Self-interaction correction and isotropic hyperfine parameter of light atoms

Numerical self-consistent field (SCF) calculations in density functional theory (DFT) and the local spin-density approximation (LSDA) were performed for the light atoms H, Li, B, C, N, O and F, in order to investigate the effect of the self-interaction correction (SIC) on the isotropic (or contact) hyperfine parameter A_ISO. In contrast to the findings for certain 3d-metals and compounds, results for light-atom SI-corrected A_ISO present no improvement over the LSDA values. We show that relatively modest changes to the correlation potential can lead to significant improvement of densities near the nucleus and the related A_ISO, suggesting a direction for future improvements in DFT functionals.     

Core-valence correlation effects for molecules containing first-row atoms. Accurate results using effective core polarization potentials

The accuracy of employing effective core polarization potentials (CPPs) to account for the effects of core-valence correlation on the spectroscopic constants and dissociation energies of the molecules B₂, C₂, N₂, O₂, F₂, CO, CN, CH, HF, and C₂H₂ has been investigated by comparison to accurate all-electron benchmark calculations. The results obtained from the calculations employing CPPs were surprisingly accurate in every case studied, reducing the errors in the calculated valence D _e values from a maximum of nearly 2.5 kcal/mol to just 0.3 kcal/mol. The effects of enlarging the basis set and using higher-order valence electron correlation treatments were found to have only a small influence on the core-valence correlation effect predicted by the CPPs. Thus, to accurately recover the effects of intershell correlation, effective core polarization potentials such as the ones used in the present work provide an attractive alternative to carrying out computationally demanding calculations where the core electrons are explicitly included in the correlation treatment. Received: 11 May 1998 / Accepted: 27 July 1998 / Published online: 28 October 1998     

Generation of ground-state structures and electronic properties of ternary AlxTiyNiz clusters (x + y + z = 6) with a two-stage density functional theory global search approach

The structural and electronic properties of ternary Al_xTi_yNi_z clusters, where x, y, and z are integers and x + y + z = 6 , are investigated. Both Slater, Vosko, Wilks, and Nusair and B3LYP exchange-correlation (XC) functionals are employed in a two-stage density functional theory (DFT) calculations to generate these clusters. In the first stage, a minimum energy cluster structure is generated by an unbiased global search algorithm coupled with a DFT code using a light XC functional and small basis sets. In the second stage, the obtained cluster structure is further optimized by another round of global minimization search coupled with a DFT calculator using a heavier XC functional and more costly basis set. Electronic properties of the structures are illustrated in the form of a ternary diagram. Our DFT calculations find that the thermodynamic stability of the clusters increases with the increment in the number of constituent nickel atoms. These results provide a new insight to the structure, stability, chemical order, and electronic properties for the ternary alloy nanoclusters.     

Relating the Composition of PtxRu100−x/C Nanoparticles to Their Structural Aspects and Electrocatalytic Activities in the Methanol Oxidation Reaction

A controlled composition‐based method—that is, the microwave‐assisted ethylene glycol (MEG) method—was successfully developed to prepare bimetallic Pt_xRu_100?x/C nanoparticles (NPs) with different alloy compositions. This study highlights the impact of the variation in alloy composition of Pt_xRu_100?x/C NPs on their alloying extent (structure) and subsequently their catalytic activity towards the methanol oxidation reaction (MOR). The alloying extent of these Pt_xRu_100?x/C NPs has a strong influence on their Pt d‐band vacancy and Pt electroactive surface area (Pt ECSA); this relationship was systematically evaluated by using X‐ray absorption (XAS), scanning electron microscopy (SEM) coupled with energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy (TEM), density functional theory (DFT) calculations, and electrochemical analyses. The MOR activity depends on two effects that act in cooperation, namely, the number of active Pt sites and their activity. Here the number of active Pt sites is associated with the Pt ECSA value, whereas the Pt‐site activity is associated with the alloying extent and Pt d‐band vacancy (electronic) effects. Among the Pt_xRu_100?x/C NPs with various Pt:Ru atomic ratios (x=25, 50, and 75), the Pt₇₅Ru₂₅/C NPs were shown to be superior in MOR activity on account of their favorable alloying extent, Pt d‐band vacancy, and Pt ECSA. This short study brings new insight into probing the synergistic effect on the surface reactivity of the Pt_xRu_100?x/C NPs, and possibly other bimetallic Pt‐based alloy NPs.     

