Main Group Atoms Research Articles

Variational versus Perturbational Treatment of Spin-Orbit Coupling in Relativistic Density Functional Calculations of Electronic g Factors: Effects from Spin-Polarization and Exact Exchange.

Different approaches are compared for relativistic calculations of electronic g factors of molecules with light atoms, transition metal complexes, and selected complexes with actinides, using density functional theory (DFT) and Hartree-Fock (HF) theory. The comparison includes functionals with range-separated exchange. Within the variationally stable zeroth-order regular approximation (ZORA) relativistic framework, g factors are obtained with a linear response (LR) method where spin-orbit (SO) coupling is treated as a linear perturbation, a spin-polarized approach based on magnetic anisotropy (MA) that includes SO coupling variationally, and a quasi-restricted variational SO method previously devised by van Lenthe, van der Avoird, and Wormer (LWA). The MA and LWA approaches were implemented in the open-source NWChem quantum chemistry package. We address the importance of electron correlation (DFT vs HF), the importance of including spin polarization in the g tensor methodology, the question of whether the use of nonrelativistic spin density functionals is adequate for such calculations, and the importance of treating spin-orbit coupling beyond first-order. For selected systems, the extent of the DFT delocalization error is explicitly investigated via calculations of the energy as a function of fractional electron numbers. For a test set of small molecules with light main group atoms, all levels of calculation perform adequately as long as there is no energetic near-degeneracy among occupied and unoccupied orbitals. The interplay between different factors determining the accuracy of calculated g factors becomes more complex for systems with heavy elements such as third row transition metals and actinides. The MA approach is shown to perform acceptably well for a wide range of scenarios.

Unusual lattice constant changes and tunable magnetic moment compensation in Mn50−xCo25Ga25+x alloys

We report on unusual lattice parameter changes and tunable magnetic moment compensation in Mn50−xCo25Ga25+x (x = 0-25) Heusler alloys by substituting Ga for Mn. The observed lattice parameter first increases with increasing Ga content x, showing a maximum at x = 12.5, and then abnormally decreases due to the enhanced covalence effect between transition-metal and main-group atoms. Moreover, a tunable magnetic moment compensation was also observed due to the diversification in role of the main magnetic contributor when the structure varies from Hg2CuTi-type Mn2CoGa to Cu2MnAl-type CoMnGa2. These results provide an alternative way to simultaneously tune both the structural and magnetic properties of Heusler alloys, which is particularly important for developing flexible spintronics devices.

Structural Diversity Within the Series of 68-Electron M4L n E2 (L = 2-Electron Ligand; M = Fe, Ru, Os, Co; E = CH, N, P, NR, PR, S) Organometallic Clusters: A Theoretical Investigation

Four different skeletal structural arrangements with very different connectivities are known for 6-vertex/68-electron of M4E2 core (M = transition metal; E = main-group atom or ligand). DFT calculations on a large number of title model compounds allow to rationalize the preferences between these structural shapes with respect to the nature of the metal and main-group elements constituting the cluster cage. In particular, the electronegativity of M and the “size” (first-row vs. second-row element) of E play an important role in the stability preference of a particular isomer. For several compounds, although only one type of structure is known, other low-energy isomeric forms are also likely to exist. Moreover, two structural types, so far unreported, are predicted to be stable enough for being synthesized.

Spline Implementation of Generalized Gradient Approximations to the Exchange-Correlation Functional and Study of the Sensitivity of Density Functional Accuracy to Localized Domains of the Reduced Density Gradient.

We present a natural cubic spline implementation of the exchange enhancement factor as a function of the reduced density gradient, and we demonstrate its performance by replicating the results of common GGA functionals. We also investigate the effect on the accuracy of various calculated properties of changing the shape of the exchange enhancement factor and an analogous factor for correlation. The properties considered are main group atomization energies, ionization potentials, electron affinities, proton affinities, alkyl bond dissociation energies, difficult hydrocarbon cases, barrier heights for chemical reactions, noncovalent interactions, atomic energies, metal bond energies, and main group bond lengths.

