Unit 1- Study Materials on Fullerene

8/7/2019 Unit 1- Study Materials on Fullerene

1/17

Fullerenes

Introduction

Until 1985, the chemical element Carbon was only known to exist in two forms -

diamond and graphite. This changed when Kroto and co-workers discovered an entirely new

form of carbon, which became known as C60 or the fullerene molecule. (This discovery later

led to their award of the 1996 Nobel Prize in Chemistry). The original discovery of C 60 was

in the soot produced from the laser ablation of graphite. Since then, other methods of

production have been developed.

The fullerene molecule consists of 60 carbon atoms arranged in pentagons and

hexagons, very like in a standard football (soccer ball). It is also known as Buckminster

Fullerene due to the resemblance of this shape to the geodesic domes designed and built by

the architect R Buckminster Fuller.It was soon discovered that C60 is not the only ball-like carbon molecule possible

(although it is the most stable and the most dominant). The rugby-ball shaped C70 molecule

is another possibility.

It was not until the early 1990s that fullerenes could be synthesized in large enough

quantities for significant research in this field to be undertaken. Since then, fullerenes have

been prepared in gas and solid phases, in inert gas matrices, in a range of different solutions,

as fulleride salts and in polymerized states. Some solids intercalated with alkali metals are

superconductors. This means that at temperatures below the transition temperature Tc, a

current can flow with no resistance. Even though Tc in the fulleride superconductors is atmost 40 K (-233C) it is only the second-known class of superconductors in which Tc is high

enough for potential applications to be a possibility. It is perhaps surprising that some

intercalated solids are insulators, i.e. are highly resistant to current flow.

Many of the early experimental results on C60 were unreliable because the samples

tended to be small and highly contaminated. However, in the last few years a very large

1


2/17

number of reliable experimental results have become available. Despite a high level of

activity, many of these results are either unexplained or subject to controversy. Analysis of

the data is far from straightforward. One major difficulty is due to the interdisciplinary nature

of the subject. The vast majority of experimental investigations are carried out by Chemists,

but the processes giving rise to the experimental observations are driven by Physics. We arecurrently trying to resolve these difficulties by combining the expertise of Chemists and

Physicists

In nanotechnology, the potential applications of carbon nanotubes (formed by

combining hexagonal rings of carbon atoms only, rather than hexagons and pentagons as in

C60) for very small electronic devices are currently the subject of much activity.

Structure of fullerenes

Fullerenes are a molecular form of pure carbon discovered in 1985. They are cage-

like structures of carbon atoms; the most abundant form produced is buckminsterfullerene

(C60), with 60 carbon atoms arranged in a spherical structure. There are larger fullerenes

containing from 70 to 500 carbon atoms. Sometimes fullerenes are mistakenly called a "new

form of carbon"; in fact, fullerenes have been found to exist in interstellar dust as well as in

geological formations on Earth.

Fullerene cages are about 7-15 angstroms in diameter ( 1A

= 10

-10

m). In atomicterms, their sizes are enormous. But fullerenes are still small compared to many organic

molecules. Chemically, they are quite stable; breaking the balls requires temperatures of over

10000 C (the exact number depends on which particular fullerene). At much lower

temperatures (a few hundred degrees C) fullerenes will "sublime," which means vapor will

form directly from the solid.

2


3/17

Pure C60 consists of 60 carbon atoms arranged as 12 pentagons and 20 hexagons

whereas C70 has got 12 pentagons and 25 hexagons. Visually C60 it is quite different from

both graphite and diamond it is a yellow powder, which turns pink when dissolved in certain

solvents such as toluene. When exposed to strong ultraviolet light, the buckyballs

polymerize, forming bonds between adjacent balls. In crystalline form C60 is cubic (at eachlattice point of a cube, there is a buckyball). Electrically, it is insulating. It shows electro-

negativity and forms compounds easily with alkali atoms.

Preparation of fullerenes by arc method

The experimental plant has the following parts: reactor chamber, ensembles of electrodes

(anode and cathode), cooling system reactor chamber, cooling system of ensembles of

electrodes, vacuum system, electrical system and electronic system. The apparatus consists

of a DC arc chamber with the following operating conditions.

