Carbon and silicon cage clusters

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Carbon and silicon cage clusters with endohedral
impurities
Frank Hagelberg Computational Center
for Molecular Structure and Interactions Jackson
State University Jackson, MS 39217 USA

• Motivation
• Metal clusters in fullerene cages the case of
  NSc3_at_C68.
• Cage-like silicon clusters enclosing metal atoms.
• IV. Polyhedral Oligomeric Silsesquioxane (POSS)
• clusters with alkali and halogen
  atom impurities.
• V. Conclusion.

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Endohedral metallofullerenes Metal impurities
encapsulated in fullerene cages.
Eu_at_C82, Gd_at_C82 Fullerene enclosure preserves
the magnetic moment of the metal impurity1. ?
Use of metallofullerenes as contrast agents
in Magnetic Resonance Imaging (MRI) ? Enhanced
magnetic effects for encapsulated small metal
clusters? 1 K.H.Mueller, L.Dunsch. D.Eckert,
M.Wolf, A.Bart, Synthetic Materials 103, 2417
(99)
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Di- and Trimetallofullerenes
From mass-spectrometric measurement
La2_at_C80, Sc2_at_C84, Sc3_at_C82, Sc3N_at_C80..
Sc2_at_C84 M. Takata et al., PRL 78, 3330 (1997)
Sc3N_at_C68 S. Stevenson et al., Nature 408,
427 (2000)
From NMR spectrum The C68 cage does not conform
the Isolated Pentagon Rule (IPR).
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The eleven C68 cage structures compatible with
the NMR spectrum. From B3LYP/6-31G
optimization
E 0.0 eV E 0.8 eV
Non-IPR structures with three fused pentagons
preferred! Seifert et al., Int.J.Quant.Chem.58,
185 (1996) DFT/tight binding approach.
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The system Sc3N.
Ground state structure of C3v symmetry
? (J.M.Campanera et al. J.Phys.Chem.A 106, 12356
(2002))
Optimization at the B3LYP/6-31G level. All
energies in Hartree.
From MP2/6-311G(3df) computation D3h ground
state geometry and instability of C3v in spin
singlet condition confirmed.
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Sc3N_at_C68
Full optimization of the two most stable C68
cages with endohedral Sc3N for two initial
orientatons.
Eclipsed geometry Staggered
geometry Most stable
structure Transition state.
E 0.0 eV E 4.5
eV
Fragment of Sc3N_at_C68 at equilibrium
geometry. Relaxation effects upon encapsulation
of Sc3N into C68 ? 1 2
Sc3N stabilized by high rotation barrier.
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Interaction between Sc3N and the pentagon
pair substructures of the C68 cage.
Comparison with C14H8

Stabilization of C14H8 by addition of
two electrons
C14H8
C14H82- ?EHL 0.03 eV
0.17 eV ?EHL HOMO LUMO gap.
Aromaticity found for Sc3N_at_C68 by Nuclear
Independent Chemical Shift (NICS) analysis.
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Sc3N_at_C68 and C686-
Sc3N_at_C68
C686-
The frontier orbitals of Sc3N_at_C68 and C686- are
nearly identical.
C68
LUMO1 LUMO2
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Comparison with Sc3N_at_C781
C78 obeys the Isolated Pentagon Rule.
Interaction of Sc atoms with pyracelene cage
subunits.

• The three lowest virtual cage orbitals occupied
  by core
• electrons.
• -Stabilization of the core towards rotation
  within the cage.

1 J.M.Campanera et al. J.Phys.Chem.A 106, 12356
(2002)
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Silicon Clusters enclosing metal atom
impurities.
Si and C share the atomic valence electron
configuration (s2 p2) Si prefers sp3 over sp2
bonding formation of compact structures. No
SiN clusters with fullerene structures detected
so far.
Silicon analogues of endohedral
metallofullerenes? Stabilization of SiN cage
structures?
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Hiura, Miyazaki, Kanayama (2001)Experimental
evidence for silicon clusters encapsulating metal
atoms. Use of ion trap method to study the
gradual emergence of various MSiN (M metal
atom) species.
Example WSi12
Observation of the growth of WSiN (N 1, 2, 3
.) .
At N 12, the growth process terminates 12 Si
atoms surround a W ion.

