Carbon nanostructures

Until the mid-1980’s pure solid carbon was thought to exist in only two physical forms, diamond and graphite.

Diamond and graphite have different physical structures and properties however their atoms are both arranged in covalently bonded networks.

These two different physical forms of carbon atoms are called allotropes.

In 1985 a group of researchers led by Richard Smalley and Robert Curl of Rice University in Houston and Harry Kroto of the University of Sussex in England discovered a molecules composed of 60 carbon atoms, C60.

The molecule they discovered had sixty equivalent carbon atoms, which formed the pattern of a football that gave it the highest symmetry.

Theoretical predictions of such structures were known since 1970.

In chemistry, there is no other molecule formed by the same atom, which is as big as buckministerfullerene.

For millennia, elemental carbon has been known to occur in two polymorphic forms, graphite and diamond.

Fullerenes constitute another allotrope of carbon.

Molecules with 70, 76, 82 and other numbers of carbon atoms were soon characterized.

The developments in 1991 showed that these molecules are found only with tens and hundreds of atoms but also with thousands of atoms.

These giant molecules of carbon occur as nanometer size tubes and balls which are called carbon nanotubes and onions respectively.

Hybridization in Carbon Compounds The electronic configuration of carbon in excited state is given as: 6C: 1s2 2s1 2px

1 2py1 2pz

1 There are four half-filled orbitals available for bond formation resulting in the formation of four covalent bonds in this state.

However, the four half-filled valence orbitals are not equivalent, there being three p orbitals and one s orbital.

To account for the equivalence of the four bonds, it is assumed that the four available orbitals of carbon, the 2s and three 2p orbitals are mixed or hybridized in a manner as to result in four equivalent orbitals (hybridization).

Carbon atom can undergo three types of hybridization in its compounds.

sp3 hybridization (Tetrahedral hybridization) In this type of hybridization four orbitals (one 2s and three 2p) of the excited carbon atom hybridize to form four orbitals of equivalent energy and same shape. Each orbital is called sp3 hybrid orbital.

=31/2

Each of the sp3 hybrid orbitals consists of a big lobe and a very small lobe that has (25%) s-character and (75%) p-character. The four hybrid orbitals are directed towards the corners of a regular tetrahedron. The hybridization is called tetrahedral hybridization and the angle between the hybridized orbitals is 109° 28'.

Four sp3 Hybrid Orbitals eg. Methane CH4

sp2 hybridization (Trigonal hybridization) In this type of hybridization, the 2s orbital and two of three 2p orbitals of excited carbon atom hybridize to form three sp2 hybridized orbitals of equivalent energy and identical shape. Each orbital is called sp2 hybrid orbital.

=21/2

The three sp2 orbitals lie in one plane making an angle of 120° with each other. A sp2 hybridized carbon is called trigonal carbon atom and the hybridization is known as trigonal hybridization. The unhybridized orbital is orientated in a plane at right angle to the plane of three hybridized orbitals.

sp2 Hybrid orbitals in carbon atom eg. Ethylene C2H4

sp Hybridization or Diagonal hybridization (Linear hybridization)

In this type of hybridization, the 2s and only one of the 2p (say 2p) orbitals hybridize forming two equivalent orbitals called sp or diagonal orbitals. The remaining two 2p orbitals are left in their original state.

The two 'sp' hybrid oibitals lie along a straight line and thus make an angle of 180° with each other. The two 2p orbitals, which are left in their original state lie in different planes at right angles to each other and also to the hybridized orbitals.

=1

eg. Acetylene C2H2

By observing the structure of a molecule it is possible to predict the state of hybridization of carbon.

A carbon atom that is directly linked to four other atoms is sp3 hybridized.

A carbon atom that is directly linked to three other atoms is sp2

hybridized.

A carbon atom is sp hybridized if it is directly linked to two other atoms.

Cubane (C8H8) is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom.

1983:

It is formed by joining carbon pentagons and has C-C bond angles ranging from 108o to 110o.

C20H20

Mass spectrum of carbon clusters.For N<30 there are cluster size for all NMolecular orbital theory predicts:Open structures for N odd and closed structures for N even.The large mass peak at N=60 correspond to C60.

Bucky Balls or fullerenes

Buckminster Fuller's Dome - Expo '67 Montreal

This new form of carbon was named after Richard Buckminster Fuller who was a pioneer in the design of the Geodesic dome often used in large building structures.

Doping with potassium, corresponding to the stoichiometry K3C60, produces a superconductor with a transition temperature of 19.8 K.

Intercalation is a process by which other atoms are put between the planes of the graphene sheets. Doping graphite with potassium also results in superconductivity, but at extremely low temperatures.

However, the doping of Na and Li does not result in superconductivity, but other alkali metals do produce superconductors with a high transition temperature (Tc).

The highest Tc organic superconductor, Rb2CsC60, has a Tc of 31 K.

They can be used as insulators, conductors, and semiconductors applied with other atoms and molecules.

The buckyballs do form crystal structures that work as insulators or semiconductors.

When it is dosed with alkaline metals like potassium, the solids become extreme electricity-conducting metals.

A fullerene, or C60, is the roundest molecule discovered to date.

As the hexagonal shapes are removed from the ball, it starts to lose its roundness, but one thing that is certain is that as the buckyball starts to lose its shape, the more unstable it becomes.

IMPORTANCE OF C60

“Window mechanism” for the encapsulation of atoms inside the fullerene cage. (A) C60 or buckminsterfullerine. (B) C60 with an atom inside. (C) C60 with an atom inside and with a bond broken (open window). (D) Same molecule as in C but with the atom moving out through the window.

The C60 molecule is large enough to enclose all of the noble gases He, neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe)

The icosahedral fullerene C540

The highly symmetric Boron-80

The shape of this very stable molecule is very similar to the original fullerene made of 60 carbon atoms placed on the corners of hexagons. But in order to get a stable structure, they've added another boron atom in the center of each hexagon.

The stability of a dodecahedral cage of nitrogen atoms (N20, “dodecahedrazane”) is examined using semi-empirical and density functional theory methods. It is shown that the nitrogen cage is stable in all vibrational modes. Protonation of the N20 structure allows hydrogen bonding with another N20 unit, leading to the possibility of extended structures. Endohedral systems including an enclosed hydrogen atom, hydride ion and hydrogen molecule are all stable.

Predicted structure of N20 Theoretical prediction for N20