Structure and properties of non-classical polymers II. Band structure and spin densities

Theoretical investigation of the band structure of three types of nonclassical polymers, namely alternant (one- and two-dimensional), nonalternant and heteroatomic, are carried out. Although polyradicals, these polymers have a considerable delocalization energy which may determine their relative stability.The spin-density distribution of the alternant type of non-classical polymers corresponds to a ferrimagnetic ground state at 0 K.The non-classical polymers represent a new class of organic systems as their band structure and magnetic properties essentially differ from those of common polymers.     

Effect of Zr and C Co-doping on the Optical Properties and Electronic Structure of TiO_2

The systematic trends and effect introduced by Zr and C co-doping to TiO2 of electronic structure and optical properties of anatase TiO2 have been calculated by the plane-wave ultra-soft pseudopotential density functional theory (DFT) method within the generalized gradient approximation (GGA) for the exchange-correlation potential. Through the current calculations, the density of states (DOS), energy band structure and optical absorption coefficients have been obtained for TiO2 and compared with the doped TiO2, and the influence of electronic structure and optical properties caused by Zr and C co-doping has been presented qualitatively together. The results revealed that the energy band gap has been decreased owing to the doped Zr and C, whereas the optical absorption coefficients have been increased in the region of 400～800 nm and a red shift of absorption band can be found. Accordingly, photo catalytic activity of TiO2 has been enhanced. The current calculations are in good agreement with the experimental data.     

Cluster expansion of electronic excitations: application to fcc Ni-Al alloys

The cluster expansion method is applied to electronic excitations and a set of effective cluster densities of states (ECDOS) is defined, analogous to effective cluster interactions (ECIs). The ECDOSs are used to generate alloy thermodynamic properties as well as the equation of state (EOS) of electronic excitations for the fcc Ni-Al systems. When parent clusters have a small size, the convergence of the expansion is not so good but the electronic density of state (DOS) is well reproduced. However, the integrals of the DOS such as the cluster expanded free energy, entropy, and internal energy associated with electronic excitations are well described at the level of the tetrahedron-octahedron cluster approximation, indicating that the ECDOS is applicable to produce electronic ECIs for cluster variation method (CVM) or Monte Carlo calculations. On the other hand, the Gruneisen parameter, calculated with first-principles methods, is no longer a constant and implies that the whole DOS profile should be considered for EOS of electronic excitations, where ECDOS adapts very well for disordered alloys and solid solutions.     

Toward absolute density of states calculations for proteins

The density of states (DOS), which gives the number of conformations with a particular energy E, is a prerequisite in computing most thermodynamic quantities and in elucidating important biological processes such as the mechanism of protein folding. However, current methods for computing DOS of large systems such as proteins generally yield only the ratios of microstate counts for different energies, which could yield absolute conformation counts if the total number of conformations in phase space is known, thus motivating this work. Here, the total number of energy minima of 50-mer polyalanine, whose size corresponds to naturally occurring small proteins, was estimated under an all-atom potential energy function based on the cumulative distribution function (CDF) of conformational differences to be approximately 10(38). This estimate can place any DOS function, such as the Gaussian DOS distribution in the random energy model, on an absolute scale. Comparing the absolute conformational counts from a Gaussian DOS function with those from the CDF derived from quenched molecular dynamics ensembles shows that the former are far greater than the latter, indicating far fewer low-energy minima actually exist. In addition to showing how CDF and relative DOS calculations can yield absolute DOS for a discrete system, we also show how they can yield absolute DOS for continuous variable systems to a specified atomic variance. In the context of protein folding, knowing this phase-space "volume" of conformations in a DOS function, as well as characteristic transition times, constrains the set of possible folding mechanisms.     

ChemInform Abstract: Theoretical versus Experimental Charge and Spin-Density Distributions in trans-(Ni(NH3)4(NO2)2).

A comparison of data from three different theoretical models (ab initio UHF and DVXα MO calculations as well as cellular ligand field analysis (clf)) with experimental charge and spin-density distributions, obtained from X-ray and polarized neutron diffraction results, leads to a consistent picture of the electronic structure and M-L bonding in the title complex.     

A first-principles DFT study of UN bulk and (001) surface: comparative LCAO and PW calculations

LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribution, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low temperatures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and electronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approaches.     

In Situ X-ray Absorption Spectroscopy of Metal/Nitrogen-doped Carbons in Oxygen Electrocatalysis

Metal/nitrogen-doped carbons (M−N−C) are promising candidates as oxygen electrocatalysts due to their low cost, tunable catalytic activity and selectivity, and well-dispersed morphologies. To improve the electrocatalytic performance of such systems, it is critical to gain a detailed understanding of their structure and properties through advanced characterization. In situ X-ray absorption spectroscopy (XAS) serves as a powerful tool to probe both the active sites and structural evolution of catalytic materials under reaction conditions. In this review, we firstly provide an overview of the fundamental concepts of XAS and then comprehensively review the setup and application of in situ XAS, introducing electrochemical XAS cells, experimental methods, as well as primary functions on catalytic applications. The active sites and the structural evolution of M−N−C catalysts caused by the interplay with electric fields, electrolytes and reactants/intermediates during the oxygen evolution reaction and the oxygen reduction reaction are subsequently discussed in detail. Finally, major challenges and future opportunities in this exciting field are highlighted.     