Core-valence correlation potentials based on density functional theory. Applications to valence-electron-only calculations on Na and K diatomics

The core-valence correlation potential has been derived for Na and K employing atomic calculations which make use of the density functional formula worked out by Lee, Yang and Parr based on Colle-Salvetti approach. The numerical potential is fitted with a small number of Gaussians leading to a very simple expression for an one-electron corevalence correlation operator? _cv . The core-valence correlation corrections can be computed by applying? _cv on a quite general class of wavefunctions. Applications of the? _cv operator within the framework of valence-electron-only calculations using effective Hamiltonians are presented for Na and K atoms, for Na₂, K₂, NaK and their cations. Almost all the corrections calculated for the physical properties due to the core-valence correlation lead to results which are in good agreement with those obtained from much more sophisticated treatments and experimental data.     

Theoretical and experimental 1H, 13C and 15N NMR studies of N‐alkylation of substituted tetrazolo[1,5‐a]pyridines

The ¹⁵N as well as ¹H and ¹³C chemical shifts of nine substituted tetrazolopyridines and their corresponding tetrazolopyridinium salts have been determined by using NMR spectroscopy at the natural abundance level of all nuclei in CD₃CN. In this paper, we report, for the first time, the N‐alkylation reaction of electron deficient tetrazolopyridines. The treatment of tetrazolopyridines 5–13 with one equivalent of trialkyloxonium tetrafluoroborate leads to a mixture of two isomers, i.e. N3‐ and N2‐alkyl tetrazolo[1,5‐a]pyridinium salts. It has been observed that the N3‐isomer is always the major isomer, except in the case of the CF₃ substituent, where the two isomers are obtained in the same amount. The quaternary tetrazolopyridinium nitrogen N3 is shielded by around 100 ppm (parts per million) with respect to the parent tetrazolopyridine. Experimental data are interpreted by means of density functional theory (DFT) calculations, including solvent‐induced effects, within the conductor‐like polarizable continuum model (CPCM). Good agreements between theoretical and experimental ¹H, ¹³C and ¹⁵N NMR were found. The combination of multinuclear magnetic resonance spectroscopy with gauge including atomic orbital (GIAO) DFT calculations is a powerful tool in the structural elucidation for both neutral and cationic heterocycles and in the determination of the orientation of N‐alkylation of tetrazolopyridines. Copyright © 2009 John Wiley & Sons, Ltd.     

A density‐functional study of the phase diagram of cementite‐type (Fe,Mn)3C at absolute zero temperature

The phase diagram of (Fe_1?x Mn_x)₃C has been investigated by means of density‐functional theory (DFT) calculations at absolute zero temperature. The atomic distributions of the metal atoms are not random‐like as previously proposed but we find three different, ordered regions within the phase range. The key role is played by the 8d metal site which forms, as a function of the composition, differing magnetic layers, and these dominate the physical properties. We calculated the magnetic moments, the volumes, the enthalpies of mixing and formation of 13 different compositions and explain the changes of the macroscopic properties with changes in the electronic and magnetic structures by means of bonding analyses using the Crystal Orbital Hamilton Population (COHP) technique. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

The Structure of Gas Phase Tin Oxide Ions Generated by Laser Ablation: A Combined Fourier Transform Mass Spectrometry and Density Functional Theory Study

The radical ion series (SnO)⁺ _2-6, (SnO)^– _2-6, (SnO)_0-5Sn⁺ and (SnO)_1-6O^– have been generated by the high power laser ablation of SnO and SnO₂ targets positioned inside an ICR cell. In all ablation spectra obtained, and for any particular size Sn _x core, the tin-rich clusters (SnO) _x Sn⁺ were more abundant than the corresponding oxygen-equivalent clusters (SnO)⁺ _x , while the oxygen-rich clusters (SnO) _x O^– were always more abundant than the oxygen-equivalent clusters (SnO)^– _x . High yields of the ions (SnO)_1,3Sn⁺, (SnO)_3,6O^– and (SnO)^– ₆ suggest high stabilities for these species. Low energy CID studies revealed that loss of neutral (SnO) _x units is the preferred, and for most ions investigated the exclusive, dissociation pathway. Global minima for the smaller cations and anions are proposed on the basis of local density functional theory (DFT) calculations. Calculated dissociation energies for the neutral and charged clusters were found to compare well with effusion cell and FTICR results. DFT also predicts that, for any cluster with the same size Sn _x core, IE(SnO) _x x Sn and EA(SnO) _x O>EA(SnO) _x . A correlation between ion abundances and DFT heats of formation is evident, and the ground state geometries provide insight into the evolution of structural versus size trends. Without assistance from the calculations, erroneous conclusions regarding the structures of the experimentally-sampled clusters might have been drawn from the low energy CID results.     

