B

Figure 9. The two stable isomers of the atomic oxygen chemisorptiony in a (4,4) SWNT forming epoxides. In the first (A) the C-C bond opens.

94.6 kcal/mol while that for the 9b is 62.6 kcal/mol. From the binding energies it is clearly shown that both additions are exothermic and the high energy gain indicates a chemisorp-tion procedure.

Comparing these two different bridge positions of the atomic oxygen on carbon nanotubes we observe an energy difference of 33.8 kcal/mol. In order to explain the stabilization of bridge 9a over bridge 9b we have to look closer at the geometric features of the two isomers. In the 9a isomer the oxygen atom is actually breaking the C-C bond (2.062 A) while the O-C bond is 1.397 A. In the 9b case the C-C bond opens slightly, 1.522 from 1.415 A of a bare tube, and the O-C bond is 1.450 A. In both cases in the chemisorption procedure we have the addition of one C-O bond to each of two neighboring sp2-hybridized carbon atoms of the tube. This lead to overcoordinated C atoms that either have to pass to sp3 hybridization or to break one bond. In the 9a isomer the breaking of the C-C bond is geometrically possible and energetically favorable. The cyclical shaped tube turned locally to an oval-shaped distorted one, and this allowed the C atoms that are connected to the O to keep the sp2 hybridization (Fig. 9a). The O is coming to the same plane with both the hexagonal rings that it is connected and the angle of these two hexagonal rings with respect to the oxygen is 95°.

In the bridge 9b isomer distortion of the tube parallel to the tube axis is not possible and that is why the C atoms change to unfavorable sp3 hybridization with weaker bonds (Fig. 9b). Thus the isomer 9b is energetically higher from the 9a by 33.8 kcal/mol. The same interesting phenomenon was observed also in the case of H adsorption in SWNTs (Section 5.3.2).

6.3. Molecular Oxygen Interaction with Carbon Nanotubes

Since the oxygen in the atmosphere appears in molecular form, it is of great importance to examine the molecular oxygen adsorption to carbon nanotubes and to analyze the nature of this procedure [48]. Using our ab initio method we perform geometry optimization of all the possible adducts of O2 molecules and (4,4) SWNTs. We tried both parallel and vertical orientations of the O2 molecule to the tube axis. Two different pathways of oxygen approach were tested (one oxygen atom attaching the tube walls; both oxygen atoms attaching the tube walls). Finally all these were combined with binding positions on top of a carbon atom, in the hollow position of C hexagons, and bridging C-C bonds.

After the geometries were optimized we found two stable adducts presented in Figure 10. In both of these local minima the oxygen molecule is bridging C-C bonds—the two different kinds of bonds that exist in the tube—making square rings. In the first case the bridged C-C bond is vertical to the tube axis (Fig. 10a) and in the second it is semiparallel (Fig. 10b). All other possibilities of O2 approaching the tube walls were either unstable or after the geometry optimization end up in the bridge positions (Fig. 10a or b).

In the first bridge isomer the C-C bond opens from 1.40 to 1.68 A, the O-O is 1.51 A, and the two C-O are 1.45 A while in the second case the C-C bond opens from 1.41 to 1.55 A, the O-O is 1.51 A, and the two C-O are 1.48 A.

In the second isomer the formed square ring is not totally planar but slightly twisted (dihedral angle 19°). The binding energies of these two species are +13 and +18 kcal/mol respectively and the addition is endothermic in both cases. The electron density of the bonding area of isomer 10a is presented in Figure 10c.

From our fully optimized calculations, without any symmetry constrains, it is clear that both bridge adducts are local minima. Nevertheless for the discussion of the binding energy we have to take into account the electronic configuration of the reactants together with the one of the product cluster (SWNT + O2). Since for both adducts (bridge 1 & 2) the ground state configuration is singlet, we consider first the singlet reaction.

For bridge 1 (9a) the addition *SWNT + *O2 ^ 1(SWNT + O2) is exothermic by 25 kcal/mol. Despite this the ground state of the O2 is triplet. The analogous triplet addition *SWNT + 3O2 ^ 3(SWNT+O2) is endothermic by 29 kcal/mol. These two energy curves are separated from the complicated multidimensional energy hypersurface of the system and presented in Figure 11 (solid lines). Since they are of different symmetry they can cross each other (diabatic curves) and some more reaction possibilities arise. This also explains the controversial results existing in the literature.

Figure 10. The two stable isomers of the molecular oxygen adsorption in a (4,4) SWNT forming orthogonal rings. The electron density of the bonding area of isomer 10a is presented in (C).

Figure 10. The two stable isomers of the molecular oxygen adsorption in a (4,4) SWNT forming orthogonal rings. The electron density of the bonding area of isomer 10a is presented in (C).

aE(kcal/mol)

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