Quantum-chemical model evaluations of thermodynamics and kinetics of oxygen atom additions to narrow nanotubes

This paper reports a computational study of oxygen additions to narrow nanotubes, a problem frequently studied with fullerenes. In fact, fullerene oxides were the first observed fullerene derivatives, and they have naturally attracted the attention of both experiment and theory. C ₆₀O had represented a long-standing case of experiment-theory disagreement, and there has been a similar problem with C ₆₀O ₂. The disagreement has been explained by kinetic rather than thermodynamic control. In this paper a similar computational approach is applied to narrow nanotubes. Recently, very narrow nanotubes have been observed with a diameter of 5 Å and even with a diameter of 4 Å. It has been supposed that the narrow nanotubes are closed by fragments of small fullerenes like C ₃₆ or C ₂₀. In this report we perform calculations for oxygen additions to such model nanotubes capped by fragments of D _2d C ₃₆, D _4d C ₃₂, and I _h C ₂₀ fullerenic cages (though the computational models have to be rather short). The three models have the following carbon contents: C ₈₄, C ₈₀, and C ₈₀. Both thermodynamic enthalpy changes and kinetic activation barriers for oxygen addition to six selected bonds are computed and analyzed. The lowest isomer (thermodynamically the most stable) is never of the 6/6 type, that is, the enthalpically favored structures are produced by oxygen additions to the nanotube tips. Interestingly enough, the lowest energy isomer has, for the D _2d C ₃₆ and D _4d C ₃₂ cases, the lowest kinetic activation barrier as well.