Unusual Low-Vibrational C═O Mode of COOH Can Distinguish between Carboxylated Zigzag and Armchair Single-Wall Carbon Nanotubes

Vibrational frequency analysis using density functional theory resolves some ambiguities in interpretation of IR spectra of carboxylated single-wall carbon nanotubes (o-SWNTs). Armchair (n,n) and zigzag (m,0) tubes, with diameter varying between 0.5 and 1.4 nm, were populated with different numbers of COOH groups at one terminus. While armchair-COOH tubes exhibit C═O stretching frequencies in the standard range of 1720–1760 cm–1, zigzag-COOH tubes display a new C═O band around 1650 cm–1 with much higher intensity, in addition to the higher-frequency C═O mode. The COOH groups exhibiting this low-frequency (lf-COOH) are displaced well off the lines of the tube walls, to the outside. This new band is in the same range as one observed experimentally, which has on occasion been attributed to quinone formation. The C═O bond length in the relevant COOH groups is longer by about 0.01 Å than regular C═O bonds. Such low-frequency C═O band is unusual and not common for any stand-alone COOH group and seems characteristic of carboxylated zigzag tubes irrespective of their diameter and the number of acid groups at the terminus. The lower frequency is not a result of H-bonding, nor can it be reproduced by small models such as benzoic acid or extended carbon networks as in graphene-COOH. Its origin is attributed instead to the curvature of the zigzag tubes in addition to their structural arrangement.