Deformation Density Analysis of Trapped Hydrogen Molecule in Carbon Nanotube

Since the discovery of carbon nanotubes (CNTs) by Iijima in early 1990, much attention has been paid to the properties of this nanometer scale materials. The advantages of CNTs, such as, their small size, low power, low weight, and high performance have led to an increase in the number of applications. One of the most important applications of CNTs, which we will focus on, is energy storage, for example: hydrogen storage, fuel cells, and lithium batteries. Single-walled nanotubes (SWNTs) are especially attractive for hydrogen gas storage applications. Conceptually, both the inner and outer surface of open-ended SWNTs could be accessible for gas storage applications.The virtual pressure imposing upon the orbitals to achieve mutual orthogonality is generally called kinetic energy pressure (K.E.P) [1]. In a typical closed-shell intermolecular interaction, the molecular orbitals of each fragment distort from their original shape in order to provide a proper nodal behavior which further gives rise to the mutual orthogonality. This phenomenon is an indirect result of Pauli repulsion which states that the electronic wave function must be antisymmetric with respect to the interchange of electrons.The K.E.P in intramolecular interactions is known as steric effect. The steric effect plays a central role in conformational analysis of chemical systems with the help of localized orbitals. Weinhold and co-workers [2,3] have studied the steric effect in several chemical systems by means of natural bond orbitals as localized orbitals. They have also performed energetic decomposition in order to extract the energy component due to steric effect [2,3]. In spite of all efforts in energy quantification of K.E.P, the study of this quantity based on the magnitude of displaced charges has not been carried out yet.The excellent molecular hydrogen storage capacity of SWNTs, reported for the first time by Dillon et. al [4], was a good motivation to study the K.E.P in intermolecular interactions between hydrogen molecule and carbon nanotube.