Hung Tan Pham

A Systematic Investigation on CrCun Clusters with n = 9–16: Noble Gas and Tunable Magnetic Property

A systematic investigation on structure, dissociation behavior, chemical bonding, and magnetic property of Cr-doped Cun clusters (n = 9-16) is carried out using the mean of density functional theory calculations. It is found that CrCu12 is a crucial size, preferring an icosahedral Cu12 cage with the central Cr dopant. Smaller cluster sizes appear as on the way to form the CrCu12 icosahedron while larger ones are produced by attaching additional Cu atoms to the CrCu12 core. The presence of Cr dopant obviously enhances the stability of CrCun clusters in comparison to that of pure counterparts. Exceptionally stable CrCu12 has an 18-electron closed-shell electronic structure, mimicking a noble gas in the viewpoint of superatom concept. Analysis on cluster electronic structure shows that the interplay between 3d orbitals of Cr and 4s orbitals of Cu has a vital role on the magnetic properties of CrCun clusters.

Boron Teetotum: Metallic [Ti(B6CxNy)]q and Bimetallic [Ti2(B6CxNy)]q Nine-Membered Heterocycles with x + y = 3 and −1 ≤ q ≤ 3

We investigated the geometry, stability, and aromaticity of a series of singly and doubly titanium-doped boron clusters. Ti dopants bring in planar cyclic form with a nine-membered boron ring B9- and B93- and C and N isoelectronic derivatives where perfectly planar B6N3, B6CN2, B6C2N, and B6C3 heterorings are coordinated with one and two Ti atoms. The presence of both C and N atoms induces bimetallic heterocycles while Ti2B9q clusters are not stable in cyclic form. Doubly Ti doped clusters have the shape of a teetotum toy. High thermodynamic stability of these bimetallic boron heterocycles, that are global equilibrium structures of corresponding systems, can be understood as the result of a stabilizing overlap between bonding and antibonding MOs of Ti2 with different eigenstates of B6C xN y cycles. Both C and N elements, which are more electronegative than the B atom, also enjoy the formation of planar nine-membered ring via classical 2c-2e bonding, rather than occupancy of high coordination position. A double aromaticity feature which comprises both σ and π aromaticity is supported by magnetic responses of electron density within a planar cycle. Such an aromatic character is also in line with the classical electron count for both sets of delocalized σ and π electron systems.

Aromaticity of Some Metal Clusters: A Different View from Magnetic Ring Current

The aromatic character of some small planar metallic clusters was revisited with an emphasis on their σ electrons. In contrast to previous reports, our approach based on magnetic ring current as an indicator for aromaticity points out that the σ electron delocalization in molecules behaves as an important contributor to their thermodynamic stability. Ring current maps were constructed using electron densities obtained from density functional theory calculations with the B3LYP functional and the 6-311G(d) basis set. Diatropic currents were further confirmed by an analysis of the symmetry of electronic excitations involved. The triatomic B3+ cycle is found to maintain a double σ and π aromaticity when it forms the [B3(NN)3]+ and [B3(CO)3]+ complexes. The planar pentacoordinated carbon clusters including C@Al5+, C@Al5-xBex1-x, C@Be5Hnn-4, C@Be5Linn-4, and C@Be5HxLi5-x+ are σ aromatic rather than π aromatic as previously assigned. The mixed copper clusters Cu3Si3+ and Cu3Ge3+ are found to be σ aromatic compounds. The copper hydrides CunHn can better be regarded as nonaromatic rather than aromatic compounds. The ring current indicator reveals the σ aromatic feature for Be2@Be5H5+ and Be2@Be6H62+ clusters, whereas this criterion shows a double aromaticity of Be2@B7- and Be2@B8. Overall, the present study points out again the importance of σ electrons in determining the bonding characteristics of metallic clusters, and they should equally be considered as a key element.

Au 19 M (M=Cr, Mn, and Fe) as magnetic copies of the golden pyramid

An investigation on structure, stability, and magnetic properties of singly doped Au19M (M=Cr, Mn, and Fe) clusters is carried out by means of density functional theory calculations. The studied clusters prefer forming magnetic versions of the unique tetrahedral Au20. Stable sextet Au19Cr is identified as the least reactive species and can be qualified as a magnetic superatom. Analysis on cluster electronic structures shows that the competition between localized and delocalized electronic states governs the stability and magnetic properties of Au19M clusters.

Theoretical Study of Small Scandium-Doped Silver Clusters ScAgn with n = 1–7: σ-Aromatic Feature

Geometry, chemical bonding, and aromatic feature of a series of small silver clusters doped by an Sc atom (ScAgn with n = 1-7) were investigated by means of density functional theory calculations. A planar shape is found for ScAgn including n from 4 to 7. The growth mechanism is established for the formation of the hexagonal and heptagonal metallic cycles following increase of the number of Ag atoms. Particularly, both clusters ScAg6- and ScAg7 present a planar cyclic form in which the Sc atom is situated at the central position of the Ag6 and Ag7 cycles. The σ aromaticity is unambiguously demonstrated by the existence of strongly diatropic current flows within the ring in both ScAg6- and ScAg7. The isovalent ScCu7 cluster has a similar ring current characteristic. In the Sc-doped ScAgn clusters, a delocalized bonding pattern is found as a connector between the dopant Sc and the Agn host, as indicated by an ELI_D analysis.

Theoretical Investigation of Metallic Heterofullerenes of Silicon and Germanium Mixed with Phosphorus and Arsenic Atoms M-A8E6, A = Si, Ge; E = P, As; and M = Cr, Mo, W

Recently, metallic heterofullerenes were experimentally prepared from mixed Ge-As clusters and heavier elements of groups 14 and 15. We found that the shape of these heterofullerenes doped by transition metals appears to be a general structural motif for both silicon and germanium clusters when mixing with phosphorus and arsenic atoms. Structural identifications for MSi8P6, MSi8As6, MGe8P6, and MGe8As6 clusters, with M being a transition metal of group 6 (Cr, Mo and W), showed that most MA8E6 clusters, except for Cr-doped derivatives CrSi8As6, CrGe8P6, and CrGe8As6, exhibit a high-symmetry fullerene shape in which metal dopant is centered in a D3h A8E6 heterocage consisting of six A3E2 pentagonal faces and three A2E2 rhombus faces. The stability of the MA8E6 metallic heterofullerene is significantly enhanced by formation an electron configuration of [1S2 1P6 1D10 1F14 1G18 2S2 2P6 2D10] enclosing 68 electrons. The A8E6 heterocages give a great charge transfer (∼4 electrons) to centered dopant, establishing subsequently a d10 configuration for metal, and as a consequence, it induces an additional stabilization of the resulting ME8P6 fullerene in a high-symmetry D3h shape and completely quenches the high spin of the metal atom, finally yielding a singlet spin ground state.