Theoretical study of structure and Raman spectra for models of carbon nanotubes in their pristine and oxidized forms

Changes upon oxidization of carbon nanotubes (CNTs) in structure and Raman spectra are investigated using geometry optimizations and frequency calculations with density functional theory and ONIOM(MO:MO) methods. Macrocycles consisting of six and 12 fused C₆ hexagon units, C₂₄H₁₂ and C₄₈H₂₄ or [6]- and [12]- cyclacene, respectively, were chosen as model systems for zigzag single wall carbon nanotubes. A vertically extended belt model with three fused [12]-cyclacenes, C₉₆H₂₄, was also studied as well. The oxidation was modeled by 1,2- or 1,4-cycloaddition poducts of two oxygen atoms at the furthest ends of each macrocycle; 1,4-adducts are found to be much more stable than 1,2-adducts and are possibly the actual oxidation products. It is shown for all these cases that, when oxygen atoms react, the macrocycle undergoes large deformation from the totally symmetric cyclic structure toward an oval shape. This is due to the introduction of sp³ centers on the ring and subsequent relaxation of the adjacent C₆ hexagon units. Upon oxidation calculated Raman spectra show large reduction in peak intensities, which can be attributed to the loss of cylindrical symmetry due to structural deformation. The study provides a novel explanation for the experimental observation that Raman spectra of individual CNTs are highly sensitive to oxidization.