Electronic Structure and Anion Photoelectron Spectroscopy of Uranium-Gold Clusters UAun-, n = 3-7.

A collaborative effort between experiment and theory toward elucidating the electronic and molecular structures of uranium-gold clusters is presented. Anion photoelectron spectra of UAun-(n = 3-7) were taken at the third (355 nm) and fourth (266 nm) harmonics of a Nd:YAG laser, as well as excimer (ArF 193 nm) photon energies, where the experimental adiabatic electron affinities and vertical detachment energies values were measured. Complementary first-principles calculations were subsequently carried out to corroborate experimentally determined electron detachment energies and to determine the geometry and electronic structure for each cluster. Except for the ring-like neutral isomer of UAu6 where one unpaired electron is spread over the Au atoms, all other neutral and anionic UAun clusters (n = 3-7) were calculated to possess open-shell electrons with the unpaired electrons localized on the central U atom. The smaller clusters closely resemble the analogous UFn species, but significant deviations are seen starting with UAu5 where a competition between U-Au and Au-Au bonding begins to become apparent. The UAu6 system appears to mark a transition where Au-Au interactions begin to dominate, where both a ring-like and two heavily distorted octahedral structures around the central U atom are calculated to be nearly isoenergetic. With UAu7, only ring-like structures are calculated. Overall, the calculated electron detachment energies are in good agreement with the experimental values.

Confinement boosts CO oxidation on an Ni atom embedded inside boron nitride nanotubes.

To date, most studies of heterogeneous catalysis have focused on metal particles supported on the surface of substrates. However, studies of the catalytic properties of metallic nanoparticles supported on the interior surface of nanotubes are rare. Using first-principles calculations based on density functional theory, we have studied the CO oxidation on a single nickel atom confined in a nitrogen vacancy on the inside surface of boron nitride nanotubes (BNNT). By exploring the Eley-Rideal mechanism, we find that an Ni atom embedded on the interior surface of BNNTs exhibits a much higher catalytic activity for CO oxidation when compared with Ni doped on their outside surface. In addition, the energy barriers of the rate-determining step for CO oxidation on Ni embedded on the inside wall of BNNT(5,5), BNNT(6,6) and BNNT(7,7) are 0.39, 0.29 and 0.33 eV, respectively. The results illustrate the merit of confinement for CO oxidation.

18-Electron rule inspired Zintl-like ions composed of all transition metals.

Zintl phase compounds constitute a unique class of compounds composed of metal cations and covalently bonded multiply charged cluster anions. Potential applications of these materials in solution chemistry and thermoelectric materials have given rise to renewed interest in the search for new Zintl ions. Up to now these ions have been mostly composed of group 13, 14, and 15 post-transition metal elements and no Zintl ions composed of all transition metal elements are known. Using gradient corrected density functional theory we show that the 18-electron rule can be applied to design a new class of Zintl-like ions composed of all transition metal atoms. We demonstrate this possibility by using Ti@Au12(2-) and Ni@Au6(2-) di-anions as examples of Zintl-like ions. Predictive capability of our approach is demonstrated by showing that FeH6(4-) in an already synthesized complex metal hydride, Mg2FeH6, is a Zintl-like ion, satisfying the 18-electron rule. We also show that novel Zintl phase compounds can be formed by using all transition metal Zintl-like ions as building blocks. For example, a two-dimensional periodic structure of Na2[Ti@Au12] is semiconducting and nonmagnetic while a one-dimensional periodic structure of Mg[Ti@Au12] is metallic and ferromagnetic. Our results open the door to the design and synthesis of a new class of Zintl-like ions and compounds with potential for applications.

Structure and properties of the aluminum borates Al(BO2)n and Al(BO2)n(-), (n = 1-4).