LMTO法研究NdFe10M2的电子结构与磁性能

采用能带论中线性Ｍｕｆｆｉｎ－ｔｉｎ轨道方法,计算了ＮｄＦｅ１０Ｍ２（Ｍ＝Ｔｉ,Ｖ,Ｃｒ,Ｍｎ,Ｍｏ,Ｗ）及渗Ｎ和Ｃ原子后材料的电子结构,得到了相应的总体态密度和各晶位的局域态密度,并由净自旋数得出了相应的磁矩及其变化,计算值与实验值相符合。     

Investigation of structural and magnetoelectronic properties of new half-metallic Heusler alloys Ru2VGexSb1−x (x = 0, 0.5 and 1): A density functional theory study

We performed an ab initio study using a method named linearized augmented plane wave with a full potential (FP-LAPW) based on the density functional theory. We predicted the physical properties of Ru₂VGe_xSb_1−x (x = 0, 0.5 and 1) Heusler alloys in L2₁ structure. We computed the magnetic and structural properties using the general gradient approximation. The modified Becke-Johnson scheme was used to study the electronic structure of these compounds. The obtained results show that the lattice constants and the spin magnetic moments are in favorable agreement compared with theoretical values and experimental data. The computed densities of state (DOSs) of these compounds indicate a half-metallic behavior with a real gap for the ternary materials, which gives perfect spin polarization, while for the quaternary one, the DOS indicate a nearly half-metallic character with a pseudogap in the minority spin close to the Fermi level E_F.     

Aromaticity, optical properties and zero field splitting of homo- and hetero-bimetallic (C8H8)M(μ2-,η8═C8H8)M(C8H8) where M = Ti, Zr, Th Complexes

This work presents a relativistic calculation of electron delocalization, optical properties, and zero field splitting in a group of molecules with the structure (C(8)H(8))M(μ(2)-,η(8)═C(8)H(8))M(C(8)H(8)), where M = Ti, Zr and Th. Additionally we also studied the heterobimetallic combinations (Ti-Th and Zr-Th). The molecular properties are discussed based on their electronic structure and the influence of the electron mobility in metal-metal communication. Nucleus independent chemical shift (NICS) was determined via the gauge-including-atomic-orbital (GIAO) method with the OPBE functional. The time-dependent density functional theory (TDDFT) was employed to calculate excitation energies, and the electronic transitions over 500 nm are presented with the objective to analyze the transition metal role as an antenna effect in the absorption band in the near-IR region. Finally the ZFS was calculated using Pederson-Khana and coupled perturbed DFT approaches implemented in the ORCA code. The contributions to spin-spin coupling (SS) and spin-orbit coupling (SOC) were analyzed, and the spin-density over the metal centers is discussed employing our scheme of metal-metal communication. Our aim is to determine the influence of the electronic structure over the optical and magnetic properties in a group of model compounds to understand the transition metals effect over these properties.     

Two-dimensional superdegeneracy and structure-magnetism correlations in strong ferromagnet, Mn2Ga5

The ferromagnetic intermetallic compound Mn2Ga5 has been synthesized and characterized by single-crystal X-ray diffraction study and by magnetic property measurements. The compound, with the Mn2Hg5-type structure, exhibits a saturated magnetic moment of 2.71 muB per formula unit with T(C) approximately = 450 K. The electronic structure of the compound was analyzed by employing FPLO, LMTO, and the extended Hückel tight-binding calculations. The nonmagnetic electronic structure of Mn2Ga5 reveals remarkably flat degenerate (superdegenerate) bands in the vicinity of the Fermi level but only in the a*b*-plane of the Brillouin zone. The resulting high density of the states at the Fermi level, DOS(E(F)), is consistent with the Stoner condition for the observed itinerant electron ferromagnetism. Detailed orbital analysis shows an intriguing structure-magnetism correlation, as the superdegeneracy is found to be a consequence of the unique atomic arrangement and bond angles in the structure.     

