Adduct 9a Adduct 9b
Figure 11. Four energy curves separated from the complicated multidimensional energy hypersurface of the (4,4) SWNT when O2 is approaching from the left side of the diagram. Solid lines end up in the adduct 9a with dashed lines for 9b. Blue lines represent triplet states and green lines singlet states.
The curve connecting the two ground states starts from the separate XSWNT and 3O2, presents a barrier where the total symmetry changes to singlet (crossing point), and falls down to the adduct X(SWNT + O2). This barrier is approximately 15 kcal/mol.
According to this figure there are two different possibilities for O2 adsorption to the SWNT: In the first the O2 is excited to its singlet electronic configuration and then it automatically reacts exothermically with the tube. In the second case we start with O2 in its triplet ground state and energy of ~15 kcal/mol has to be offered to the system in order to overpass the energy barrier and react with the tube.
For the bridge 2 (9b) adsorption a picture completely analogous to bridge 1 (9a) exists and is presented in Figure 11 (dashed lines). The only difference is the relative energy values. Finally, since all these cases (singlet and triplet, bridge 1 and 2) are coexisting, a combination of all these four energy curves is needed in order to have the complete adsorption picture (Fig. 11).
In real—and not ideal—systems, when the tubes possess defects and the temperature is not absolute zero—as in quantum mechanical calculations—the triplet O2 adsorption is possible. The barrier is relatively small and the defects of the tube can lower it even more. Also the temperature "movements" can help the procedure.
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