Solvation Enhances the Distinction between Carboxylated Armchair and Zigzag Single-Wall Carbon Nanotubes (SWNT-COOH)

The effect of various solvents on the structures and properties of carboxylated SWNTs has been explored using the same level different basis set approach (SLDB), where B3LYP functional of density functional theory (DFT) was applied. Armchair (4,4) and zigzag (8,0) and (9,0) tubes were considered as the testbed. In order to simulate varying concentration of −COOH groups, one to five acids groups were placed at one end of these tubes. These samples were placed in different solvents (namely, CS₂, THF, and water) with varying polarity and results were compared with gas-phase properties. Similar to the gas-phase, zigzag tubes also exhibit both regular (r-COOH, ν­(CO) above 1700 cm^–1) and low-frequency (lf-COOH, ν­(CO) below 1700 cm^–1) acid groups. Characteristics of the r-COOH group are not affected much in solvents, but lf-COOH of the zigzag tube is the one that makes these tubes distinguishable from its armchair cousin. Stability and charge distribution of SWNT-COOH strongly depend on the number of acid groups in different solvents which may help controlling further functionalization. Vibrational analyses reveal certain features in the 1400–1600 cm^–1 range that are characteristic of lf-COOH and in different solvents, which may help in the assignment of experimental spectra of oxidized SWNT in solvents.