Improving the Accuracy of Hybrid Meta-GGA Density Functionals by Range Separation

The Minnesota family of exchange–correlation functionals, which consists of meta generalized gradient approximations (meta-GGAs) and global-hybrid meta-GGAs, has been successful for density functional calculations of molecular structure, properties, and thermochemistry, kinetics, noncovalent interactions, and spectroscopy. Here, we generalize the functional form by using range-separated hybrid meta-GGA exchange. We optimize a functional, called M11, with the new form against a broad database of energetic chemical properties and compare its performance to that of several other functionals, including previous Minnesota functionals. We require the percentage of Hartree–Fock exchange to be 100 at large interelectronic distance, and we find an optimum percentage of 42.8 at short range. M11 has good across-the-board performance and the smallest mean unsigned error over the whole test set of 332 data; it has especially good performance for main-group atomization energies, proton affinities, electron affinities, ...

New mixed palladium—lead and palladium—tin tellurides of the NiAs type

New ternary mixed nickel arsenide-type palladium and platinum—Group 14 metal tellurides were searched for by the high-temperature ampoule synthesis combined with the X-ray powder diffraction analysis. As a result, new phases of the ternary Pd—Pb—Te and Pd—Sn—Te systems belonging to the NiAs structure type were synthesized. The crystal structure of the compound PdPb0.304(5)Te0.695(5) was determined by the Rietveld method (P63/mmc, a = 4.151(1) A, c = 5.680(1) A, Z = 2, R1 = 0.032, Rp = 0.075, wRall = 0.025). It was shown that no ordering exists in the occupancy of the main-group atom site by lead and tellurium. In the Pd—Sn—Te system, a phase isostructural with the above-mentioned compound was found. In the latter phase, there is also no ordering in the occupancy of the main-group atom site. According to the electron diffraction data, a superstructure derived from the NiAs type exists in a narrow range of the compositions PdSn0.3Te0.7—PdSn0.4Te0.6.

Supramolecular copper halide networks using the coordinative flexibility of the cyclo-P 2S 3 middle deck in (C 5Me 5) 2Mo 2P 2S 3: Phosphorus versus sulfur coordination

Reaction of solutions of [ Cp 2 ∗ Mo 2P 2S 3] ( 1; Cp ∗ = η 5-C 5Me 5) in CH 2Cl 2 with solutions of CuHal (Hal = Cl, Br, I) in CH 3CN gave after slow diffusion the novel coordination polymers [ Cp 2 ∗ Mo 2P 2S 3(CuCl) 4] ( 2), [ Cp 2 ∗ Mo 2P 2S 3(CuBr) 3] ( 3) and [ Cp 2 ∗ Mo 2P 2S 3(CuI) 3] ( 4). The products were characterized by elemental analysis, 31P magic angle spinning (MAS) NMR spectroscopy and single-crystal X-ray crystallography. The solid-state structures of 2 and 3 are composed of two-dimensional polymeric networks, whereas that of 4 contains parallel stacks of one-dimensional ribbons. In all cases the planar cyclo-P 2S 3 middle deck of the organometallic building block 1 provides one or two P atoms of the PS dumbbells along with the singly bridging sulfur atom as coordination sites for the various copper halide subunits. The assignment of P atoms follows from the correlation of 31P MAS NMR spectra with X-ray crystallographic data. In all compounds the distribution of main group atoms in the cyclo-P 2S 3 middle deck is different from that previously found in the starting material 1.

Transition metals as Lewis bases: “Z-type” boron ligands and metal-to-boron dative bonding

While the vast majority of inorganic chemistry involves electron donation from main-group atoms to metals, an intriguing yet flip-side exists: where Lewis-basic metals donate electron density to Lewis-acidic main-group atoms (most often boron). These so-called "Z-type" ligands, along with other less clear-cut complexes, are examples of this metal-ligand role reversal. This perspective article offers an introduction to metal-to-boron dative bonding, and attempts to correlate spectroscopic and structural data from the complexes.

