Hyper trumps super: A central atom, typically a metal, surrounded by halogen or oxygen atoms is called a superhalogen. Theoretical calculations and experiments reveal that a new class of highly electronegative species can be created when the central metal atom is surrounded by superhalogen moieties. These hyperhalogens can have electron affinities even larger than those of their superhalogen building blocks.
Whereas boron has many hydrides, aluminum has been thought to exhibit relatively few. A combined anion photoelectron and density functional theory computational study of the Al4H-6 anion and its corresponding neutral, Al4H6, showed that Al4H6 can be understood in terms of the Wade-Mingos rules for electron counting, suggesting that it may be a borane analog. The data support an Al4H6 structure with a distorted tetrahedral aluminum atom framework, four terminal Al-H bonds, and two sets of counter-positioned Al-H-Al bridging bonds. The large gap between the highest occupied and the lowest unoccupied molecular orbitals found for Al4H6, together with its exceptionally high heat of combustion, further suggests that Al4H6 may be an important energetic material if it can be prepared in bulk.
Equilibrium geometries and ground state spin multiplicities of neutral and anionic coinage metal fluoride XF n clusters (X ) Cu, Ag, and Au; n ) 1-7) are obtained from density functional theory-based calculations. Our results show that in the case of neutral and anionic CuF n and AgF n clusters, a maximum of 4 F atoms (n max ) 4) can be bound atomically to the metal atoms, while remaining F atoms bind to the other F atoms to form F 2 units. In contrast, a Au atom can bind up to six F atoms dissociatively. This contrasting binding scenario observed for these metal fluoride clusters is explained using the natural bond orbital analysis. The neutral XF n (X ) Cu, Ag) clusters are stable against dissociation into X and F atoms up to n ) 6, while AuF n clusters are stable up to n ) 7. Similarly, with the exception of AgF 7 and AuF 6 , all neutral clusters studied are stable against dissociation into F 2 molecules. On the other hand, XF n clusters are stable against dissociation into F atoms and F 2 molecules over the entire size range, indicating the increased stability of anionic species over their neutral counterparts. Even more striking is the fact that the electron affinities of these clusters can be as large as 8 eV, far exceeding the electron affinity of Cl that has the highest value in the periodic table. These clusters are thus classified as superhalogens. † Part of the special issue "Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules".
Using density functional theory (DFT), we have systematically calculated the equilibrium geometries, electronic structure, and electron detachment energies of Al(BH(4))(n=1→4) and Al(BF(4))(n=1→4) at the B3LYP/6-311+G(2d,p) level of theory. The electron affinities of Al(BH(4))(n) not only exhibit odd-even alternation, just as seen in (BH(4))(n), but also, for n = 3 and 4, show a remarkable behavior: whereas the electron affinities of BH(3) and BH(4) are, respectively, 0.06 and 3.17 eV, those of Al(BH(4))(3) and Al(BH(4))(4) are 0.71 and 5.56 eV. Results where H is replaced by F are also very different. The electron affinities of BF(3) and BF(4) are, respectively, -0.44 and +6.86 eV, and those of Al(BF(4))(3) and Al(BF(4))(4) are 1.82 and 8.86 eV. The results demonstrate not only marked difference when H is replaced by F but also substantially enhanced electron affinities by almost 2 eV when BH(4) and BF(4) units are allowed decorate a metal atom, confirming the recently observed hyperhalogen behavior of superhalogen building blocks.
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