A photoelectron spectroscopic and computational study of sodium auride clusters, NanAun- (n = 1-3)

Negatively charged sodium auride clusters, NanAun- (n = 1-3), have been investigated experimentally using photoelectron spectroscopy and ab initio calculations. Well-resolved electronic transitions were observed in the photoelectron spectra of NanAun- (n = 1-3) at several photon energies. Very large band gaps were observed in the photoelectron spectra of the anion clusters, indicating that the corresponding neutral clusters are stable closed-shell species. Calculations show that the global minimum of Na2Au2- is a quasi-linear species with Cs symmetry. A planar isomer of D2h symmetry is found to be 0.137 eV higher in energy. The two lowest energy isomers of Na3Au3- consist of three-dimensional structures of Cs symmetry. The global minimum of Na3Au3- has a bent-flake structure lying 0.077 eV below a more compact structure. The global minima of the sodium auride clusters are confirmed by the good agreement between the calculated electron detachment energies of the anions and the measured photoelectron spectra. The global minima of neutral Na2Au2 and Na3Au3 are found to possess higher symmetries with a planar four-membered ring (D2h) and a six-membered ring (D3h) structure, respectively. The chemical bonding in the sodium auride clusters is found to be highly ionic with Au acting as the electron acceptor.     

Gold apes hydrogen. The structure and bonding in the planar B7Au2- and B7Au2 clusters

We produced the B7Au2- mixed cluster and studied its electronic structure and chemical bonding using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of B7Au2- were observed to be relatively simple with vibrational resolution, in contrast to the complicated spectra observed for pure B7-, which had contributions from three isomers (Alexandrova et al. J. Phys. Chem. A 2004, 108, 3509). Theoretical calculations show that B7Au2- possesses an extremely stable planar structure, identical to that of B7H2-, demonstrating that Au mimics H in its bonding to boron, analogous to the Au-Si bonding. The ground-state structure of B7Au2- (B7H2-) can be viewed as adding two Au (H) atoms to the terminal B atoms of a higher-lying planar isomer of B7-. The bonding and stability in the planar B7Au2- (B7H2-) clusters are elucidated on the basis of the strong covalent B-Au (H) bonding and the concepts of aromaticity/antiaromaticity in these systems.     

Al(6)(2-) - fusion of two aromatic Al(3)(-) units. A combined photoelectron spectroscopy and ab initio study of M(+)[Al(6)(2-)] (M = Li,Na, K,Cu, and Au)

Photoelectron spectroscopy is combined with ab initio calculations to elucidate the structure and chemical bonding of a series of MAl(6)(-) (M = Li, Na, K, Cu, and Au) bimetallic clusters. Well-resolved photoelectron spectra were obtained for MAl(6)(-) (M = Li, Na, Cu, and Au) at several photon energies. The ab initio calculations showed that all of the MAl(6)(-) clusters can be viewed as an M(+) cation interacting with an Al(6)(2-) dianion. Al(6)(2-) was found to possess an O(h) ground-state structure, and all of the MAl(6)(-) clusters possess a C(3v) ground-state structure derived from the O(h) Al(6)(2-). Careful comparison between the photoelectron spectral features and the ab initio one-electron detachment energies allows us to establish firmly the C(3v)ground-state structures for the MAl(6)(-) clusters. A detailed molecular orbital (MO) analysis is conducted for Al(6)(2-) and compared with Al(3)(-). It was shown that Al(6)(2-) can be considered as the fusion of two Al(3)(-) units. We further found that the preferred occupation of those MOs derived from the sums of the empty 2e' MOs of Al(3)(-), rather than those derived from the differences between the occupied 2a(1)' and 2a(2)' ' MOs of Al(3)(-), provides the key bonding interactions for the fusion of the two Al(3)(-) into Al(6)(2-). Because there are only four bonding MOs (one pi and three sigma MOs), an analysis of resonance structures was performed for the O(h)Al(6)(2-). It is shown that every face of the Al(6)(2-) octahedron still possesses both pi- and sigma-aromaticity, analogous to Al(3)(-), and that in fact Al(6)(2-) can be viewed to possess three-dimensional pi- and sigma-aromaticity with a large resonance stabilization.     