Fabrication of Multi-pore Structure Cu,N-codoped Porous Carbon-based Catalyst and its Oxygen Reduction Reaction Catalytic Performance for Microbial Fuel Cell

The development of a non-noble metal cathode ORR catalyst with low cost, high activity and high stability has become an inevitable trend in MFC. The purpose of this study is to develop an efficient and stable Cu, N-codoped porous carbons catalysts with multi-pore structure for MFC. Herein, Cu, N-codoped porous carbons materials (Cu−NC−T) with high N content and multi-pore structure were successfully developed by co-pyrolysis with MOF-199 and melamine. By contrast, Cu-doped porous carbon (Cu−C−T) without melamine was synthesized using MOF-199 as template. The results showed that Cu−NC−T possessed a rough octahedral crystal with a unique multi-mesopore structure with pore centers of 3.4 nm and 11.2 nm, respectively. Owing to high N content, abundantly exposed Cu−N_x active sites and the multi-pore structure, Cu−NC−800 had a pronounced electrochemical ORR activity in neutral solution (onset potential and limiting current density were 0.161 V and −6.256 mA ⋅ cm⁻²), which were slightly lower than 20 wt % Pt/C (0.189 V and −6.479 mA ⋅ cm⁻²). Moreover, the MFC with Cu−NC−800 showed a power density of 662.8±3.6 mW ⋅ m⁻², which was higher than that of Cu−C−800 (425.7±3.9 mW ⋅ m⁻²) and was slightly lower than that 20 wt % Pt/C (815.0±6.2 mW ⋅ m⁻²). The output voltage of MFC with Cu−NC−T had no obvious decreasing trend in 30 days, demonstrating that the Cu−NC−T had great stability.     

Prediction of Spin Density,Baird-Antiaromaticity,and Singlet–Triplet Energy Gap in Triplet-State Polybenzenoid Systems from Simple Structural Motifs

Triplet-state aromaticity has been recently proposed as a strategy for designing functional organic electronic compounds, many of which are polycyclic aromatic systems. However, in many cases, the aromatic nature of the triplet state cannot be easily predicted. Moreover, it is often unclear how specific structural manipulations affect the electronic properties of the excited-state compounds. Herein, the relationship between the structure of polybenzenoid hydrocarbons (PBHs) and their spin-density distribution and aromatic character in the first triplet excited state is studied. Although a direct link is not immediately visible, classifying the PBHs according to their annulation sequence reveals regularities. Based on these, a set of guidelines is defined to qualitatively predict the location of spin and paratropicity and the singlet–triplet energy gap in larger PBHs, using only their smaller tri- and tetracyclic components, and subsequently tested on larger systems.     

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid-State NMR Spectroscopy,Polarized Neutron Diffraction,and First-Principle Calculations

The local magnetic structure in the [Fe^III(Tp)(CN)₃]⁻ building block was investigated by combining paramagnetic Nuclear Magnetic Resonance (pNMR) spectroscopy and polarized neutron diffraction (PND) with first-principle calculations. The use of the pNMR and PND experimental techniques revealed the extension of spin-density from the metal to the ligands, as well as the different spin mechanisms that take place in the cyanido ligands: Spin-polarization on the carbon atoms and spin-delocalization on the nitrogen atoms. The results of our combined density functional theory (DFT) and multireference calculations were found in good agreement with the PND results and the experimental NMR chemical shifts. Moreover, the ab-initio calculations allowed us to connect the experimental spin-density map characterized by PND and the suggested distribution of the spin-density on the ligands observed by NMR spectroscopy. Interestingly, significant differences were observed between the pseudo-contact contributions of the chemical shifts obtained by theoretical calculations and the values derived from NMR spectroscopy using a simple point-dipole model. These discrepancies underline the limitation of the point-dipole model and the need for more elaborate approaches to break down the experimental pNMR chemical shifts into contact and pseudo-contact contributions.     

Potentials in density functional theory and the importance of sum rules

In an ab initio approach to density functional theory one needs to know the electronic pair-density averaged over the coupling strength of the pair-interaction. As this pair-function is not available without having solved the associated N-electron problem, one has to resort to universal properties of the pair-density that are independent of specific features of the ground-state wavefunction. By exploiting these universal properties and so-called sum rules for the pair-correlation factors we derive very simple approximate spin-dependent expressions for the exchange-correlation energy per particle and for the associated potential in the Kohn-Sham equations. There is some similarity of the resulting density functionals with those obtained from the widely applied local spin density approximation (LSDA) based on electron gas theory. As the application of the latter to exceedingly inhomogeneous gases in realistic systems is very debatable, the manifest similarity seems to suggest that LSDA can consistently be justified only via the above pair-density analysis, but the justification of certain electron gas refinements may remain questionable. We shortly review similar attempts made by other authors and particularly focus on the issue of self-interaction and the “overbinding problem”. We demonstrate for the 3d- and 4d-metals that our approximation yields density of states (DOS), magnetic moments and Stoner parameters that are practically identical with respective data obtained from up-to-date LSDA- or gradient corrected (GGA-)potentials. There is also an excellent agreement of the DOS for the insulators (semi-conductors) C, Si, Ge, and GaAs. We show that our approach yields cohesive energies for these materials that are very close to the GGA-values indicating a distinct improvement over the standard LSDA-values. The calculations have been performed with the aid of the WIEN 97 computer code based on the Full Potential Linear Augmented Plane Wave (FLAPW-) method.     