The geometrical and electronic structures of Al(BO(2))(n) and Al(BO(2))(n)(-) (n = 1-4) clusters are computed at different levels of theory including density functional theory (DFT), hybrid DFT, double-hybrid DFT, and second-order perturbation theory. All aluminum borates are found to be quite stable toward the BO(2) and BO(2)(-) loss in the neutral and anion series, respectively. Al(BO(2))(4) belongs to the class of hyperhalogens composed of smaller superhalogens, and should possess a large adiabatic electron affinity (EA(ad)) larger than that of its superhalogen building block BO(2). Indeed, the aluminum tetraborate possesses the EA(ad) of 5.6 eV, which, however, is smaller than the EA(ad) of 7.8 eV of the AlF(4) supehalogen despite BO(2) is more electronegative than F. The EA(ad) decrease in Al(BO(2))(4) is due to the higher thermodynamic stability of Al(BO(2))(4) compared to that of AlF(4). Because of its high EA and thermodynamic stability, Al(BO(2))(4) should be capable of forming salts with electropositive counter ions. We optimized KAl(BO(2))(4) as corresponding to a unit cell of a hypothetical KAl(BO(2))(4) salt and found that specific energy and energy density of such a salt are competitive with those of trinitrotoluol (TNT).

Functionalized boranes for hydrogen storage.

Using density functional theory, the generalized gradient approximation for the exchange-correlation potential and Møller-Plesset perturbation theory we study the hydrogen uptake of Li- and Mg-doped boranes. Specifically, we calculate the structures and binding energies of hydrogen molecules sequentially attached to LiB(6)H(7), LiB(12)H(13), Li(2)B(6)H(6), Li(2)B(12)H(12), MgB(6)H(6), and MgB(12)H(12). Up to three H(2) molecules can be bound quasi-molecularly to each of the metal cations with binding energies per H(2) molecule ranging between 0.07 eV and 0.27 eV. The corresponding gravimetric densities lie in the range of 3.49 to 12 wt %, not counting the H atoms bound chemically to the B atoms.

Stability and spectroscopic properties of singly and doubly charged anions.

Using density functional theory and hybrid B3LYP exchange-correlation energy functional we have studied the structure, stability, and spectroscopic properties of singly and doubly charged anions composed of simple metal atoms (Na, Mg, Al) decorated with halogens such as Cl and pseudohalogens such as CN. Since pseudohalogens mimic the chemistry of halogen atoms, our objective is to see if pseudohalogens can also form superhalogens much as halogens do and if the critical size for a doubly charged anion depends upon the ligand. The electron affinities of MCl(n) (M = Na, Mg, Al) exceed the value of Cl for n ≥ (k + 1), where k is the normal valence of the metal atom. However, for M(CN)(n) complexes this is only true when n = k + 1. In addition, while the electron affinities and vertical detachment energies of MCl(n) complexes are close to each other, they are markedly different when Cl is replaced by pseudohalogen, CN. The origin of these anomalous results is found to be due to the large binding energy of cyanogen, (NCCN) molecule. Because of the tendency of CN molecules to dimerize, the ground state geometries of the neutral and anionic M(CN)(n) complexes are very different when their number exceed the normal valence of the metal atom. While our calculations support the conclusion of Skurski and co-workers that pseudohalogens can form the building blocks of superhalogens, we show that there is a limitation on the number of CN moieties where this is true. Equally important, we find large differences between the ground state geometries of the neutral and anionic M(CN)(n) complexes for n ≥ (k + 2) which could play an important role in interpreting future experimental data on M(CN)(n) complexes. This is because the electron affinity defined as the energy difference between the ground states of the anion and neutral can be very different from the adiabatic detachment energy defined as the energy difference between the ground state of the anion and its structurally similar neutral isomer.

Evolution of superhalogen properties in PtCl(n) clusters.

