease, but the elastic anisotropy increases. This is related to the change of bond stiffness. It can be predicted that the fracture toughness of Ti₃SnC₂ would be comparable with that of Ti₃SiC₂ and Ti₃GeC₂.     

Mechanism of abstraction reactions of dimetallenes (R2X=XR2; X = C, Si, Ge, Sn, Pb) with halocarbons: a theoretical study

Potential energy surfaces for the abstraction reactions of dimetallenes with halocarbons have been studied using density functional theory (B3LYP). Five dimetallene species, (SiH(3))(2)X=X(SiH(3))(2), where X = C, Si, Ge, Sn, and Pb, have been chosen in this work as model reactants. The present theoretical investigations suggest that the relative dimetallenic reactivity increases in the order C=C < Si=Si < Ge=Ge < Sn=Sn < Pb=Pb. That is to say, for halocarbon abstractions there is a very clear trend toward lower activation barriers and more exothermic reactions on going from C to Pb. Moreover, for a given dimetallene, the overall barrier heights are determined to be in the order CF(4) > CCl(4) > CBr(4) > CI(4). That is, the heavier the halogen atom (Y), the more facile its abstraction from CY(4). Halogen abstraction is always predicted to be much faster than the abstraction of a CY(3) group irrespective of the dimetallene or halocarbon involved. Our model conclusions are consistent with some available experimental findings. Furthermore, both a configuration mixing model based on the work of Pross and Shaik and bonding dissociation energies can be used to rationalize the computational results. These results allow a number of predictions to be made.     

Heteroatom effects on allenic seven-membered ring X2C4H4C (X = CH,N, P,and As)

Stable configurations of seven-membered rings X₂C₄H₄C (1 _X , X = CH, N, P, and As) in the singlet (s) and triplet (t) states are found at B3LYB/6-311++G** level of theory. Thermal energy gaps, ΔE _s-t; enthalpy gaps, ΔH _s-t; Gibbs free energy gaps, ΔG _s-t, between the singlet and triplet states of 1 _X were estimated at the same level of theory. The ΔG _s-t gap between the singlet and triplet states of 1 _X changes in the order: 1 _P > 1 _As > 1 _N , respectively.     

Ab initio band structure calculations on polymers (C5H3X)n (X=CH,N,P,As)

The ab initio crystal orbital calculations on conjugated aromatic six-membered rings polymers,namely,poly(p-phenylene) (PPP),poly(2,5-pyridinediyl) (PPD),poly(2,5-phosphabenzene) (PPB) and ploy(2,5-arsabenzene) (PAB) are reported.The comparison of the important electronic properties of these polymers,such as band gap,bandwidth,ionization potential and electron affinity,indicates that PPP is the best intrinsic semiconductor,and PPD has the best prospects for forming n-doped conducting materials.     

A first-principle study of the stability and electronic properties of halide inorganic double perovskite Cs2PbX6 (X = Cl,I) for solar cell application

The problem of lead toxicity in perovskites materials that are currently performing with the most efficiency can be partially solved by choosing double perovskites compounds Cs₂PbX₆ (X = Cl,I), which have considerably reduced lead contents. These materials are slightly more stable, and substituting Cl and I with Br in small percentages further improves their mechanical stability and electronic properties. In this study, the properties of these promising materials were investigated in their pure and mixed forms.     

A systematic computational study on the reactions of dimethyl sulfoxide with XO (X = NO2, Cl, Br and I)

The reactions of dimethyl sulfoxide (DMSO) with XO (X = NO₂, Cl, Br, I) have been studied at CCSD/6-311G(d,p)//B3LYP/6-311G(d,p) level. Two reaction channels have been considered: (1) the oxygen-atom transfer (OAT) from XO (X = NO₂, Cl, Br, I) to DMSO and (2) the hydrogen-abstraction by XO (X = NO₂, Cl, Br, I). The reaction mechanisms of DMSO with NO₃, ClO and BrO are similar: the OAT channel is the dominant channel and DMSO₂ is the primary product; the hydrogen-abstraction channel is not likely to be competitive with the OAT channel. The DMSO + IO reaction, because two reaction channels have the overall negative reaction activation energies and Gibbs free energies, the reaction could occur on both reaction channels. Furthermore, the formation of the stable complex may vary the yield rate of DMSO₂.     