Probing the electronic structure and chemical bonding of the "staple" motifs of thiolate gold nanoparticles: Au(SCH3)2- and Au2(SCH3)3-

Thiolate-protected gold nanoparticles have been found recently to be coordinated by the so-called "staple" bonding motifs, consisting of quasi-linear [RS-Au-SR] and V-shaped [RS-Au-(SR)-Au-SR] units, which carry a negative charge formally. Using photoelectron spectroscopy (PES) in conjunction with ab initio calculations, we have investigated the electronic structure and chemical bonding of the simplest staples with R = CH(3): Au(SCH(3))(2)(-) and Au(2)(SCH(3))(3)(-), which were produced by electrospray ionization. PES data of the two Au-thiolate complexes are obtained both at room temperature (RT) and 20 K. The temperature-dependent study reveals significant spectral broadening at RT, in agreement with theoretical predictions of multiple conformations due to the different orientations of the -SCH(3) groups. The Au-S bonds in Au(n)(SCH(3))(n+1)(-) (n = 1, 2) are shown to be covalent via a variety of chemical bonding analyses. The strong Au-thiolate bonding and the stability of the Au-thiolate complexes are consistent with their ubiquity as staples for gold nanoparticles and on gold surfaces.     

Electronic structure and chemical bonding in MO(n)- and MO(n) clusters (M = Mo, W; n = 3-5): a photoelectron spectroscopy and ab initio study

Photoelectron spectroscopy (PES) and ab initio calculations are combined to investigate the electronic structure of MO(n)- clusters (M = W, Mo; n = 3-5). Similar PES spectra were observed between the W and Mo species. A large energy gap between the first and second PES bands was observed for MO3- and correlated with a stable closed-shell MO3 neutral cluster. The electron binding energies of MO4- increase significantly relative to those of MO3-, and there is also an abrupt spectral pattern change between MO3- and MO4-. Both MO4- and MO5- give PES features with extremely high electron binding energies (>5.0 eV) due to oxygen-2p-based orbitals. The experimental results are compared with extensive density functional and ab initio [CCSD(T)] calculations, which were performed to elucidate the electronic and structural evolution for the tungsten oxide clusters. WO3 is found to be a closed-shell, nonplanar molecule with C3v symmetry. WO4 is shown to have a triplet ground state (3A2) with D2d symmetry, whereas WO5 is found to be an unusual charge-transfer complex, (O2-)WO3+. WO4 and WO5 are shown to possess W-O* and O2-* radical characters, respectively.     

Gold as hydrogen. An experimental and theoretical study of the structures and bonding in disilicon gold clusters Si2Au(n)- and Si2Au(n) (n = 2 and 4) and comparisons to Si2H2 and Si2H4

In a previous communication, we showed that a single Au atom behaves like H in its bonding to Si in a series of Si-Au clusters, SiAu(n) (n = 2-4) (Kiran et al. Angew. Chem., Int. Ed. 2004, 43, 2125). In this article, we show that the H analogy of Au is more general. We find that the chemical bonding and potential energy surfaces of two disilicon Au clusters, Si(2)Au(2) and Si(2)Au(4), are analogous to Si(2)H(2) and Si(2)H(4), respectively. Photoelectron spectroscopy and ab initio calculations are used to investigate the geometrical and electronic structures of Si(2)Au(2)(-), Si(2)Au(4)(-), and their neutral species. The most stable structures for both Si(2)Au(2) and Si(2)Au(2)(-) are found to be C(2)(v), in which each Au bridges the two Si atoms. For Si(2)Au(4)(-), two nearly degenerate dibridged structures in a cis (C(2)(h)) and a trans (C(2)(v)) configuration are found to be the most stable isomers. However, in the neural potential energy surface of Si(2)Au(4), a monobridged isomer is the global minimum. The ground-state structures of Si(2)Au(2)(-) and Si(2)Au(4)(-) are confirmed by comparing the computed vertical detachment energies with the experimental data. The various stable isomers found for Si(2)Au(2) and Si(2)Au(4) are similar to those known for Si(2)H(2) and Si(2)H(4), respectively. Geometrical and electronic structure comparisons with the corresponding silicon hydrides are made to further establish the isolobal analogy between a gold atom and a hydrogen atom.     