3


4/17

The operating conditions are:

- the diameter of the electrodes: 6 mm;

- the discharge voltage: 25 V;- the continuous current on the electrodes: 60 A;

- the cooling water flow of the discharge chamber: aprox.300l/h;

- the cooling oil flow of the electrodes: 60l/h;

- the control gas pressure, helium, in a range between 100 to 200 mmHg.

The main part is extrapure spectroscopic grade graphite rods of 6mm dia and 150 mm

length, fixed in copper holders. The facing sides of both the graphite rods are parallel. The

discharge chamber has a cooling coil around it, attached to a chiller to remove the heat

produced in the arc chamber. The power to the generator is provided from a high current low

voltage power supply.

After evacuating discharge chamber to 10-3 torr, it is isolated from the pump and filled

with 99.99 % pure He gas to a pressure of 100 to 200 mm Hg. An arc is strike between two

graphite electrodes kept at a distance of 2mm apart by maintaining a voltage of 25 V.

The plasma obtained can be viewed through a view part. At the end of each reaction

the apparatus is filled with He gas to atmospheric pressure and the chamber is allowed to

cool down. After this, the soot was collected from all regions of the discharge chamber and

C60 is separated from the soot by chromatographic technique. The soot extract contains

carbon clusters mixture of 25-30%, and the content of C60 and C70 fullerenes is 1.2-2.5%.

To extract the fullerenes from the soot is used the Soxhlet extraction with toluene

followed by extraction in a sonic bath. The C60+C70 mixture obtained is separated into its

components using a chromatographic column with neutral alumina like stationary phase and

hexane like mobile phase.

Purifying the Fullerenes - by Chromatography

4


5/17

Separation of pure allotropes by chromatography is believed to be unique to

fullerenes. The collected soot is placed in a small flask and 20 to 30 ml of fullerene is added.

Flask is closed with a stopper and shaken vigorously and then it is filtered.

This solution is coloured (deep red) due to mixture of C60 and C70. Next step is to

separate the fullerenes into fractions using chromatography. The column is first filled with

carbon granules. Then toluene is filled into the column until the level of toluene equals to the

height of carbon granules. Then the solution containing C60 and C70 is added into the column

through a dropper flask.

The solution first coming out of the column is collected separately in a conical flask

and thrown into waste. When the colour starts to change i.e. to magenta after 20 to 25

minutes, the fraction coming out is collected in a clean flask. This is C60 fraction, this will

continue for another 20 to 30 minutes after which magenta colour solution will stop coming

out.

At this stage dichlorobenzene is added to the column, after a while red C70 fraction

will appear. This fraction is collected separately in a flask, then the solvent toluene and

dichlorobenzene fractions are evaporated from the two flasks to remove the solvent to get

pure C60 and C70.

5


6/17

Solvents that are able to dissolve a fullerene extract mixture (C60 / C70) are listed below in

order from highest solubility. The value in parentheses is the approximate saturated

concentration.

1,2,4-trichlorobenzene (20 mg/ml)

Carbon disulfide (12 mg/ml)

toluene (3.2 mg/ml) - can also be used as a fullerene indicator, as fullerenes

turn toluene purple

Benzene (1.8 mg/ml)

Chloroform (0.5 mg/ml)

Carbon tetrachloride (0.4 mg/ml)

Cyclohexane (0.054 mg/ml)

n-hexane (0.046 mg/ml)

Tetrahydrofuran (0.037 mg/ml)

Acetonitrile (0.02 mg/ml)

Methanol (0.0009 mg/ml)

Spectroscopic and X-ray studies

13C NMR spectra (shown below) of C60 shows a single peak at 142 ppm showing

symmetry of the carbon atoms. While 13C NMR spectra of C70 shows 5 peaks in the ratio of

10:10:20:20:10.