We have systematically calculated the ground state geometries, relative stability, electronic structure, and spectroscopic properties of PtCl(n) (n = 1-7) clusters. The bonding in these clusters is dominated by covalent interaction. In neutral clusters, chlorine atoms are chemically bound to Pt up to n = 5. However, in neutral PtCl(6) and PtCl(7) clusters, two of the chlorine atoms bind molecularly while the remaining bind as individual atoms. In the negative ions, this happens only in the case of PtCl(7) cluster. The geometries of both neutral and anionic clusters can be considered as fragments of an octahedron and are attributed to the stabilization associated with splitting of partially filled d orbitals under the chloride ligand field. The electron affinity of PtCl(n) clusters rises steadily with n, reaching a maximum value of 5.81 eV in PtCl(5). PtCl(n) clusters with n ≥ 3 are all superhalogens with electron affinities larger than that of chlorine. The accuracy of our results has been verified by carrying out photoelectron spectroscopy experiments on PtCl(n)(-) anion clusters.

Borane derivatives: a new class of super- and hyperhalogens.

Super- and hyperhalogens are a class of highly electronegative species whose electron affinities far exceed those of halogen atoms and are important to the chemical industry as oxidizing agents, biocatalysts, and building blocks of salts. Using the well-known Wade-Mingos rule for describing the stability of closo-boranes B(n)H(n)(2-) and state-of-the-art theoretical methods, we show that a new class of super- and hyperhalogens, guided by this rule, can be formed by tailoring the size and composition of borane derivatives. Unlike conventional superhalogens, in which a central metal atom is surrounded by halogen atoms, the superhalogens formed according to the Wade-Mingos rule do not have to have either halogen or metal atoms. We demonstrate this by using B(12)H(13) and its isoelectronic cluster CB(11)H(12) as examples. We also show that while conventional superhalogens containing alkali atoms require at least two halogen atoms, a single borane-like moiety is sufficient to give M(B(12)H(12)) clusters (M=Li, Na, K, Rb, Cs) superhalogen properties. In addition, hyperhalogens can be formed by using the above superhalogens as building blocks. Examples include M(B(12)H(13))(2) and M(CB(11)H(12))(2) (M=Li-Cs). This finding opens the door to an untapped source of superhalogens and weakly coordinating anions with potential applications.

Au(CN)n complexes: superhalogens with pseudohalogen as building blocks.

Electron affinity (EA) is one of the most important factors that govern reactivity of atoms and molecules. Chlorine, with the highest electron affinity (3.6 eV) of all elements in the periodic table, is a classic example of reactive elements. Over past thirty years, much research has been done to expand the scope of molecules with electron affinities even larger than that of Cl. These molecules, called superhalogens, have the general formula MX(n+1) where M is a metal atom, X is a halogen atom, and n is the valency of the metal. In this paper we explore the potential of pseudohalogens such as CN, which mimic the chemistry of halogens, to serve as building blocks of new superhalogens. Using calculations based on density functional theory, we show that when a central Au atom is surrounded by CN moieties, superhalogens can be created with electron detachment energies as high as 8.4 eV. However, there is a stark contrast between the stability of these superhalogens and that of conventional AuF(n) superhalogens. Whereas AuF(n) complexes are stable up to n = 5 for neutrals and n = 6 for anions, Au(CN)(n) complexes (with CN moieties attached individually) are metastable beyond n = 1 for neutrals and n = 3 for anions. We investigate the nature and origin of these differences. In addition, we elucidate important distinctions between electron affinity (EA) and adiabatic detachment energy (ADE), two terms that are often used synonymously in literature.

Potential candidates for hyperhalogens: a comparative study of BO2, AlO2, and VO3 species.

Recent work has shown that BO(2) which is a superhalogen with an electron affinity of 4.46 eV, can be used as building block of a new class of molecules/clusters whose electron affinities can exceed that of BO(2). This class of molecules was named hyperhalogens and the concept was illustrated by focusing on Au(BO(2))(2). Here we explore other superhalogens besides BO(2) to see if they too can be used to form hyperhalogens. We have chosen to focus on AlO(2) which is valence isoelectronic with BO(2) as well as VO(3) which involves a transition metal atom. The results obtained using density functional theory show unexpected behavior: Although AlO(2) and VO(3) are both superhalogens such as BO(2), only Na(BO(2))(2) is a hyperhalogen while Na(AlO(2))(2) and Na(VO(3))(2) are not. The origin of this anomalous result is traced to the large binding energy of the dimers of AlO(2) and VO(3).