Low-lying isomers and finite temperature behavior of (H2O)6 -

(H2O)(6) (-) appears as a "magic" number water cluster in (H2O)(n) (-) mass spectra. The structure of the (H2O)(6) (-) isomer dominating the experimental population has been established only recently [N. I. Hammer et al., J. Phys. Chem. A 109, 7896 (2005)], and the most noteworthy characteristic of this isomer is the localization of the excess electron in the vicinity of a double-acceptor monomer. In the present work, we use a quantum Drude model to characterize the low-energy isomers and the finite temperature properties of (H2O)(6) (-). Comparison with ab initio calculations shows that the use of a water model employing distributed polarizabilities and distributed repulsive sites is necessary to correctly reproduce the energy ordering of the low-lying isomers. Both the simulations and the ab initio calculations predict that there are several isomers of (H2O)(6) (-) significantly lower in energy than the experimentally observed species, suggesting that the experimental distribution is far from equilibrium.     

On the aromaticity of square planar Ga4(2-) and In4(2-) in gaseous NaGa4- and NaIn4- clusters

We investigated the electronic structure and chemical bonding of two bimetallic clusters NaGa4- and NaIn4-. Photoelectron spectra of the anions were obtained and compared with ab initio calculations. We found that the ground state of the two anions contains a square planar dianion interacting with a Na+ cation. The Ga4(2-) and In4(2-) dianions both possess two delocalized pi electrons and are considered to be aromatic, similar to that recently found in Al4(2-). Using calculations for a model compound, we showed that a recently synthesized Ga4-organometallic compound also contains an aromatic -Ga4(2-)- unit, analogous to the gaseous clusters.     

Observation of triatomic species with conflicting aromaticity: AlSi2- and AlGe2-

We created mixed triatomic clusters, AlCGe(-), AlSi(2)(-), and AlGe(2)(-), and studied their electronic structure and chemical bonding using photoelectron spectroscopy and ab initio calculations. Excellent agreement between theoretical and experimental photoelectron spectra confirmed the predicted global minimum structures for these species. Chemical bonding analysis revealed that the AlSi(2)(-) and AlGe(2)(-) anions can be described as species with conflicting (sigma-antiaromatic and pi-aromatic) aromaticity. The AlCGe(-) anion represents an interesting example of chemical species which is between classical and aromatic.     

Planar nitrogen-doped aluminum clusters AlxN- (x=3-5)

The electronic and geometrical structures of three nitrogen-doped aluminum clusters, Al(x)N(-) (x=3-5), are investigated using photoelectron spectroscopy and ab initio calculations. Well-resolved photoelectron spectra have been obtained for the nitrogen-doped aluminum clusters at four photon energies (532, 355, 266, and 193 nm). Global minimum structure searches for Al(x)N(-) (x=3-5) and their corresponding neutrals are performed using several theoretical methods. Vertical electron detachment energies are calculated using three different methods for the lowest energy structures and low-lying isomers are compared with the experimental observations. Planar structures have been established for all the three Al(x)N(-) (x=3-5) anions from the joint experimental and theoretical studies. For Al(5)N(-), a low-lying nonplanar isomer is also found to contribute to the experimental spectra, signifying the onset of two-dimensional to three-dimensional transition in nitrogen-doped aluminum clusters. The chemical bonding in all the planar clusters has been elucidated on the basis of molecular orbital and natural bond analyses.     