Intramolecular spin alignment in photomagnetic molecular devices: a theoretical study

Ground- and excited-state magnetic properties of recently characterized pi-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient ((3)An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin-density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed.     

Classification of organic molecules to obtain electron affinities from half-wave reduction potentials: cytosine, uracil, thymine, guanine and adenine

A procedure for obtaining the adiabatic electron affinities (AEA) of organic molecules from half-wave reduction potentials in aprotic solvents is presented. Molecules are placed into groups according to their structure. Each group has a different solution energy difference. Calculations of AEA and charge distributions with AM1-multiconfiguration configuration interaction are used to support the intuitive classification of the molecules. The procedure is illustrated for Vitamins A and E, riboflavin, the azines, polyenes, hydroxy-pyrimidine, oxo-guanine, the hydrogen bonded cytosine-oxo-guanine as well as the AEA, and vertical EA (VEA) of Cytosine (C), Uracil (U), Thymine (T), Guanine (G) and Adenine (A). The latter values are: (VEA) G, 0.10; A, -0.49; U, 0.33; T, 0.31; C, -1.48 and (AEA) G, 1.51 +/- 0.05; A, 0.95 +/- 0.05; U, 0.80 +/- 0.05; T, 0.79 +/- 0.05; C, 0.56 +/- 0.05 in eV.     

Intriguing diaza effects on magnetic coupling characteristics in diaza‐benzo[k]tetraphene‐bridged nitroxide diradicals

We theoretically design four diaza‐benzo[k]tetraphene‐based diradical isomers ( 1, 2, 3 , and 4 ) with two nitroxide (NO) radical groups as spin sources. The calculations at the B3LYP/6‐311++G(d,p) level suggest that the diaza doping can induce the aromaticity changes and the C C bond rearrangements and, thus, remarkably affect their magnetic coupling magnitudes and even characteristics (ferromagnetic vs. antiferromagnetic). More interestingly, different diaza‐doping positions can lead to distinctly different effects, and further dielectron‐oxidation can also noticeably change the magnetic coupling magnitudes from −919.9 cm⁻¹ ( 1 ) to −158.3 cm⁻¹ ( 1 ²⁺ ) or from −105.1 cm⁻¹ ( 3 ) to −918.9 cm⁻¹ ( 3 ²⁺ ) or induce the magnetic conversions from nonmagnetism ( 2 ) to antiferromagnetism ( 2 ²⁺ , −140.1 cm⁻¹) or from ferromagnetism ( 4 , 108.9 cm⁻¹) to antiferromagnetism ( 4 ²⁺ , −462.5 cm⁻¹). Good matching of two singly occupied molecular orbitals (SOMOs) of the NO groups with the highest occupied molecular orbital (HOMO) of the coupler (for 1 ), or with the lowest unoccupied molecular orbital (LUMO) of the coupler (for 3 ²⁺ and 4 ²⁺ ), available Kekulé structure (for 2 ), aromaticity variations are responsible to strong magnetic couplings. Besides, the HOMO‐LUMO energy gaps of the couplers also considerably affect the magnetic couplings. This work may open a new route for the rational design of the diaza‐benzo[k]tetraphene‐based magnetic molecular modulators or switches.     

Model for dynamics of inhomogeneous and bulk fluids

An accurate model for the density of states (DOS) for strongly inhomogeneous and bulk fluids has been proposed based on gamma distributions. The contribution to the density of states from the collective dynamics is modeled as an incomplete gamma distribution and the high frequency region is obtained from the solution of the memory equation using a sech memory kernel. Using only the frequency moments as input, the model parameters for the collective dynamics are obtained by matching moments of the resulting distribution. The model results in an analytical expression for the self-diffusivity of the fluid. We present results for soft sphere fluids confined in slit-shaped pores as well as bulk soft sphere liquids. Comparisons of the DOS, velocity autocorrelation functions, and memory kernels with molecular dynamics simulations reveal that the model predicts features in the DOS over the entire frequency range and is able to capture changes in the DOS as a function of fluid density and temperature. As a result the predicted VACFs, memory kernels, and self-diffusivities are accurately predicted over a wide range of conditions. Since the frequency moments for bulk liquids can be obtained from pair correlation functions, our method provides a direct route from fluid structure to dynamics. For fluids confined in slit-shaped pores, where the frequency moments are obtained from molecular dynamics simulations, the predicted self-diffusivities capture the resulting oscillations due to variations in the solvation pressure, and in the case of smooth walled pores, the predictions are superior to those obtained using kinetic theory.