Interactions of a Mn atom with halogen atoms and stability of its half-filled 3d-shell.

Using density functional theory with hybrid exchange-correlation potential, we have calculated the geometrical and electronic structure, relative stability, and electron affinities of MnX(n) compounds (n = 1-6) formed by a Mn atom and halogen atoms X = F, Cl, and Br. Our objective is to examine the extent to which the Mn-X interactions are similar and to elucidate if/how the half-filled 3d-shell of a Mn atom participates in chemical bonding as the number of halogen atoms increases. While the highest oxidation number of the Mn atom in fluorides is considered to be +4, the maximum number of halogen atoms that can be chemically attached in the MnX(n)(-) anions is 6 for X = F, 5 for X = Cl, and 4 for X = Br. The MnCl(n) and MnBr(n) neutrals are superhalogens for n ≥ 3, while the superhalogen behavior of MnF(n) begins with n = 4. These results are explained to be due to the way different halogen atoms interact with the 3d electrons of Mn atom.

A systematic study of neutral and charged 3d-metal trioxides and tetraoxides.

Using density functional theory with generalized gradient approximation, we have performed a systematic study of the structure and properties of neutral and charged trioxides (MO(3)) and tetraoxides (MO(4)) of the 3d-metal atoms. The results of our calculations revealed a number of interesting features when moving along the 3d-metal series. (1) Geometrical configurations of the lowest total energy states of neutral and charged trioxides and tetraoxides are composed of oxo and∕or peroxo groups, except for CuO(3)(-) and ZnO(3)(-) which possess a superoxo group, CuO(4)(+) and ZnO(4)(+) which possess two superoxo groups, and CuO(3)(+), ZnO(3)(+), and ZnO(4)(-) which possess an ozonide group. While peroxo groups are found in the early and late transition metals, all oxygen atoms bind chemically to the metal atom in the middle of the series. (2) Attachment or detachment of an electron to∕from an oxide often leads to a change in the geometry. In some cases, two dissociatively attached oxygen atoms combine and form a peroxo group or a peroxo group transforms into a superoxo group and vice versa. (3) The adiabatic electron affinity of as many as two trioxides (VO(3) and CoO(3)) and four tetraoxides (TiO(4), CrO(4), MnO(4), and FeO(4)) are larger than the electron affinity of halogen atoms. All these oxides are hence superhalogens although only VO(3) and MnO(4) satisfy the general superhalogen formula.

Origin of the unusual properties of Au(n)(BO2) clusters.

We report the discovery of a new class of clusters consisting of Au(n)(BO(2)) that forms during the oxygenation of gold clusters when boron nitride is used as insulation in a pulsed-arc cluster ion source (PACIS). Photoelectron and mass spectroscopy of these clusters further revealed some remarkable properties: instead of the expected Au(n)O(m) peaks, the mass spectra contain intense peaks corresponding to Au(n)(BO(2)) composition. Some of the most predominant features of the electronic structure of the bare Au clusters, namely even-odd alternation in the electron affinity, are preserved in the Au(n)(BO(2)) species. Most importantly, Au(n)(BO(2)) [odd n] clusters possess unusually large electron affinity values for a closed-shell cluster, ranging from 2.8-3.5 eV. The open-shell Au(n)(BO(2)) [even n] clusters on the other hand, possess electron affinities exceeding that of F, the most electronegative element in the periodic table. Using calculations based on density functional theory, we trace the origin of these species to the unusual stability and high electron affinity of the BO(2) moiety. The resulting bond formed between BO(2) and Au(n) clusters preserves the geometric and electronic structure of the bare Au(n) clusters. The large electron affinity of these clusters is due to the delocalization of the extra electron over the Au(n) cluster.

Negative ions of transition metal-halogen clusters.

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MX(n) clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n≤n(max), where the maximal valence n(max) equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>n(max), two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=n(max), reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.