Photoelectron spectroscopy and ab initio study of the doubly antiaromatic B(6) (2-) dianion in the LiB(6) (-) cluster

A metal-boron mixed cluster LiB(6) (-) was produced and characterized by photoelectron spectroscopy and ab initio calculations. A number of electronic transitions were observed and used to compare with theoretical calculations. An extensive search for the global minimum of LiB(6) (-) was carried out via an ab initio genetic algorithm technique. The pyramidal C(2v) ((1)A(1)) molecule was found to be the most stable at all levels of theory. The nearest low-lying isomer was found to be a triplet C(2) ((3)B) structure, 9.2 kcal/mol higher in energy. Comparison of calculated detachment transitions from LiB(6) (-) and the experimental photoelectron spectra confirmed the C(2v) pyramidal global minimum structure. Natural population calculation revealed that LiB(6) (-) is a charge-transfer complex, Li(+)B(6) (2-), in which Li(+) and B(6) (2-) interact in a primarily ionic manner. Analyses of the molecular orbitals and chemical bonding of B(6) (2-) showed that the planar cluster is twofold (pi- and sigma-) antiaromatic, which can be viewed as the fusion of two aromatic B(3) (-) units.     

Aluminum avoids the central position in AlB9- and AlB10-: photoelectron spectroscopy and ab initio study

The structures and the electronic properties of two Al-doped boron clusters, AlB(9)(-) and AlB(10)(-), were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (σ- and π-) aromatic systems. The nona-coordinated wheel-type structure of AlB(9)(-) was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB(9) cluster was previously shown to be either a global minimum or a low-lying isomer.     

Electronic and structural evolution and chemical bonding in ditungsten oxide clusters: W2O(n)- and W2O(n) (n = 1-6)

We report a systematic and comprehensive investigation of the electronic structures and chemical bonding in a series of ditungsten oxide clusters, W2O(n)- and W2O(n) (n = 1-6), using anion photoelectron spectroscopy and density functional theory (DFT) calculations. Well-resolved photoelectron spectra were obtained at several photon energies (2.331, 3.496, 4.661, 6.424, and 7.866 eV), and W 5d-based spectral features were clearly observed and distinguished from O 2p-based features. More complicated spectral features were observed for the oxygen-deficient clusters because of the W 5d electrons. With increasing oxygen content in W2O(n)-, the photoelectron spectra were observed to shift gradually to higher binding energies, accompanied by a decreasing number of W 5d-derived features. A behavior of sequential oxidation as a result of charge transfers from W to O was clearly observed. A large energy gap (2.8 eV) was observed in the spectrum of W2O6-, indicating the high electronic stability of the stoichiometric W2O6 molecule. Extensive DFT calculations were carried out to search for the most stable structures of both the anion and neutral clusters. Time-dependent DFT method was used to compute the vertical detachment energies and compare to the experimental data. Molecular orbitals were used to analyze the chemical bonding in the ditungsten oxide clusters and to elucidate their electronic and structural evolution.     

In search of covalently bound tetra- and penta-oxygen species: a photoelectron spectroscopic and Ab initio investigation of MO4- and MO5- (M = Li,Na, K,Cs)

Although neutral and ionic O4(0/-/+) species have been observed experimentally and considered for energetic materials, O4(2-) and O5(2-) dianions have not yet been explored. O4(2-) is valent isoelectronic to the well-known ClO3- and SO3(2-) anions, and O5(2-) is valent isoelectronic to ClO4- and SO4(2-). All are stable, common anions in solutions and inorganic salts. In this article, we explore the possibility of making covalently bound O4(2-) and O5(2-) species stabilized in the forms of M+O4(2-) and M+O5(2-) (M = Li, Na, K, Cs) in the gas phase. Laser vaporization experiments using M-containing targets and an O2-seeded carrier gas yielded very intense mass peaks corresponding to MO4- and MO5-. To elucidate the structure and bonding of the newly observed MO4- and MO5- species, we measured their photoelectron spectra and then compared them with ab initio calculations and the spectra of ClO3-, Na+SO3(2-), ClO4-, and Na+SO4(2-). Careful analyses of the experimental and ab initio results showed, however, that the observed species are of the forms, O2-M+O2- and O2-M+O3-. The more interesting M+O4(2-) and M+O5(2-) species were found to be higher-energy isomers, but they are true minima on the potential energy surfaces, which suggests that it might be possible to synthesize bulk materials containing covalently bound tetra- and pentatomic oxygen building blocks.     