From H. Hiura, T. Miyazaki and T. Kanayama,
Phys. Rev. Lett. 86, 1733 (2001).
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WSi12
CuSi12
Si12 (D6h)
Highest occupied molecular orbital for Si12
(D6h) HOMO Eg ?
? -gt Shell closing
through addition or subtraction of two electrons.
F. Hagelberg, C.Xiao, W.A.Lester, Jr., Phys. Rev.
B 67, 035426 (2003).
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WSi12 Natural Charge Q on W Q -1.74 e
HOMO of Si12
HOMO of WSi12
Strong ionic component in the bonding between the
W impurity and the Si12 cage.
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WSi12
CuSi12
Si12 (D6h)
Highest occupied molecular orbital for Si12
(D6h) HOMO Eg ?
? -gt Shell closing
through addition or subtraction of two electrons.
F. Hagelberg, C.Xiao, W.A.Lester, Jr., Phys. Rev.
B 67, 035426 (2003).
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Energetic properties of MeSi12 (Me Cu, Mo, W)
MeSi12 Symmetry group ?Ee ?EHL
AIP CuSi12 C2h
- 3.84 2.09 6.59 MoSi12
D6h - 7.80 2.19
7.66 WSi12 D6h
- 9.96 2.59 7.79
?Ee Embedding energy
E(MeSi12) E(Me) E(Si12) ?EHL HOMO LUMO
energy difference. AIP Adiabatic Ionization
potential. All values are given in eV. High
values of the embedding energy and the HOMO
LUMO gap for Me Mo, W.
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Me2Si18 (Me Mo, W)
Experimentally detected W2Si18. (H. Hiura, T.
Miyazaki and T. Kanayama, Phys. Rev. Lett. 86,
1733 2001) Periodic change of Si Si bond
lengths in each Si6 layer ? Reduction of D6h to
D3h symmetry.
Charge density distribution of W2Si18
W2 is bonded to the two terminating Si6 layers
but does not interact with the intermediate one.
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Polyhedral Oligomeric Silsesquioxane (POSS)
clusters and analogous systems.
Species of the form (RSiO3/2)N
POSS core molecule Si8O12R8 , R organic group.
Spherosiloxane Si8O12H8

• Interest in POSS units motivated by
• A wide range of applications from polymer
• modifiers to lubricants.
• Addition of POSS compounds results in polymers
• with
• - extended temperature ranges,
• - reduced flammability,
• - lower thermal conductivity.

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Study of geometry, stability and electronic
structure of - POSS monomers with atomic
impurities. - POSS analogous systems (Y8O12R8
with Y C, Ge)
POSS monomers with endohedral alkali and halogen
impurities. Impurities Li, Na, K, F-, Cl-,
Br-, I- Matrix Spherosiloxane.
Na_at_Si8O12H8
Li_at_Si8O12H8
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Energetic Parameters of A_at_Si8O12H8. Impurity
?E (HOMO LUMO) ?Eea
QAb Li 8.83
- 0.89 0.87 Na
8.73 0.47
0.88 K
8.59 3.14
0.94 F - 9.02
-4.22 - 0.79 Cl -
8.32 0.01
- 0.60 None
8.75 a
Embedding energy ? Ee E(A_at_Si8O12H8) - E(A)
- E(Si8O12H8) . b Natural charge of impurity A.
All energies in eV.
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From ion mobility studies Na adopts an
exohedral position
Comparison between Si8O12H8 with endohedral and
with exohedral alkali impurities.
Example A Li. ? Ee (Li_at_Si8O12H8) - 1.24
eV ? Ee (Li Si8O12H8) - 2.32 eV
The exohedral isomer of AY8O12H8 is found more
stable than the endohedral one for A Li, Na,
K, and Y C, Si.
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Embedding and adsorption energies for
AX8O12H8 with A Li, Na, K and X C, Si, Ge.
Endohedral structures From C to Ge Increase of
stability From Li to K Decrease of stability
For Li Ge8O12H8 crossover between endohedral
and exohedral structures.
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POSS monomers with halogen impurities Endohedral
coordination is strongly preferred. Weak
exohedral bonding.
For F-_at_ Y8O12H8 with Y Si, Ge, halogen atom
encapsulation is an exothermic process. These
species have been detected experimentally1.
1A.R. Bassindale et al., Angew. Chem. Int. Ed.
42, 3488 (2003) F-_at_ Si8O12R8 L. A.
Villaescusa et al., Chem. Comm. 2002, 2220 F-_at_
Ge8O12R8
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Reaction paths for incorporation of an alkali or
halogen- impurity into the POSS cage.
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Summary. ?
Fullerenes with endohedral Sc3N metal clusters
- NSc3_at_C68 adopts an eclipsed geometry that
maximizes the interaction between Sc atoms
and fused pentagons. - NSc3 is strongly
stabilized towards rotation within the C68 and
C78 cages. ? Silicon clusters with metal atom
impurities - Critical cluster size for
endohedral coordination of M_at_SiN N 12
with M W, Mo, Cu. - Stabilization of M_at_SiN
by electron transfer to(from) M W, Mo (Cu).
? Polyhedral Oligomeric Silsesquioxane (POSS)
clusters - Exohedral coordination preferred
for alkali, endohedral for halogen doped
systems. - Most stable endohedral structures
identified X_at_ Y8O12H8 with X Li, F-, Y
Si, Ge - Dependence of cage symmetry on the
alkali center.
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With thanks to
Research Associates
Graduate Student Dr. Jian-Ge Zhou
Yashesh Pandya Dr. Jianhua Wu Dr.
Sung Soo Park
Undergraduate student
Kendrick
Walker And special thanks to NIH-SCORE,
NSF-CREST, NSF-EPSCoR.
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Transition States Between endohedral and
Exohedral at B3LYP/6-311G