ChemInform Abstract: Theoretical Aspects of Metal Cluster Chemistry

During the last twenty years the Polyhedral Skeletal Electron Pair Theory and the isolobal analogy have provided a theoretical basis for the rapid experimental developments, which have occurred in metal cluster chemistry. These theoretical principles have been underpinned by m.o. calculations on specific molecules and more generally by the Tensor Surface Harmonic Theory. This paper will review the important theoretical developments and relate them to the experimental and structural data which have been obtained for cluster compounds in our own and other laboratories. In particular the application of theoretical principles for rationalising and predicting the structures of cluster compounds of the platinum metals and gold are discussed. The bonding requirements of interstitial atoms and fragments are discussed, particularly in the context of interstitial C, B, N and transition metal atoms and diatomic fragments, e.g. C2 and C-H. POLYHEDRAL SKELETAL ELECTRON PAIR THEORY The historical development of the theoretical ideas which have contributed to the Polyhedral Skeletal Electron Pair Theory can be traced back to the pioneering work of Longuet-Higgins forty years ago. In the 1950's the contributions of Longuet-Higgins proved decisive since he not only provided a molecular orbital description of the bonding in diborane (ref. l), but also pioneered the application of molecular orbital theory ideas to deltahedral borides and boranes (ref. 2). The three-centre two-electron description of B-H-B and B-B-B bonds was elegantly generalised into the styx formalism and applied to all known boron hydrides by Lipscomb (ref. 3). The molecular orbital analysis of polyhedral boranes resulted in the successful prediction of the octahedral and icosahedral borane anions some years before they were structurally characterised (ref. 4). Hoffmann and Lipscomb (ref. 5) also developed the molecular orbital methodology of L nguet-Higgins in a general molecular orbital analysis of polyhedral borane anions, BnH;-, in 1962. In the 1960's Cotton and Haas (ref. 6) pioneered the development of molecular orbital ideas to metal cluster compounds of the early transition metals stabilised by halide ligands. Their analysis emphasised the important interactions which can result in such clusters from the overlap of the transition metal d orbitals. An alternative localised description of the bonding in such clusters, which resembled the styx methodology, was proposed by Kettle (ref. 7). These theoretical models tended to emphasise the inherent differences between metal and main group clusters and in common with ligand field theory tended to stress the role of the metal d orbitals. By the mid-nineteen sixties the major classes of transition metal IT -donor and acceptor clusters, and main group polyhedral borane and Zintl 'naked' clusters had been established but were viewed as distinct areas of inorganic chemistry. Experimental studies had, however, begun to indicate the artificiality of these subdivisions. The synthesis of polyhedral organometallic compounds from the reactions of acetylenes with metal carbonyls by HUbel and Braye and transition metallocarboranes by Hawthorne's group (ref. 8) clearly demonstrated that it was possible to synthesise polyhedral molecules with transition metal and main group atoms at the vertices. In addition the structural characterisation of Rh6(C0Il6 by Dahl (ref. 9) provided a real difficulty for the theoretical models which were most widely used. The bonding in Rh6(C0)16 could not be explained by the effective atomic number rule and was not amenable to a molecular orb'tal

Crossed Molecular Beam Study on the Ground State Reaction of Atomic Boron [B(2Pj)] with Hydrogen Cyanide [HCN(X1Σ+)

The linear boronisocyanide species, [BNC(X(1)Sigma(+))], represents the simplest triatomic molecule with three distinct, neighboring main group atoms of the second row of the periodic table of the elements: boron, carbon, and nitrogen. This makes boronisocyanide a crucial benchmark system to understand the chemical bonding and the electronic structure of small molecules, in particular when compared to the isoelectronic tricarbon molecule, [CCC(X(1)Sigma(g)(+))]. However, a clean, directed synthesis of boronisocyanide-a crucial prerequisite to study the properties of this molecule-has remained elusive so far. Here, we combine crossed molecular beam experiments of ground state boron atoms ((2)P(j)) with hydrogen cyanide with electronic structure calculations and reveal that the boronisocyanide molecule, [BNC(X(1)Sigma(+))], is formed as the exclusive product under gas phase single collision conditions. We also show that higher energy isomers such as the hitherto unnoticed, ring-strained cyclic BNC(X(3)A') structure, which is isoelectronic to the triplet, cyclic tricarbon molecule, [C(3)(X(3)A(2)')], do exist as local minima. Our studies present the first directed synthesis and observation of gas phase boronisocyanide providing a doorway for further fundamental studies on one of the simplest triatomic molecules composed solely of group III-V elements.