Structural Investigation of Small Nickel-Ethanol Clusters Using Vibrational Spectroscopy in a Molecular Beam

The structural identification of small nickel clusters with ethanol can help to understand fundamental steps for heterogenous catalysis. We investigate the rows [Ni_x(EtOH)₁]⁺ with x=1–4, and [Ni₂(EtOH)_y]⁺ with y=1–3 via IR photodissociation spectroscopy in a molecular beam experiment. Analyzing the CH- and OH-stretching frequencies and comparing these experimental results with density functional theory (DFT) calculations on the PW91/6-311+G(d,p) level leads to the identification of intact motifs for all clusters and hints for C−O cleavage of the ethanol in two particular cases. Furthermore, we analyze the effects of frequency shifts with the increasing clusters sizes using the results of natural bond orbitals (NBO) analyses and an energy decomposition method.     

Essential Factors for Control of the Equilibrium in the Reversible Rearrangement of M3N@Ih‐C80 Fulleropyrrolidines: Exohedral Functional Groups versus Endohedral Metal Clusters

The effects of exohedral moieties and endohedral metal clusters on the isomerization of M₃N@I_h‐C₈₀ products from the Prato reaction through [1,5]‐sigmatropic rearrangement were systematically investigated by using three types of fulleropyrrolidine derivatives and four different endohedral metal clusters. As a result, all types of derivatives provided the same ratios of the isomers for a given trimetallic nitride template (TNT) as the thermodynamic products, thus indicating that the size of the endohedral metal clusters inside C₈₀ was the single essential factor in determining the equilibrium between the [6,6]‐isomer (kinetic product) and the [5,6]‐isomer. In all the derivatives, the [6,6]‐ and [5,6]‐Prato adducts with larger metal clusters, such as Y₃N and Gd₃N, were equally stable, which is in good agreement with DFT calculations. The reaction rate of the rearrangement was dependent on both the substituent of exohedral functional groups and the endohedral metal‐cluster size. Further DFT calculations and ¹³C NMR spectroscopic studies were employed to rationalize the equilibrium in the rearrangement between the [6,6]‐ and [5,6]‐fulleropyrrolidines.     

Reaction of an N‐Heterocyclic Carbene‐Stabilized Silicon(II) Monohydride with Alkynes: [2+2+1] Cycloaddition versus Hydrogen Abstraction

An in depth study of the reactivity of an N‐heterocyclic carbene (NHC)‐stabilized silylene monohydride with alkynes is reported. The reaction of silylene monohydride 1 , tBu₃Si(H)Si←NHC, with diphenylacetylene afforded silole 2 , tBu₃Si(H)Si(C₄Ph₄). The density functional theory (DFT) calculations for the reaction mechanism of the [2+2+1] cycloaddition revealed that the NHC played a major part stabilizing zwitterionic transition states and intermediates to assist the cyclization pathway. A significantly different outcome was observed, when silylene monohydride 1 was treated with phenylacetylene, which gave rise to supersilyl substituted 1‐alkenyl‐1‐alkynylsilane 3 , tBu₃Si(H)Si(CH?CHPh)(C?CPh). Mechanistic investigations using an isotope labelling technique and DFT calculations suggest that this reaction occurs through a similar zwitterionic intermediate and subsequent hydrogen abstraction from a second molecule of phenylacetylene.     

Achieving High‐Temperature Stability of Metastable α‐MoC1‐x by Suppressing Phase Transformation with Mounted Atoms for Lithium Storage Performance

Despite a significant advancement in preparing metastable materials, one common problem is the strict and precious reaction conditions due to their metastable structures. Herein, we achieved the preparation of high‐temperature stabilized metastable α‐MoC_1?x by mounting zinc atoms into its lattice structure. Such a structural construction could suppress the phase transformation from α‐MoC_1?x to β‐Mo₂C through restricting the displacement of Mo atoms upon increased temperature. The resultant metastable α‐MoC_1?x can be stabilized up to 1000 °C and this stability temperature is the highest for the metastable α‐MoC_1?x so far. Synchrotron X‐ray absorption spectroscopy (XAS) and X‐ray photoelectron spectroscopy (XPS) confirm the structure of Zn‐mounted α‐MoC_1?x. Density functional theory (DFT) calculations reveal that the introduction of the Zn atoms in the lattice structure of α‐MoC_1?x could significantly decrease the energy difference (ΔE) between α‐MoC_1?x and β‐Mo₂C, thus effectively suppressing the phase transformation from α‐MoC_1?x to β‐Mo₂C and accordingly maintaining the high‐temperature stability of α‐MoC_1?x. This novel strategy can be used as a universal method to be extended to synthesize metastable α‐MoC_1?x from different precursors or other mounted elements. Moreover, the optimal product exhibits excellent lithium storage performances in terms of the cycling stability and rate performance.     

