Multiple bonds to gold: a theoretical investigation of XAuC (X=F, Cl, Br, I) molecules

Probing the structures and bonding of auropolyynes, Au-(C≡C) n -Au- (n = 1-3), using high-resolution photoelectron imaging

We report an investigation of a series of auropolyynes, Au-(C≡C) _n -Au^- (n = 1-3), using high-resolution photoelectron imaging and ab initio calculations. Vibrationally resolved photoelectron spectra are obtained, allowing the electron affinities of Au-(C≡C) _n -Au to be accurately measured as 1.651(1), 1.715(1), and 1.873(1) eV for n = 1-3, respectively. Both the Au-C symmetric stretching and a bending vibrational frequency are observed for each neutral auropolyyne. Theoretical calculations find that the ground state of Au₂C₂ ^- has a linear acetylenic Au-C≡C-Au^- structure, whereas the asymmetric Au-Au-C≡C^- structure is a low-lying isomer. However, for Au₂C₄ ^- and Au₂C₆ ^-, our calculations show that the asymmetric Au-Au-(C≡C) _n ^- isomers are the global minima and the Au-(C≡C) _n -Au^- symmetric structures become low-lying isomers. All the asymmetric Au-Au-(C≡C) _n ^- isomers are found computationally to have much higher electron binding energies and are not accessible at the detachment photon energies used in the current study. For neutral Au₂C_2n , the Au-(C≡C) _n -Au auropolyyne structures are found to be the global minima for n = 1-3. The electronic structures and bonding for Au-(C≡C) _n -Au (n = 1-3) are compared with the corresponding Au-(C≡C) _n and Au-(C≡C) _n -H species.

Comparison of Hydrogen and Gold Bonding in [XHX](-) , [XAuX](-) , and Isoelectronic [NgHNg](+) , [NgAuNg](+) (X=Halogen, Ng=Noble Gas)

Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.

Investigation into the metallophilic interaction in coinage-metal halides: an ab initio study of CMX (CM = Cu and Ag, X = F - I)

Investigation of the metallophilic interactions of the title coinage-metal halide series, CMX (CM = Ag and Cu, X = F - I), and their cationic and anionic systems, were performed at CCSD(T) theoretical level with extended basis sets. Natural bond orbital analysis shows that the interactions come mainly from the overlap of the sp hybrid on the halogen and the spd hybrid on the coinage-metal atom. Electron density deformation analysis demonstrates a pronounced electron accumulation in the interaction region between the heavier X and the coinage-metal atoms, and suggests a covalent character of the interaction. Positive Laplacian values and negative total energy densities at bond critical points (BCPs) show the "intermediate" character of the interactions. Reduced density gradient analysis visualizes the interaction; a linear relationship between energy densities and eigenvalues can be found at BCPs.

Probing the nature of gold-carbon bonding in gold-alkynyl complexes

Homogeneous catalysis by gold involves organogold complexes as precatalysts and reaction intermediates. Fundamental knowledge of the gold–carbon bonding is critical to understanding the catalytic mechanisms. However, limited spectroscopic information is available about organogolds that are relevant to gold catalysts. Here we report an investigation of the gold–carbon bonding in gold(I)–alkynyl complexes using photoelectron spectroscopy and theoretical calculations. We find that the gold–carbon bond in the ClAu–CCH⁻ complex represents one of the strongest gold–ligand bonds—even stronger than the known gold–carbon multiple bonds, revealing an inverse correlation between bond strength and bond order. The gold–carbon bond in LAuCCH⁻ is found to depend on the ancillary ligands and becomes stronger for more electronegative ligands. The strong gold–carbon bond underlies the catalytic aptness of gold complexes for the facile formation of terminal alkynyl–gold intermediates and activation of the carbon–carbon triple bond.

Fundamental understanding of gold–carbon bonding in homogeneous catalysts is vital for improved catalyst design, although spectroscopic information is limited. Here, the authors probe the bonding in gold–alkyne complexes using a combination of photoelectron spectroscopy and ab initio calculations.