NMR spectra of silatranes and M ← N (M = C, Si, Ge, Sn, Pb) bond lengths in atranes: Substituent effects

The literature data on X substituent influence on the ¹H, ²⁹Si and ¹⁵N NMR chemical shifts (δ) and coupling constants (J) of Si-substituted silatranes , as well as M-N bond lengths (d) in atranes (M = C, Si, Ge, Sn, Pb) have been analyzed. It was established for the first time that the δ, J and d values depend not only on the inductive and resonance effects but also on the polarizability of X substituents. The polarizability contribution ranges from 8% to 25%.     

Crystal structure and magnetic properties of the new cobalt tellurite halide Co5(TeO3)4X2 (X = Cl,Br)

Two new cobalt tellurite halides Co₅(TeO₃)₄Cl₂ and Co₅(TeO₃)₄Br₂ have been synthesized and found to be iso-structural with Ni₅(TeO₃)₄X₂ (X = Cl, Br). Co₅(TeO₃)₄X₂ crystallizes in the monoclinic system space group C2/c, and the Br-phase has the lattice parameters a = 20.440(1) Å, b = 5.2760(2) Å, c = 16.4710(7) Å, β = 124.790(5)°, and Z = 4. The crystal structures were solved from single-crystal X-ray data, R1 = 1.90 and 1.77, respectively, for the Cl- and Br-phases. The crystal structure is layered with only weak van der Waals' interactions in between the layers. The layers are built by large [Co₅O₁₆X₂] groups consisting of five edge- and face-sharing Co-octahedra. Each group is connected to adjacent groups via corner sharing through common oxygen atoms as well as through [TeO₃E] groups. Magnetic susceptibility measurements on oriented single crystals reveal pronounced anisotropy in a broad temperature range and clear signs of antiferromagnetic ordering at low temperatures. Anisotropic susceptibility of an iso-structural Ni-based compound was also studied and compared with the corresponding results of Co₅(TeO₃)₄X₂. Magnetic anisotropy is discussed in framework of single-ion anisotropy effects.     

Coordination of niobium and tantalum oxides by Ar, Xe and O2: Matrix isolation infrared spectroscopic and theoretical study of NbO2(Ng)2 (Ng = Ar, Xe) and MO4(X) (M = Nb, Ta; X = Ar, Xe, O2) in solid argon

The combination of matrix isolation infrared spectroscopic and quantum chemical calculation results indicate that the NbO₂ molecule is coordinated by two noble gas atoms in forming the NbO₂(Ng)₂ (Ng = Ar, Xe) complexes in solid noble gas matrixes. In contrast, the TaO₂ molecule is not able to form similar noble gas complex. The niobium and tantalum dioxides further react with dioxygen to form the side-on bonded superoxo-dioxide complexes MO₄ (M = Nb, Ta), which are coordinated by one argon atom in solid argon matrix. The coordinated Ar atom in MO₄(Ar) can be replaced by O₂ or Xe in forming the MO₆ and MO₄(Xe) complexes. The results indicate that the NbO₂, NbO₄ and TaO₄ molecules trapped in solid noble gas matrixes should be regarded as the NbO₂(Ng)₂ and MO₄(Ng) (Ng = Ar, Xe; M = Nb, Ta) complexes instead of “isolated” metal oxide species.     

Structures and Aromaticity of Planar XY2Z （X = Li,K, Y - P,As and Z = C,Si, Ge） Clusters

Clusters XY2Z species are theoretically investigated with density functional theory （DFT） method. The results show that for LiP2C, LiAs2Ge and KAs2C species, the C2v isomer is the most stable planar structure, while for other species the Cs isomer is the most stable planar structure at the B3LYP/6-311＋G＊ level. Wiberg Bond Index （WBI） and Nucleus-Independent Chemical Shift （NICS） values indicate the existence of delocalization in stable planar structures. A detailed Molecular Orbital （MO） analysis further reveals that planar isomers of these species have strong aromatic character, which strengthens the structural stability and makes them closely connect with the concept of aromaticity.