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2 Large Inorganic Hollow Clusters

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496



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Nanochemistry



Fig. 10.22 Structure of the Mo368 hedghog perpendicular to the C4 axis (a) and along the

C4 axis (b). Building units {Mo1 }≡[MoO(H2 O)] (yellow), {Mo2 }≡{Mo2 O3 (H2 O)2 } (red),

and {Mo(Mo5 )} (blue) with blue-turquoise pentagonal bipyramids. The cavity has a size of

2.5×2.5×4.0 nm3 . (Reprinted with permission from [10.42]. © 2002 Wiley-VCH)



(see Fig. 10.23). These clusters are stabilized by van der Waals attraction, electrostatic repulsion, and hydrogen bonding involving water molecules. The ensemble of

the spherically arranged nanowheels yields a soft and flexible “surface membrane”

of the vesicles as analyzed by light scattering and transmission electron microscopy.

These vesicles partially collapse during the drying process. The vesicle built by

the 1,165 nanowheels (Fig. 10.23c) is modeled by an icosadeltahedron which is a



10.2



Large Inorganic Hollow Clusters



497



Fig. 10.23 Spherical 90 nm vesicles composed of 1,165{Mo154 } nanowheels. (a) Smallest fragment with a Mo atom and its coordinated sphere including one of the 70 H2 O ligands causing the

hydrophilic nature. (b) Space-filling representation of the 3.6 nm size {Mo154 }-type nanowheel

(blue and light blue, Mo atoms; red, O atoms). (c) Schematic representation of the vesicle structure

formed of nanowheels in aqueous solution with the inset showing enlarged nanowheels. (Reprinted

with permission from [10.28]. © 2003 Nature Publishing Group)



polytope with icosahedral symmetry where all faces are equilateral triangles and

either five or six triangles are found adjacent to each vortex [10.28].



10.2.3 Nitride–Phosphate Clathrate

Clathrates are formed when a host compound encloses guest molecules without

using strong ionic interactions or covalent bonds. A nitride–phosphate clathrate

framework has been synthesized [10.43] (Fig. 10.24) which can trap neutral



Fig. 10.24 Exploded view

of a cavity via a nitride–

phosphate clathrate

framework, formed from a

ring of eight tetrahedra

capped with two rings of four

tetrahedra. The arrows point

between nitrogen atoms that

are shared by the central and

the capping rings. (Reprinted

with permission from [10.43].

© 2006 Wiley-VCH)



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Nanochemistry



ammonia molecules in small cages. The framework is constructed from tetrahedra

which consist of a phosphorus atom (red) at the center of four nitrogen atoms (blue).

To form a cage, eight of these tetrahedra form a ring which is capped above and

below by two smaller rings, each formed from four tetrahedra (Fig. 10.24). This

compound with a remarkable thermal stability up to 550◦ C could be useful in membrane reactors where the trapped molecules would participate in chemical reactions

at solid–gas interfaces. The nitride may even exhibit electrical protonic conduction.



10.3 Chemistry on the Nanoscale

The availability of carbon nanotubes and nanodroplets as “nano test tubes” and

“nanoreactors” allows for the investigation of novel chemical reactions in lowdimensional environments and confined geometries. Chemical reactions can be

performed in attoliter (10−18 l) volumes on a zeptomole (10−21 mol) scale [10.44].

In addition, by employing nano-manipulation techniques [10.45], the interaction of

single molecules can be studied, as outlined in the following.



10.3.1 Nano Test Tubes

Single-walled carbon nanotubes (SWNTs) were used as templates for forming covalent polymeric chains from C60 O [10.46]; the resulting polymer topology is different

from the 3D bulk polymer in that it is linear and unbranched. Above 250◦ C in the

bulk solid state, C60 O polymerizes via epoxide ring opening, with a rigid furan-type

bridge linking the cages. The bulk fullerene oxide polymer forms a face-centered

cubic lattice with an interfullerene spacing of 0.997 nm, close to that of C60 . For

the reaction in the nanotubes, the C60 O molecules were filled into the SWNTs and

heated to 260◦ C.

The confinement of 1-heptene molecules inside of SWNTs was shown to result

in lowering of their reactivity to atomic hydrogen compared to 1-heptene absorption

on external SWNT sites [10.47]. The reaction of atomic hydrogen with 1-heptene to

heptane

˙ − CH3

˙ ⇒ C5 H11 CH

C5 H11 CH = CH2 + H

˙ − CH3 ⇒ C5 H11 CH2 − CH3 + C5 H11 CH = CH2

2C5 H11 CH

is nearly nonactivated and occurs readily even below 100 K. At SWNT temperatures

of 270 K, when only the interior of the nanotube is populated with 1-heptene, a

lower rate of reaction with atomic hydrogen is observed, compared to experiments

in which both interior and surface sites are occupied. This suggests that the graphene

framework shields the 1-heptene molecules from the incident hydrogen atoms and

that nanotubes can be used to control the chemical reactivity of a molecule [10.47].



10.3



a



Chemistry on the Nanoscale



499



b



c



Fig. 10.25 (a) High-resolution transmission electron micrograph (HRTEM) of tris (cyclopentadienyl)cerium (CeCp3 ) in a single-walled carbon nanotube SWNT (CeCp3 @SWNT). (b), (c)

HRTEM of the sample transformed at 1000◦ C in vacuo into double-walled carbon nanotubes

(DWNT) containing cerium (Ce@DWNT). The undulating lines in between the outer tube wall

indicate an inner tube, while the dark dots are from cerium ions. Scale bar is 1 nm. (Reprinted with

permission from [10.48]. © 2009 American Physical Society)



For the application of nano test tube chemistry, the 1D quantized electronic

levels of SWNTs can be manipulated by doping [10.48], e.g., with tris(cyclopentadienyl)cerium (CeCp3 ; see Fig. 10.25a).



10.3.2 Dynamics in Water Nanodroplets

There are many examples in the fields of biology, geochemistry, tribology, and

nanofluidics, where water molecules are not present as a bulk liquid, but in confined geometries [10.49]. Near a surface, ordering of water molecules into layers

occurs [10.50] as shown by x-ray diffraction. This ordering was found to extend up

to several molecular diameters into the liquid. In the case of small water droplets, the

confinement is 3D, and the overall structure and dynamics of the water are affected.

A solution of nanometer-sized droplets (see Sect. 7.9) forms when preparing an emulsion of water in an apolar solvent by addition of a surfactant. The

anionic lipid surfactant sodium bis(2-ethylhexyl)sulfosuccinate (AOT) is known

to form monodisperse micelles with radii ranging from 0.2 to 4.5 nm [10.49],

depending of the water-to-AOT ratio, conventionally denoted by the parameter

w0 = [H2 O] [AOT]. The dynamics of water molecules in nanodroplets was studied

by using ultrafast mid-infrared pump-probe spectroscopy on the OH-stretch vibration of isotopically diluted water (HDO in D2 O) contained in reverse micelles. In



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