A Rare Example of a Krypton Difluoride Coordination Compound: [BrOF2][AsF6]·2KrF2

The synthesis of [BrOF(2)][AsF(6)] x 2 KrF(2), its structural characterization, and bonding are described in this study. Although several KrF(2) adducts with transition metal centers have been previously reported, none have been crystallographically characterized. The solid-state Raman spectrum of [BrOF(2)][AsF(6)] x 2 KrF(2) has been assigned with the aid of quantum-chemical calculations. The low-temperature (-173 degrees C) X-ray crystal structure of [BrOF(2)][AsF(6)] x 2 KrF(2) consists of isolated molecular units and represents an example of KrF(2) coordinated to a main-group atom. The coordination geometry around the BrOF(2)(+) cation renders the free valence electron lone pair more compact than in free BrOF(2)(+). The KrF(2) ligands are coordinated trans to the fluorine atoms of BrOF(2)(+) with the AsF(6)(-) anion coordinated trans to oxygen. The quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) analyses have been carried out in order to define the nature of the bonding in the complex. A significant amount of charge (0.25 e) is transferred to BrOF(2)(+) from the two KrF(2) ligands (0.10 e each) and from the AsF(6)(-) anion (0.05 e). Significant polarization also occurs within the KrF(2) ligands, which enhances the anionic characters of the fluorine bridges. The interaction energy is mostly governed by the electrostatic interaction of the positively charged bromine atom with the surrounding fluorine atoms.

Room-temperature methane activation by a bimetallic oxide clusterAlVO4+

The spin density of AlVO 4 + clusters is localized on one terminal O atom bonded with Al rather than V atom. This cluster can abstract one H-atom from CH 4 at room temperature. Gas-phase room-temperature methane activation by various clusters draws increasing attention. In this Letter, we produce a series of bimetallic oxide clusters which contain both main-group metal aluminum atoms and transition-metal vanadium atoms. We provide solid experimental and theoretical evidence that bimetallic AlVO 4 + cluster can draw a hydrogen atom from methane to produce CH 3 at room temperature. The Al 3 VO 7 + cluster which can be considered as a system formed from Al 2 O 3 + AlVO 4 + does not break C–H bonds of methane. The results enrich the chemistry of gas phase methane activation and provide possible molecular level mechanism to couple with heterogeneous catalysis.

Theoretical study of the mechanisms of [3+2] cycloaddition reactions of trimetallaallenes [MMM] (M = C, Si, Ge, Sn, and Pb)

The potential energy surfaces for the cycloaddition reactions of 16 valence-electron trimetallaallene species have been studied using density functional theory (B3LYP/LANL2DZ). Seven trimetallaallene species of the form [double bond splayed left]M=M=M[double bond splayed right] with five-membered-rings where M = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, the alkene cycloaddition has been used to study the chemical reactivities of these 16 valence-electron trimetallaallene species. The present theoretical investigations suggest that their relative reactivity increases in the order: C=C=C < Si=Si=Si < Si=Ge=Si < Ge=Si=Ge < Ge=Ge=Ge < Sn=Sn=Sn < Pb=Pb=Pb. That is, less electronegative and heavier main group atoms will lead to a smaller ΔE(st) and, in turn, will facilitate the [3 + 2] cycloaddition reactions to alkenes. Furthermore, the singlet-triplet energy splitting of the 16 valence-electron trimetallaallene species, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.

Structural, magnetic, and thermal characteristics of the phase transitions in Gd 5Ga xGe 4−x magnetocaloric materials

Temperature-dependent, single crystal and powder X-ray diffraction studies as well as magnetization, and heat capacity measurements were carried out on two phases of the Gd 5Ga x Ge 4− x system: for x=0.7 and 1.0. Gd 5Ga 0.7Ge 3.3 shows three structure types as a function of temperature: (i) from 165 K to room temperature, the orthorhombic Sm 5Ge 4-type structure exists; (ii) below 150 K, it transforms to a orthorhombic Gd 5Si 4-type structure; and (iii) a monoclinic Gd 5Si 2Ge 2-type component is observed for the intermediate temperature range of 150 K≤ T≤165 K. This is the first time that all these three structure types have been observed for the same composition. For Gd 5Ga 1.0Ge 3.0, the room temperature phase belongs to the orthorhombic Pu 5Rh 4-type structure with interslab contacts between main group atoms of 2.837(4) Å. Upon heating above 523 K, it transforms to a Gd 5Si 4-type structure with this distance decreasing to 2.521(7) Å before decomposing above 573 K.

