Effects of different caps on model nanotube surface properties

Earlier work has shown that the most negative electrostatic potentials and the lowest local ionization energies on the surfaces of closed model nanotubes are on the outside of their caps. Conversely, the most positive electrostatic potentials and highest local ionization energies are on the insides. These regions should accordingly display enhanced reactivity, in both noncovalent and covalent interactions, the latter especially toward electrophiles on the outside and nucleophiles inside. We have now investigated in detail, for caps of three different structures, how these properties vary from one to another. Several types and compositions of tubes were considered: (5, 5) and (6, 0) carbon, (5, 5) and (6, 0) BxNx, and (6, 0) C2xBxNx. We found that the structural features of the cap can have quite a significant effect upon the surface potential and local ionization energies; this is most pronounced for the carbon systems. An interesting observation was the remarkable change in the electrostatic potential of (6, 0) C52B26N26 when a boron atom on one cap and a nitrogen on the other were replaced by carbons, giving stoichiometries of C6B4 and C6N4 at the tips. This produced a strong potential gradient along the entire length of the tube, from positive at one end to negative at the other.