A short introduction on: Carbon nano tube and the reaction dynamics inside it-Part 2

Anant Babu Marahatta
Ph.D. student in Chemistry
Tohoku University,Japan

A carbon nanotube may be considered as a hollow cylinder formed by rolling up a graphite sheet. The chirality and diameter of a carbon nanotube is uniquely defined by a vector (n, m), na + mb; where a and b denote the unit vectors of the hexagonal lattice and n and m are integers.

Fig ; 2.  A unit cell of a system of twelve carbon nano tubes and 1540 water molecules.

Computational studies have suggested that CNTs can be designed as molecular channels to transport water. A single-walled CNT, with a diameter of 8.1 Å, has been studied by molecular dynamics (MD) simulations. The simulations revealed that the CNT was spontaneously filled with a single file of water molecules and that water diffused through the tube concertedly at a fast rate. The transportation of water molecules across nanometer water channels in membranes plays a key role in biological activities. It has been recognized that the existence of the charged residues in these water channels greatly reduces the permeation of protons across the channel but maintains quite stable water flows. Moreover, because charges are indispensable in both membrane proteins and physiological solutions inside and outside the cells, it is also important to understand how external charges influence the water permeation.

The electronic polarizability of carbon nanotubes has been the subject of numerous studies because of their potential use as novel photonic materials and molecular electronic elements. The axial polarizability of carbon nanotubes has been found to be much larger than the radial polarizability and is dependent on nanotube length. Dipolar species confined within or in the interstitial spaces between carbon nanotubes can interact with an induced image dipole becoming stabilized relative to the gas phase.

Experimentally, carbon nanotubes have been filled with a variety of materials. Carbon nanotubes have been filled with molten AgNO₃ and then created pure Ag particles inside the nanotubes by electron-beam-mediated reduction. Carbon nanotubes have also been filled with materials such as KI, Ag, Au, AuCl, ZrCl₄, and even C60 and higher fullerenes. Inorganic nanorods have been synthesized through carbon nanotube confined reactions. For example, Ga₂O vapor and NH₃ were reacted inside carbon nanotubes to create gallium nitrides (GaN) nanorods with diameters determined by the radius of the nanotubes. Similarly, silicon nitrides (Si₃N₄) nanorods have also been synthesized.

Due to their unique physical properties, carbon nanotubes are a novel nano scale environment to carry out chemical reactions in it. Reaction energetics, mechanism and dynamics could be significantly altered inside carbon nanotubes due to their large intrinsic polarizabilities and due to the severely decreased reaction volume. In an effort to examine the effect on reaction enthalpies and activation energies of confining reacting systems inside carbon nanotubes, calculations using hybrid density functional theory have been carried out for a model reaction.

In the studies of interaction of CNTs with organic compounds, especially amines, it has been found that the amine groups in the molecules are electron donating and responsible for charge transfer to the semi conducting nanotubes. Terminal carbon atom forms a covalent bond with the N atom of the amine.

In theoretical studies examining the effect of local environment present inside the carbon nano tube on chemical reactivity, the Menshutkin SN2 reaction has often been studied. It is the simplest system in which an amine is alkylated by an alkyl halide. Choosing ammonia as the nucleophile and methyl chloride as the methyl transfer reagent gives chloride as the anionic leaving group.

H₃N + H₃CCl           H₃NCH₃⁺  +   Cl^-

Menshutkin SN2 reactions, in which the reactants are neutral and the product species are formally charged, are quite sensitive to the polarity of the surrounding environment, becoming more favorable with increasing polarizability, which stabilizes the separation of charge throughout the reaction. Medium effects result in a reduction in reaction barrier.

 The effect of confinement of the simplest Menshutkin SN2 reaction inside carbon nanotubes on chemical reaction enthalpies and activation energies has also been investigated. It has been found that in comparison to the gas phase, the potential energy surface changes dramatically.

Fig; Menshutkin SN2 ion pair product structure inside the carbon nanotube as viewed down the symmetry axis of the nanotube (top) and from the side (bottom).

At first, the ion pair product is significantly stabilized, making the overall process more favorable. And in the second step, the transition state shifts towards the reactants and is stabilized, giving a lower reaction barrier. This result indicates that the effect of nanotube confinement on relative reaction energies closely resembles solvation and the chemical reactions in which there is a separation of charge along the reactio coordinate will be enhanced inside fullerene based materials due to their large electronic polarizabilities.

Thus the research on “reaction dynamics on carbon nanotubes” has been expected to contribute some roles in the field of nanotechnology. Modification of the surfaces of the nano tubes for studying several disciplines is not very common and convenient to every where. So the knowledge of the research is definetly applicable and will add some crucial points in the days to come.