Structure and properties of small aurocarbons: a selective study

Gold clusters are versatile catalysts, and adding nonmetal dopants can allow tuning of their electronic properties via both shape and composition alteration. In the present work, mixed clusters of carbon and gold atoms are studied in terms of structure, stability, and the correlation between the shape and electronic properties by using a density functional theory approach. Four series of isomers (hydrocarbon analogues, carbon chains and cycles on gold surface, and carbon cores encapsulated by gold atoms) are investigated, exhibiting variation of the relative stability with the system size. Calculated vertical ionization energies, vertical electron affinities, and HOMO-LUMO energy gaps of the mixed clusters show a considerable change relative to the values for the pure gold clusters, the properties generally altering more strongly for the gold-encapsulated-carbon isomers. Also discussed are the structure, stability, and properties of larger clusters with a few such encapsulated-carbon units, with pronounced effects due to aggregation.

Investigation of gold fluorides and noble gas complexes by matrix-isolation spectroscopy and quantum-chemical calculations

Noble with a difference: Matrix-isolation experiments and quantum-chemical calculations have led to the characterization of two new compounds, namely first open-shell binary gold fluoride, AuF(2), and a NeAuF complex. Moreover, ArAuF, AuF(3), Au(2)F(6), and monomeric AuF(5) have been produced and identified under cryogenic conditions in neon and argon matrices.

Interaction and Electron Density Properties of MKr42+ (M=Cu, Ag and Au): ab initio Calculation

Quantum chemical calculations of the structures and stabilities of the MKr42+ series at the CCSD(T) theoretical level have been performed. The role of the interaction was investigated using the natural bond orbital (NBO), Laplacian, electron density deformation, electron localization function and reduced density gradient analysis. The results show that a covalent contribution occurs in the Kr-M2+ bonding.

Extrapolation to the complete basis set limit of structural parameters: comparison of different approaches

For the first time, different procedures for extrapolating structural parameters to the complete basis set limit are compared. More precisely, a well-tested scheme, based on extrapolated gradients, is employed for validating the extrapolation procedure directly applied to geometrical parameters by means of different extrapolation functions. For this purpose, a set of small systems (mostly triatomics), also containing second-row elements, has been chosen.

The chemical bond between Au(I) and the noble gases. Comparative study of NgAuF and NgAu+ (Ng = Ar, Kr, Xe) by density functional and coupled cluster methods

The nature of the chemical bond between gold and the noble gases in the simplest prototype of Au(I) complexes (NgAuF and NgAu+, where Ng = Ar, Kr, Xe), has been theoretically investigated by state of art all-electron fully relativistic DC-CCSD(T) and DFT calculations with extended basis sets. The main properties of the molecules, including dipole moments and polarizabilities, have been computed and a detailed study of the electron density changes upon formation of the Ng-Au bond has been made. The Ar-Au dissociation energy is found to be nearly the same in both Argon compounds. It almost doubles along the NgAuF series and nearly triples in the corresponding NgAu+ series. The formation of the Ng-Au(I) bonds is accompanied by a large and very complex charge redistribution pattern which not only affects the outer valence region but reaches deep into the core-electron region. The charge transfer from the noble gas to Au taking place in the NgAu+ systems is largely reduced in the fluorides but the Ng-Au chemical bond in the latter systems is found to be tighter near the equilibrium distance. The density difference analysis shows, for all three noble gases, a qualitatively identical nature of the Ng-Au bond, characterized by the pronounced charge accumulation in the middle of the Ng-Au internuclear region which is typical of a covalent bond. This bonding density accumulation is more pronounced in the fluorides, where the Au-F bond is found to become more ionic, while the overall density deformation is more evident and less localized in the NgAu+ systems. Accurate density difference maps and charge-transfer curves help explain very subtle features of the chemistry of Au(I), including its peculiar preference for tight linear bicordination.

