Demonstration of Covalent Sidewall Functionalization of Single Wall Carbon Nanotubes by NMR Spectroscopy: Side Chain Length Dependence on the Observation of the Sidewall sp³ Carbons

Carboxylic acid-functionalized single walled carbon nanotubes (SWNTs) prepared via the reaction of an amino acid, NH₂(CH₂)_nCO₂H where n = 1 (glycine, GLY), 5 (6-aminohexanoic acid, AHA), 10 (11-aminoundecanoic acid, AUDA), with fluorinated single walled carbon nanotubes (F-SWNTs) have been characterized by MAS ¹³C NMR spectroscopy. The ease of observing the aliphatic CH₂ groups and the resolution of the signal are dependent on the length of the amino acid's aliphatic chain. We have proposed that where substituent chains are short (making NMR data collection difficult) chemical modification to extend the chain length should alleviate analysis problems. In this regard, we have investigated the esterification of the carboxylic acid termini. The amino acid-functionalized SWNTs were esterified with an appropriate alcohol to ensure parity of the overall substituent length, i.e., GLY-SWNT (C₁) + 1-dodecanol (C₁₂) = DOD-GLY-SWNT (1), AHA-SWNT (C₅) + 1-octanol (C₈) = OCT-AHA-SWNT (2), and AUDA-SWNT (C₁₀) + 1-propanol (C₃) = PRO-AUDA-SWNT (3). The ¹³C NMR shift for the sp³ nitrogen-substituted carbon atoms of the SWNT sidewall is observed at δ ≈ 75 ppm. Increasing the length of SWNT sidewall functional groups enhances the ability to observe the sidewall sp³ carbon. The methylene carbon signal intensity is less attenuated in the dipolar dephasing spectrum of the ester-functionalized SWNTs than their associated amino acid derivatives, suggesting more motional freedom of the side chain in the solid state. The confirmation of the dipolar dephasing spectral effects was assisted by the characterization of the ester of AUDA-SWNT with 1, 3-propanediol: PPD-AUDA-SWNT (4).