Validation study of the ability of density functionals to predict the planar-to-three-dimensional structural transition in anionic gold clusters

As gold clusters increase in size, the preferred structure changes from planar to three-dimensional and, for anionic clusters, Au(n)-, the two-dimensional(2D)-->three-dimensional (3D) transition is found experimentally to occur between n=11 and n=12. Most density functionals predict that planar structures are preferred up to higher n than is observed experimentally, an exception being the local spin density approximation. Here we test four relatively new functionals for this feature, in particular, M05, M06-L, M06, and SOGGA. We find that M06-L, M06, and SOGGA all predict the 2D-->3D transition at the correct value of n. Since the M06-L and M06 functionals have previously been shown to be reasonably accurate for transition metal bond energies, main group atomization energies, barrier heights, and noncovalent interaction energies, and, since they are here shown to perform well for the s-d excitation energy and ionization potential of Au atoms and for the size of Au(n)- clusters at which the 2D-->3D transition occurs, they are recommended for simulating processes catalyzed by gold clusters.

Theoretical study of the energies and activation barriers of elementary reactions of hydrogenation of the Ti-doped closo-aluminide cluster Al@TiAl11 and its anion Al@TiAl 11 −

The potential energy surfaces, energies E, and activation barriers h of elementary reactions of addition of an H2 molecule to the Ti-doped closo-aluminide cluster Al@TiAl11 and its anion Al@Ti11− with an icosahedral and marquee structure in the states with different multiplicity were calculated within the B3LYP approximation of the density functional theory using the 6–31G* and 6–311+G* basis sets. The results were compared with the data calculated at the same level of theory for the related reactions of hydrogenation of bare closo-aluminides Al13 and Al13− and their B-, C-, Si-, and Ge-doped derivatives. The computations demonstrated that, depending on the structure, charge, and multiplicity of the Al@TiAl11 cluster, the hydrogenation energy varies in the range 15–23 kcal/mol. At the first stage of addition (chemisorption) of H2, a μ-H2 complex at the Ti atom (intermediate) forms with the distance R(Ti-H2) ∼ 1.9–2.0 A, which is accompanied by an energy decrease of ∼4–10 kcal/mol. The H-H bond in the μ-H2 complex is ∼0.1 A longer and the stretching vibration frequency Vval(HH) is ∼700–1500 cm−1 (or more) lower than the corresponding characteristics of the isolated H2 molecule. In the transition state with an imaginary frequency of ∼600i–1100i, the H2 molecule is coordinated to the attacked edge Ti-Alr, and its length increases to ∼0.9–1.1 A. The activation barrier height h varies from a few kcal/mol to ∼8–10 kcal/mol when measured from the μ-H2 complex and is within 18–22 kcal/mol when measured from the product (dihydride Al@TiAl11H2). The latter barrier (to the reverse reaction of dehydrogenation) is considerably higher than the barriers to migration of hydrogen atoms around the metal cage in the Al@TiAl11H2 dehydrides. There is a correlation between the energies E and barriers h of hydrogenation reactions and the structure, external charge, and multiplicity of the Al@TiAl11 cluster. In all cases, the hydrogenation should occur significantly more readily than dehydrogenation. It was shown that these reactions can be both irreversible (for example, for an icosahedron in the singlet state) and reversible (for a marquee in the triplet state and others). The conclusion was drawn that the elementary reactions of hydrogenation and dehydrogenation for Ti-doped aluminides should occur considerably faster and under milder conditions than for bare aluminides or their analogues doped with main-group atoms.