Aqueous Microsolvation of Mercury Halide Species†

The effects of aqueous solvation on the thermochemistry of reactions between mercury and small halogen molecules has been investigated by the microsolvation approach using ab initio and density functional theory (DFT) calculations. The structures, vibrational frequencies, and binding energies of 1, 2, and 3 water molecules with mercury-halide (HgBr2, HgBrCl, HgCl2, HgBr, and HgCl) and related mercury and halogen species (Br2, BrCl, Cl2, Cl, Hg, and Br) have been computed with second order Møller-Plesset perturbation theory (MP2) and the B3LYP density functional method. Accurate incremental water binding energies have been obtained at the complete basis set (CBS) limit using sequences of correlation consistent basis sets, including higher order correlation effects estimated from coupled cluster calculations. The resulting energetics were used to calculate the influence of water molecules on the thermochemistry of a number of reactions between mercury and small halogen-containing molecules. In general, the presence of water favors the formation of oxidized mercury halide species.

Chemically Accurate Thermochemistry of Cadmium: An ab Initio Study of Cd + XY (X = H, O, Cl, Br; Y = Cl, Br)

Using a composite coupled cluster method employing sequences of correlation consistent basis sets for complete basis set (CBS) extrapolations and with explicit treatment of core-valence correlation and scalar and spin-orbit relativistic effects, the 0 K enthalpies of a wide range of cadmium-halide reactions, namely, Cd + (HCl, HBr, ClO, BrO, Cl2, BrCl, Br2) have been determined to an estimated accuracy of +/-1 kcal/mol. In addition, accurate equilibrium geometries, harmonic frequencies, and dissociation energies have been calculated at the same level of theory for all the diatomic (e.g., CdH, CdO, CdCl, CdBr) and triatomic (CdHCl, CdHBr, CdClO, CdBrO, CdCl2, CdBrCl, CdBr2) species involved in these reactions, some for the very first time. Like their mercury analogues, all of the abstraction reactions are predicted to be endothermic, while the insertion reactions are strongly exothermic with the formation of stable linear, Cd-centric complexes. With the exception of CdH and the reactions involving this species, the present results for the remaining Cd-containing systems are believed to be the most accurate to date.

Ligand-Stabilized Aromatic Three-Membered Gold Rings and Their Sandwichlike Complexes

Electronic structure calculations (DFT) suggest that ligand-stabilized three-membered gold(I) rings constituting the core structure in a series of cyclo-Au3L(n)H(3-n) (L = CH3, NH2, OH and Cl; n = 1, 2, 3) molecules exhibit aromaticity, which is primarily due to 6s and 5d cyclic electron delocalization over the triangular Au3 framework (s- and d-orbital aromaticity). The aromaticity of the novel triangular gold(I) isocycles was verified by a number of established criteria of aromaticity. In particular, the nucleus-independent chemical shift, NICS(0), the upfield changes in the chemical shifts for Li+, Ag+, and Tl+ cations over the Au3 ring plane, and their interaction with electrophiles (e.g., H+, Li+, Ag+, and Tl+) are indicative for the aromaticity of the three-membered gold(I) rings. Interestingly, unlike the respective substituted derivatives of cyclopropenium cation and the bora-cyclopropene carbacyclic analogues, the aromatic Au3 rings, although exhibit comparable diatropicity, react with electrophiles in a different way affording 1:1 and 2:1 sandwichlike complexes. The bonding in the three-membered gold(I) rings is characterized by a common ring-shaped electron density, more commonly seen in aromatic organic molecules and in "all-metal" aromatics, such as the cyclo-[Hg3]4- tetraanion. Moreover, the cation-pi interactions in the 1:1 and 1:2 sandwichlike complexes formed upon reacting the Au3 rings with electrophiles, depending on the nature of the cation, are predicted to be predominantly electrostatic (Li+, Tl+) or covalent (H+, Ag+). The 1:2 complexes constitute a new class of sandwichlike complexes, which are expected to have novel properties and applications.