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

We have examined the effects of substituents on the computed electrostatic potentials V-S(r) and average local ionization energies (I) over bar (S)(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 (I) over bar (S)(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; (I) over bar (S)(r) correspondingly goes from higher to lower values. Replacing OH by another resonance-donor, NH-(2), 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 (I) over bar (S)(r). The first hyperpolarizability of this system was found to be nine times that of para-nitroaniline, suggesting possible nonlinear optical applications.