6
http://en.wikipedia.org/wiki/Chlorobenzenehttp://en.wikipedia.org/wiki/Carbon_disulfidehttp://en.wikipedia.org/wiki/Toluenehttp://en.wikipedia.org/wiki/Benzenehttp://en.wikipedia.org/wiki/Chloroformhttp://en.wikipedia.org/wiki/Carbon_tetrachloridehttp://en.wikipedia.org/wiki/Cyclohexanehttp://en.wikipedia.org/wiki/Hexanehttp://en.wikipedia.org/wiki/Tetrahydrofuranhttp://en.wikipedia.org/wiki/Acetonitrilehttp://en.wikipedia.org/wiki/Methanolhttp://en.wikipedia.org/wiki/Chlorobenzenehttp://en.wikipedia.org/wiki/Carbon_disulfidehttp://en.wikipedia.org/wiki/Toluenehttp://en.wikipedia.org/wiki/Benzenehttp://en.wikipedia.org/wiki/Chloroformhttp://en.wikipedia.org/wiki/Carbon_tetrachloridehttp://en.wikipedia.org/wiki/Cyclohexanehttp://en.wikipedia.org/wiki/Hexanehttp://en.wikipedia.org/wiki/Tetrahydrofuranhttp://en.wikipedia.org/wiki/Acetonitrilehttp://en.wikipedia.org/wiki/Methanol


7/17

F

I

Figure : Idealized C NMR spectra and structural drawings of C (top) and C

(bottom). In C , all carbon atoms are identical and a single C NMR peak is observed. In C

, there are five sets of inequivalent carbon atoms (labelled a-e), giving rise to five C NMR

signals.

Mass spectra of C60

shows a peak at m/e value of 720 indicating it is made up of 60carbon atoms (60 X 12 = 720).Most molecules absorb in the Infra-red (IR) and the number of

absorptions is dependant on the number of atoms in the molecule and how symmetrical the

structure of the molecule is. In general an N atom molecule will have about 3N absorptions

in the IR. As the symmetry increases the number of IR absorptions falls. For a molecule

having 60 atoms one would expect 3 x 60 = roughly 180 IR absorptions. However because of

the unique symmetry of Buckminsterfullerene, C60 has just 4 IR absorptions. This incredibly

7


8/17

simple spectral fingerprint for C60 made IR spectroscopy a particularly effective method by

which to probe for C60 in the arc made materials.

8


9/17

9


10/17

10


11/17

11


12/17

The powder XRD pattern of the soot, C60 and mixed fullerenes is shown in fig.2. The

soot has an amorphous pattern whereas C60 has a cubic structure with a= 14.179. The XRD

pattern of mixed fullerenes contains additional peaks around 2 = 30.

Pure C60 in toluene shows absorption at 338 nm and also at higher leading to themagenta colour (fig.1). The spectra of soot extract (extracting from different region of arc

chamber) in toluene shows absorption maxima in the range of 330 to 715 nm.

The chemical transformations that are possible with C60 could be classified in five

main groups:

a) Addition reactions. Formation of exohedral compounds by addition of nucleophiles or

radicals, cycloadditions, complexations with transition metals and others.

b) Electron transfer reactions. Chemical reduction of fullerenes can easily be achieved byreaction with electropositive alkali and alkaline earth metals or organic donor molecules.

c) Heterofullerenes. Substitution of a carbon atom of the fullerene skeleton for a

heteroatom, for example nitrogen or boron.

d) Ring opening reactions. Producing a hole in the C60 skeleton while breaking a discrete

number of bonds.

e) Formation of endohedrals. Introducing and trapping of atoms inside the spherical carbon

cage.

Phase transitions in hexagonal close-packed phase of C60

C60 fullerite is known to exist as three polymorphic forms: the face-centered cubic

modification (fcc), transforming into the simple cubic modification (260 K, 1 atm), and the

hexagonal close-packed (hcp) modification. However, no data concerning the phase

transitions of hexagonal modification are available in the literature. The cubic and hexagonal

structures modeled as close-packed balls can be considered as polytypic modifications,

because their crystal lattice are distinguished only in packing mode of identical planes (three

layers are alternated in the case of cubic modification, and two layers, in the case of

hexagonal modification).

