Encapsulated Metal Atoms Research Articles

Charged Fullerenes as High-Capacity Hydrogen Storage Media

Using first-principles calculations within density functional theory, we explore systematically the capacity of charged carbon fullerenes Cn (20 <or= n <or= 82) as hydrogen storage media. We find that the binding strength of molecular hydrogen on either positively or negatively charged fullerenes can be dramatically enhanced to 0.18-0.32 eV, a desirable range for potential room-temperature, near ambient applications. The enhanced binding is delocalized in nature, surrounding the whole surface of a charged fullerene, and is attributed to the polarization of the hydrogen molecules by the high electric field generated near the surface of the charged fullerene. At full hydrogen coverage, these charged fullerenes can gain storage capacities of up to approximately 8.0 wt %. We also find that, contrary to intuitive expectation, fullerenes containing encapsulated metal atoms only exhibit negligible enhancement in the hydrogen binding strength, because the charge donated by the metal atoms is primarily confined inside the fullerene cages. These predictions may prove to be instrumental in searching for a new class of high-capacity hydrogen storage media.

Characterizing and Manipulating Individual Molecules by Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) can provide us the special means to characterize the locally physical and chemical properties of individual molecules, and even help us to manipulate the individual molecules for constructing new molecule-scale devices. Here we have adopted two new types of STM techniques to characterize the encapsulated metal atom inside a fullerene cage, and to construct a molecule-device with strong Kondo effect, respectively. The spatially dI/dV mapping spectra were used to unveil the energy-resolved metal-cage hybrid states of individual Dy@C82 molecule, and the important information about the spatial position of Dy atom inside the cage and the Dy-cage interaction was revealed. The high voltage pulse by STM tip was controlled to induce the dehydrogenation of Co phthalocyanine molecule and change its adsorption configuration on Au(111) surface, so as to recover Kondo effect that disappears in the case of intact adsorbed molecule.

Reduction of activation energy barrier of Stone-Wales transformation in endohedral metallofullerenes

We examine effects of encapsulated metal atoms inside a C$_{60}$ molecule on the activation energy barrier to the Stone-Wales transformation using {\it ab initio} calculations. The encapsulated metal atoms we study are K, Ca and La which nominally donate one, two and three electrons to the C$_{60}$ cage, respectively. We find that isomerization of the endohedral metallofullerene via the Stone-Wales transformation can occur more easily than that of the empty fullerene owing to the charge transfer. When K, Ca and La atoms are encapsulated inside the fullerene, the activation energy barriers are lowered by 0.30, 0.55 and 0.80 eV, respectively compared with that of the empty C$_{60}$ (7.16 eV). The lower activation energy barrier of the Stone-Wales transformation implies the higher probability of isomerization and coalescence of metallofullerenes, which require a series of Stone-Wales transformations.

Electronic structure and ground states of transition metals encapsulated in aSi12hexagonal prism cage

We report on a computational study of the electronic structure of recently discovered clusters with an encapsulated transition metal (TM) atom in a ${\mathrm{Si}}_{12}$ hexagonal prism cage. The cage geometry is remarkably stable regardless of the type of doping TM atom from $3d,4d,$ and $5d$ series. We predict and quantify the stability for several other TM dopings besides the experimentally observed ones. The multiplicity of the ground states can be ``tuned'' between singlets and triplets by varying the type of TM atom (even number of electrons), while they are doublets for odd number of electrons. We also explore the possibility of forming solids with hexagonal structure from selected clusters.

Many Reactive Fullerenes Tend To Form Stable Metallofullerenes

It was found that very many isolable or extractable metallofullerenes employ kinetically unstable isomers of C 7 8 , C 8 0 , and C 8 2 as carbon cages. These fullerene isomers are supposed to be stabilized by accepting one or more electrons from the encapsulated metal atoms. It seems that nature prefers to generate metallofullerenes with chemically reactive fullerene isomers. Some kinetically stable isomers of C 8 4 likewise form isolable metallofullerenes. All stability considerations were made using a bond resonance energy (BRE) model.

Structures, Stabilities, and Ionization Potentials of Dodecahedrane Endohedral Complexes

The equilibrium geometries and frequencies of endohedral complexes between H, He, Ne, Ar, Li, Li+, Be, Be+, Be2+, Na, Na+, Mg, Mg+, and Mg2+ and dodecahedrane (X@C20H20) were computed at B3LYP/ 6-311+G(d,p). The majority have Ih minima; the exceptions, X@C20H20 (X = Be, Be+, Be2+), have C5v symmetry with X localized against an inner cage face. Cage C−C bonds shorten slightly (<0.01 A) and cage C−H bonds lengthen slightly (≤0.02 A) in the series: M2+@C20H20 → M+@C20H20 → M@C20H20 (M = Li, Na, Be, Mg). These subtle changes in dodecahedrane geometry are due to donation of electron density from the encapsulated metal atom into the C−C bonding and C−H antibonding endohedral complex HOMO, which has a structure closely resembling the LUMO (A1g) of dodecahedrane. The zero point-corrected inclusion energies of Li+@C20H20 (Ih; −12.7 kcal/mol), Be+@C20H20 (C5v; −1.3 kcal/mol), Be2+@C20H20 (C5v; −236.3 kcal/mol) and Mg2+@C20H20 (Ih; −118.0 kcal/mol) are exothermic relative to their isolated components. However, all ...

Closed-shell electronic requirements for small fullerene cage structures

Closed-shell electronic requirements for small fullerene cage structures