The resolution of the identity approximation applied to the correlation consistent composite approach

A new implementation of the correlation consistent composite approach (ccCA), denoted RI-ccCA, utilizing both the resolution of the identity (RI) and local methods is presented. A set of 102 molecules composed of first and second row, main group atoms is employed to compare total energies, atomization energies, and enthalpies of formation between the original ccCA implementation and those of RI-ccCA. Relative CPU time and disk space requirements of RI-ccCA as compared to ccCA, demonstrate that on the average, employing the RI approximation in ccCA affords CPU time savings over 70% and disk space requirements diminished by well over 90% without introducing significant error in energetic properties.

A CCSD(T) and ccCA study of mixed silicon hydrides and halides: Structures and thermochemistry

A set of 24 mixed silicon hydrides and halides (SiH n X m − n , where X = F, Cl; m = 1–4; and n = 0– m) has been investigated using; (1) CCSD(T) with the correlation consistent basis sets of double-through quintuple-ζ quality; (2) CCSD(T) with the augmented tight d form of the correlation consistent basis sets for the second row atoms (Si and Cl); and (3) the recently developed correlation consistent composite approach (ccCA). Optimized geometries and harmonic vibrational frequencies are reported, as well as enthalpies of formation, free energies of formation, and dissociation reaction enthalpies. Relativistic effects, including atomic spin-orbit coupling and scalar relativistic effects, and core-valence correlation, have been accounted for via the ccCA formulism. As demonstrated in previous studies, the tight d basis sets accelerate the convergence, as compared to the standard correlation consistent basis sets, of CCSD(T)-computed geometries and energetics of molecular species containing second row, main group atoms. The ccCA enthalpies of formation are shown to be similar and, on average, better than the corresponding CCSD(T) enthalpies of formation for the silicon species. Further, large DKH-CCSD(T) computations with explicit inclusion of core-valence and scalar relativistic effects have been performed to predict the enthalpies of formation of SiHF, SiHCl, SiH 2F, SiHF 2, SiH 2Cl, and SiHCl 2.

Recent Developments in the Organic Chemistry of Calcium – An Element with Unlimited Possibilities in Organometallic Chemistry?

AbstractThe organocalcium chemistry developed vastly during the last decade. The preparation of the organocalcium compounds via direct synthesis (insertion of Ca into a C‐X bond of phenyl halides, Grignard reaction) affords skilful procedures due to the inertia of the calcium metal and the extreme reactivity of the organocalcium derivatives. Further suitable preparative methods include metathesis reactions of CaX2 with KR or LiR, metallation reactions of H‐acidic substrates, metal‐halogen exchange reactions, and transmetallation of heavy main group atoms in their compounds with calcium metal. Possibilities to stabilize organocalcium compounds include steric shielding by bulky ligands at the periphery and electronic reduction of the nucleophilicity of the calcium‐bound carbanions. Selected applications in catalysis such as hydrophosphination are also mentioned. Very recent developments and challenges in the preparation of alkaline earth metal(I) compounds are presented as well. Concepts to overcome the rather large atomization energies of the metals are discussed.

Versatile Behavior of Conjugated Diynes with Zirconocene Reactive Species

Thermal decomposition of Cp2ZrPh2 in the presence of the buta-1,3-diynes RC≡CC≡CR (R = Ph, SiMe3) can lead to seven- or five-membered metallacycles. In both cases a stable benzo-fused seven-membered zirconacyclocumulene arising from a 2-fold insertion of the triple bonds of the dialkyne in the in situ generated zirconocene benzyne is formed. In the case of Me3SiC≡CC≡CSiMe3 a second minor complex is isolated: a 3-alkynyl-substituted zirconaindene arising from a β monoinsertion of one acetylenic function of the conjugated diyne in the zirconocene benzyne. No stable 2-alkynyl-substituted zirconacycle was isolated. This α monoinsertion complex is an intermediate in the exchange of the metalated moiety with a main-group atom (e.g., antimony) in a zirconacyclocumulene, explaining the formation of a 2-alkynyl-substituted stibaindene. In the thermal decomposition of Cp2Zr(Me)SCH2R4 (R4 = Ph, p-MeOC6H4) in the presence of PhC≡CC≡CPh, only the α monoinsertion of one triple bond of the diacetylenic reagent in the tr...