The high-temperature solid phases of the pure fullerene C70 are orientationally

disordered and show one of the close packed structures: fcc or hcp. Both structures have been

observed depending on the preparation procedures. The samples grown from solution

12


13/17

crystallize preferentially in the hexagonal structure hcp. The orientational distribution of the

C70 molecules is practically spherical in the high-temperature phase hcp-2. At about 337 K

the crystal C70 undergoes a discontinuous phase transition in which the long molecular axes

become ordered along the crystallographic direction (0001). The space group of the crystal

does not change at this phase transition, whilst the ratio of the lattice constants c=a jumpsabruptly from the value characteristic to spherical elective molecules: c=a = 1:63... (hcp-2

phase) to the value corresponding to the shape of a single C70 molecule c=a = 1:84 (hcp-1

phase). Thus, the phase transition hcp-2!hcp-1 falls into the category of isomorphous phase

transitions. For symmetrical reasons, such phase transitions are either of first order or there is

no transition at all, both situations being separated by a critical point.

Potential applications of Fullerenes

a. Superconductivity in K3C60

When doped with alkali atoms, solid C60 becomes a superconductor at surprisingly

high temperatures. The detailed properties of this superconductor, and most particularly

why the repulsion between electrons does not suppress the superconductivity, appear to be

related to the unusual highly symmetric molecular character of the material, plus the highly

curved metastable nature of the surface of each ball. Experimental data on the

superconducting and normal-state properties of fullerene superconductors reveals several

novel effects arising from the low Fermi velocity and molecular character of the material.A3C60 is a conductor at room temperature due to partial filling of t 1u conduction band. It

retains the basic fcc structure of C60 and lattice constant expands to accommodate the

alkali ions.

Recent reports of superconductivity in alkali-metal-doped compounds of the

icosahedral C60 (buckminsterfullerene) molecule have attracted great experimental and

theoretical interest. Superconductivity was originally discovered in samples prepared from

gas solid reactions, which made it impossible to determine the composition or structure of thesuperconducting phase. Holczer et al. demonstrated that potassium-doped C60 has only a

single stable superconducting phase, K3C60, with a transition temperature of 19.3 K.

Improvements have since resulted in the preparation of 100% bulk Superconductors.

Because of the absence of impurity phases, we have been able to perform accurate Rietveld

13


14/17

analysis of X-ray diffraction data from the superconducting phase. Rb2CsC60 has maximum

Tc of 31K.

C

T

When K C is cooled, its resistivity begins to drop sharply at about 18K, indicatingthe onset of superconductivity (Hebard, 1991). Interestingly, as larger alkali-metal cations are

incorporated into the lattice and the fcc lattice parameter (a) increases, the superconducting

transition temperature, T , also increases (see Figure ). Hence, the T for Rb C rises to

28K. This rise in T may be related to an increase in the density of states at the Fermi level

with increasing lattice constant.

Plot of superconducting transition temperature T (K) vs. lattice parameter a

(angstroms) for various compositions of A C.

14


15/17

The correlation between T and lattice constant (a) suggests that even higher T 's

could be obtained by incorporating larger and larger cations, A. There are two potentialproblems with this strategy. 1) As the C ions move apart, electron flow may be shut down.

2) If the cations, A, become too large to be accommodated in the octahedral and tetrahedral

holes of the fcc lattice, a major reorganization of the packing would be required, which might

lead to a loss of superconductivity.

Although the detailed mechanism of superconductivity in A C remains to be

established, the simplicity of the materials and the progress already made suggest a definitive

resolution of this question may be achieved more quickly than in the case of high-T copper-

oxide superconductors.

b. HIV Protease Inhibitor

C and its derivatives, because of their large size, stability, and hydrophobic character,

may prove to have value as diagnostic or therapeutic agents in medicine. For example,

derivatives of C are currently being investigated as potential inhibitors of the protease

enzyme specific to the human immunodeficiency virus 1 (HIVP) (Friedman, 1993). The

active site of this enzyme can be roughly described as an open-ended cylinder which is lined

almost exclusively by hydrophobic amino acids. Notable exceptions to this hydrophobic

trend are two catalytic aspartic acids which catalyze the attack of water on a peptide bond of

the substrate.