A Metal‐Containing N‐Heterocyclic Germylene Based on an Oxalamidine Framework

A metal‐containing N‐heterocyclic germylene based on a N‐mesityl (Mes)‐substituted oxalamidine framework is reported. The precursor (MesN=)₂C–C(–N(H)Mes)₂ ( 1 H₂) was converted into its rhodium complex [Rh(κ²N‐ 1 H₂)(cod)][OTf] ( 2 ) (cod = 1,5‐cyclooctadiene; OTf = triflate) in 62 % isolated yield. Subsequent reaction of 2 with Ge{N(SiMe₃)₂}₂ gave the crystalline N‐heterocyclic germylene [Rh(cod)(μ‐ 1 )Ge][OTf] ( 3 ) in 50 % yield. The compounds under study were fully characterized by various methods, also including X‐ray crystallographic studies on single crystals of 2 and 3 . Density functional theory (DFT) calculations revealed that π conjugation in the bridging oxalamidine framework is increased and n(N)–p(Ge) π bonding is decreased upon κ²N metal coordination; a further weakening of the Ge–N bond occurs through triflate coordination to the Ge^II atom. Nevertheless, preliminary coordination studies revealed that 3 behaves as 2‐electron (L ‐type) germylene donor ligand. Treatment of 3 with [Ir(cod)Cl]₂ furnished the heterobimetallic complex [Rh(cod)(μ‐ 1 )Ge‐Ir(cod)Cl][OTf] ( 4 ), as evidenced by NMR spectroscopic investigations and DFT calculations.     

Glycosidic linkage,N‐acetyl side‐chain,and other structural properties of methyl 2‐acetamido‐2‐deoxy‐β‐d‐glucopyranosyl‐(1→4)‐β‐d‐mannopyranoside monohydrate and related compounds

The crystal structure of methyl 2‐acetamido‐2‐deoxy‐β‐d ‐glycopyranosyl‐(1→4)‐β‐d ‐mannopyranoside monohydrate, C₁₅H₂₇NO₁₁·H₂O, was determined and its structural properties compared to those in a set of mono‐ and disaccharides bearing N‐acetyl side‐chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C—N (amide) and C—O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N—H hydrogen. Relative to N‐acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen‐bond acceptor display elongated C—O and shortened C—N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C—N and C—O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis–trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter‐residue hydrogen bonding and some bond angles in or proximal to β‐(1→4) O‐glycosidic linkages on linkage torsion angles ? and ψ. Hypersurfaces correlating ? and ψ with the linkage C—O—C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.     

Mixed silicon–germanium nanocrystals: a detailed study of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>47−<Emphasis Type="Italic">x</Emphasis></Subscript>:H

Mixed SiGe:H nanocrystals have been studied within the framework of Density Functional Theory. (DFT) using the hybrid non-local exchange-correlation functional of Becke, Lee, Parr and Yang (B3LYP). In addition to ground-state DFT/B3LYP calculations, excited-state calculations for the determination of the optical absorption spectrum have been performed employing the time-dependent density functional theory (TDDFT). In order to fully investigate the substitution of Si by Ge, on structural, cohesive, electronic and optical properties, we have used the Si _x Ge_47−x :H nanocrystal, as a representative medium size nanosystem. Our results show that the optical gap depends not only on the relative concentrations of silicon, germanium and hydrogen, but also on the relative position of the silicon and germanium shells relative to the surface of the nanocrystal. This is also true for the structural, cohesive and electronic properties. This dependence allows for the possibility of electronic and optical gap engineering.     