On the structure and chemical bonding of Si6(2-) and Si6(2-) in NaSi6(-) upon Na+ coordination

Photoelectron spectroscopy was combined with ab initio calculations to elucidate the structure and bonding in Si6 2- and NaSi6 -. Well-resolved electronic transitions were observed in the photoelectron spectra of Si6 - and NaSi6 - at three photon energies (355, 266, and 193 nm). The spectra of NaSi6 - were observed to be similar to those of Si6 - except that the electron binding energies of the former are lower, suggesting that the Si6 motif in NaSi6 - is structurally and electronically similar to that in Si6 -. The electron affinities of Si6 and NaSi6 were measured fairly accurately to be 2.23+/-0.03 eV and 1.80+/-0.05 eV, respectively. Global minimum structure searches for Si6 2- and NaSi6 - were performed using gradient embedded genetic algorithm followed by B3LYP, MP2, and CCSDT calculations. Vertical electron detachment energies were calculated for the lowest Si6 - and NaSi6 - structures at the CCSD(T)/6-311+G(2df), ROVGF/6-311+G(2df), UOVGF/6-311+G(2d), and time-dependent B3LYP/6-311+G(2df) levels of theory. Experimental vertical detachment energies were used to verify the global minimum structure for NaSi6 -. Though the octahedral Si6 2-, analogous to the closo form of borane B6H6 2-, is the most stable form for the bare hexasilicon dianion, it is not the kernel for the NaSi6 - global minimum. The most stable isomer of NaSi6 - is based on a Si6 2- motif, which is distorted into C2v symmetry similar to the ground state structure of Si6 -. The octahedral Si6 2- coordinated by a Na+ is a low-lying isomer and was also observed experimentally. The chemical bonding in Si6 2- and NaSi6 - was understood using natural bond orbital, molecular orbital, and electron localization function analyses.     

Hydrated copper and gold monovalent cations: Ab initio study

To understand the hydration phenomena of noble transition metals, we investigated the structures, hydration energies, electronic properties, and spectra of the Cu(+)(H(3)O)(1-6) and Au(+)(H(2)O)(1-6) clusters using ab initio calculations. The coordination numbers of these clusters are found to be only two, which is highly contrasted to those of Ag(+)(H(2)O)(n) (which have the coordination numbers of 3-4) as well as the hydrated alkali metal ions (which have the coordination numbers of approximately 6). For the possible identification of their interesting hydration structures, we predict their IR spectra for the OH stretch modes.     

OH(3) (-) and O(2)H(5) (-) double Rydberg anions: predictions and comparisons with NH(4) (-) and N(2)H(7) (-)

A low barrier in the reaction pathway between the double Rydberg isomer of OH(3) (-) and a hydride-water complex indicates that the former species is more difficult to isolate and characterize through anion photoelectron spectroscopy than the well known double Rydberg anion (DRA), tetrahedral NH(4) (-). Electron propagator calculations of vertical electron detachment energies (VEDEs) and isosurface plots of the electron localization function disclose that the transition state's electronic structure more closely resembles that of the DRA than that of the hydride-water complex. Possible stabilization of the OH(3) (-) DRA through hydrogen bonding or ion-dipole interactions is examined through calculations on O(2)H(5) (-) species. Three O(2)H(5) (-) minima with H(-)(H(2)O)(2), hydrogen-bridged, and DRA-molecule structures resemble previously discovered N(2)H(7) (-) species and have well separated VEDEs that may be observable in anion photoelectron spectra.     