Because a C molecule has approximately the same radius as the cylinder that describes

the active site of HIVP and since C and its derivatives are primarily hydrophobic, an

opportunity exists for a strong hydrophobic vander Waals interaction between the nonpolar

active-site surface and the C surface. In addition, however, there is an opportunity for

increasing binding energy by the introduction of specific electrostatic interactions. One

obvious possibility involves salt bridges between the catalytic aspartic acids on the floor of

the HIVP active site and basic groups such as amines introduced on the C surface. The key

to exploiting this promising system will be the development of organic synthetic

methodology to derivative the C surface in highly selective ways.

15


16/17

c. Carbon Nanotubes and Nanowires

One potentially important application for these carbon nanotubes is in the area of

composites (Calvert, 1992). Carbon fibers, made from organic polymers, are used tostrengthen lightweight high-tech materials such as the carbon/epoxy resins used in golf clubs,

tennis racquets, bicycle frames, and yachts. Each fiber is 6,000-10,000 nm in diameter or

about five times thinner than a human hair. The carbon nanotubes are substantially thinner, 4-

30 nm in diameter, and could be an even more effective strengthening agent than carbon

fibers in carbon/epoxy resins. The reason for this is that the breaking strength of brittle

materials decreases as the size of the largest internal flaw increases. A 30 nm diameter fiber

cannot have a transverse flaw bigger than 30 nm.

A somewhat more speculative use for carbon nanotubes is as molecular wires (or

"nanowires"). Theoretical studies have suggested that the extremely small diameters of the

tubes could lead to high conductivity, comparable to that of metals at room temperature

(Mintmire, 1992).

Applications:

H2-filled C60:

Using organic synthesis as a scalpel and stitches, Japanese researchers have

performed "molecular surgery" on a buckyball. A group at Kyoto University creates an

opening in the molecule, inserts H2 into the cavity, and then, in just four steps, closes up the

C60 framework to construct the endohedral fullerene H2@C60 [Science,307, 238 (2005)].

In Medicines:

In April 2003, fullerenes were under study for potential medicinal use: binding specific

antibiotics to the structure to target resistant bacteria and even target certain cancer

cells such as melanoma. The October 2005 issue ofChemistry and Biology contains an

article describing the use of fullerenes as light-activated antimicrobial agents.

Jensen AW, Wilson SR, Schuster DI.Bioorg Med Chem. 1996 Jun;4(6):767-79 (Review).

16
http://www.kyoto-u.ac.jp/index-e.htmlhttp://en.wikipedia.org/wiki/Antibiotichttp://en.wikipedia.org/wiki/Bacteriumhttp://en.wikipedia.org/wiki/Cancerhttp://en.wikipedia.org/wiki/Melanomahttp://en.wikipedia.org/w/index.php?title=Chemistry_and_Biology&action=edithttp://en.wikipedia.org/wiki/Antimicrobialhttp://www.kyoto-u.ac.jp/index-e.htmlhttp://en.wikipedia.org/wiki/Antibiotichttp://en.wikipedia.org/wiki/Bacteriumhttp://en.wikipedia.org/wiki/Cancerhttp://en.wikipedia.org/wiki/Melanomahttp://en.wikipedia.org/w/index.php?title=Chemistry_and_Biology&action=edithttp://en.wikipedia.org/wiki/Antimicrobial


17/17

Endohedral fullerenes have ions or small molecules or positive metal ions (of La, Sc, Y..)

incorporated inside the cage atoms.

Applications:

1. Hydrogen Storage

2. Superconductors

3. Magnetic Resonance Imaging Contrast Agents

Encapsulating the gadolinium (Magnevist, a common MRI contrast agent) inside a fullerene

4. Trapping Reactive Species

5. Monitoring reactions on fullerenes

6. Geochemistry and astrochemistry

17

Date post:	09-Apr-2018
Category:	Documents
Upload:	ronnie1992
View:	226 times
Download:	0 times

Unit 1- Study Materials on Fullerene

Documents