15N and 13C NMR chemical shifts of 6‐(fluoro,chloro, bromo,and iodo)purine 2′‐deoxynucleosides: measurements and calculations

The ¹⁵N and ¹³C chemical shifts of 6‐(fluoro, chloro, bromo, and iodo)purine 2′‐deoxynucleoside derivatives in deuterated chloroform were measured. The ¹⁵N chemical shifts were determined by the ¹H? ¹⁵N HMBC method, and complete ¹⁵N chemical‐shift assignments were made with the aid of density functional theory (DFT) calculations. Inclusion of solvation effects significantly improved the precision of the calculations of ¹⁵N chemical shifts. Halogen‐substitution effects on the ¹⁵N and ¹³C chemical shifts of purine rings are discussed in the context of DFT results. The experimental coupling constants for ¹⁹F interacting with ¹⁵N and ¹³C of the 6‐fluoropurine 2‐deoxynuleoside are compared with those from DFT calculations. Copyright © 2009 John Wiley & Sons, Ltd.     

Evaluation of Non‐covalent Binding Energies and Optoelectronic Properties of New CuBr2(C6H7N)2 Complex: DFT Approaches

For the first time, the structural and optoelectronic properties of a new complex formulated as CuBr₂(C₆H₇N)₂ ( 1 ) [trans‐dibromidobis(3‐methylpyridine‐κN) copper(II)] were studied by density functional theory (DFT) calculations. They are performed using B3LYP through the Gaussian 09 program and also with full potential linearized augmented plane wave (FP‐LAPW) methods within the Generalized Gradient Approximation (GGA) and Hartree‐Fock (HF) theory by the Wien2k package. The neutral monomeric complex participates in a variety of non‐covalent interactions, including hydrogen bonding and π stacking to create a 2D coordinate plane. The binding energy value of the non‐covalent interactions responsible for the crystalline network formation of 1 were calculated using the method of dispersion corrected density functional theory (DFT‐D). In this method, the independent smallest fragment (monomer) and subsequently the related network, including seven monomers bearing all non‐covalent interactions were optimized. The results demonstrate that hydrogen bonds, especially non‐conventional C–H ··· Br interactions, govern the network formation along the a and c axes. It can be mentioned because of these directed interactions, increasing of charge transfer along x and z directions results in increasement of the absorption and refractive index along y and z directions, and vice versa. The results of band structure show indirectly and directly the nature of the bandgap within GGA and HF, respectively. The bandgap value of CuBr₂(C₆H₇N)₂ is comparable to those of binary semiconductor compounds. DOSs spectra reveal that 3d Cu, 4p Br, and 2p C states play important roles in the optical transitions of the electrons. The calculated electronic absorption of the UV/Vis spectrum shows six major electron‐transition bands derived from d → d (ligand field) n → n, n → π*, π → n, and σ → n MLCT and LMCT transitions. The calculated absorption spectrum of the titled complex through FP‐LAPW within GGA method shows good consistency with the B3LYP/def2‐TZVP/6‐311+G(d,p) method. Our calculated birefringence results show that 1 has capability of nonlinear optical, which can be used in the nonlinear optoelectronic devices.     

Solid Solutions of Grimm–Sommerfeld Analogous Nitride Semiconductors II-IV-N2 (II=Mg,Mn, Zn; IV=Si,Ge): Ammonothermal Synthesis and DFT Calculations

Grimm–Sommerfeld analogous II-IV-N₂ nitrides such as ZnSiN₂, ZnGeN₂, and MgGeN₂ are promising semiconductor materials for substitution of commonly used (Al,Ga,In)N. Herein, the ammonothermal synthesis of solid solutions of II-IV-N₂ compounds (II=Mg, Mn, Zn; IV=Si, Ge) having the general formula (II^a_1−xII^b_x)-IV-N₂ with x≈0.5 and ab initio DFT calculations of their electronic and optical properties are presented. The ammonothermal reactions were conducted in custom-built, high-temperature, high-pressure autoclaves by using the corresponding elements as starting materials. NaNH₂ and KNH₂ act as ammonobasic mineralizers that increase the solubility of the reactants in supercritical ammonia. Temperatures between 870 and 1070 K and pressures up to 200 MPa were chosen as reaction conditions. All solid solutions crystallize in wurtzite-type superstructures with space group Pna2₁ (no. 33), confirmed by powder XRD. The chemical compositions were analyzed by energy-dispersive X-ray spectroscopy. Diffuse reflectance spectroscopy was used for estimation of optical bandgaps of all compounds, which ranged from 2.6 to 3.5 eV (Ge compounds) and from 3.6 to 4.4 eV (Si compounds), and thus demonstrated bandgap tunability between the respective boundary phases. Experimental findings were corroborated by DFT calculations of the electronic structure of pseudorelaxed mixed-occupancy structures by using the KKR+CPA approach.