An unusual feature of end-substituted model carbon (6,0) nanotubes

Abstract

We have examined the effects of substituents on the computed electrostatic potentials V _S (r) and average local ionization energies ${\bar{\text I}}_{\text{S}} ({\mathbf{r}})$ on the surfaces of model carbon nanotubes of the types (5,5), (6,1) and (6,0). For the (5,5) and the (6,1), the effects upon both V _S (r) and ${\bar{\text I}}_{\text{S}} ({\mathbf{r}})$ of substituting a hydroxyl group at one end are primarily localized to that part of the system. For the (6,0) tube, however, a remarkable change is observed over its entire length, with V _S (r) showing a marked gradation from strongly positive at the substituted end to strongly negative at the other; ${\bar{\text I}}_{\text{S}} ({\mathbf{r}})$ correspondingly goes from higher to lower values. Replacing OH by another resonance- donor, NH₂, produces similar results in the (6,0) system, while the resonance withdrawing NO₂ does the opposite, but in equally striking fashion. We explain these observations by noting that the arrangement of the C−C bonds in the (6,0) tube facilitates charge delocalization over the full length and entire surface of the tube. Substituting NH₂ and NO₂ at opposite ends of the (6,0) tube greatly strengthens the gradations in both V _S (r) and ${\bar{\text I}}_{\text{S}} ({\mathbf{r}}).$ The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications. Figure HF/STO-5G electrostatic potential on outer surface of open (6,0) C₇₂H₁₀NH₂NO₂. The nitro group is at the right end of the tube, the amino group at the left. In eV: purple is less than 14, blue is between 14 and 15, green is between 15 and 16.5, yellow is between 16.5 and 17.5, and red is more than 17.5.