Reactivity of atomic gold anions toward oxygen and the oxidation of CO: experiment and theory

Results for the binding of carbon monoxide and oxygen along with the oxidation of CO in the presence of atomic Au(-) have been obtained utilizing a fast-flow reactor mass spectrometer. In addition, density functional calculations have been performed to explain the experimental findings. It was observed that upon oxygen addition to the metal plasma, gold oxide species of the form AuO(n)(-), where n = 1-3, were produced. The addition of carbon monoxide to the preoxidized gold atom revealed that AuO(-) and AuO(3)(-) promote the oxidation of CO. Density functional calculations on structures and their energetics confirmed the experimental findings and allowed us to propose mechanisms for the oxidation of carbon monoxide. The reactions of CO with AuO(1,3)(-) proceed via complex formation with CO bound to the oxygen atom, followed by either cleavage of the Au-O bond or complex rearrangement to form a weakly bound CO(2) unit, leading in both cases to the emanation of CO(2).     

Chemical bonding in Si5(2-) and NaSi5(-) via photoelectron spectroscopy and ab initio calculations

Photoelectron spectroscopy and ab initio calculations are used to investigate the electronic structure and chemical bonding of Si5(-) and Si5(2-) in NaSi5(-). Photoelectron spectra of Si5(-) and NaSi5(-) are obtained at several photon energies and are compared with theoretical calculations at four different levels of theory, TD-B3LYP, R(U)OVGF, UCCSD(T), and EOM-CCSD(T), all with 6-311+G(2df) basis sets. Excellent agreement is observed between experiment and theory, confirming the obtained ground-state structures for Si5(-) and Si5(2-), which are both found to be trigonal bipyramid with D3h symmetry at several levels of theory. Chemical bonding in Si5, Si5(-), and Si5(2-) is analyzed using NPA, molecular orbitals, ELF, and NICS indices. The bonding in Si5(2-) is compared with that in the isoelectronic and isostructural B5H5(2-) species, but they are found to differ due to the involvement of electron densities, which are supposed to be lone pairs in the skeletal bonding in Si5(2-).     

Cu3C4-: a new sandwich molecule with two revolving C2(2-) units

A combined photoelectron spectroscopy (PES) and ab initio study was carried out on a novel copper carbide cluster in the gas phase: Cu(3)C(4)(-). It was generated in a laser vaporization cluster source and appeared to exhibit enhanced stability among the Cu(3)C(n)(-) series. Its PES spectra were obtained at several photon energies, showing numerous well-resolved bands. Extensive ab initio calculations were performed on Cu(3)C(4)(-), and two isomers were identified: a C(2) structure ((1)A) with a Cu(3)(3+) triangular group sandwiched by two C(2)(2-) units and a linear CuCCCuCCCu structure (D(infinity)(h), (1)Sigma(g)(+)). A comparison of ab initio PES spectra with experimental data showed that the sandwich Cu(3)C(4)(-) cluster was solely responsible for the observed spectra and the linear isomer was not present, suggesting that the C(2) structure is the global minimum in accordance with CCSD(T)/6-311+G predictions. Interestingly, a relatively low barrier (0.4-0.6 kcal/mol) was found for the internal rotation of the C(2)(2-) units in the sandwich Cu(3)C(4)(-). To test different levels of theory in describing the Cu(m)C(n)(-) systems and lay foundations for the validity of the theoretical methods, extensive calculations at a variety of levels were also carried out on a simpler copper carbide species CuC(2)(-), where two isomers were found to be close in energy: a linear one (C(infinity)(v), (1)Sigma(+)) and a triangular one (C(2)(v), (1)A(1)). The calculated electronic transitions for CuC(2)(-) were also compared with the PES data, in which both isomers were present.