A theoretical and experimental exploration of the mechanism of microwave assisted 1,3-dipolar cycloaddition of pyridinium ylides to single walled carbon nanotubes

Abstract

Cycloaddition reactions have widely been used for surface functionalization of single walled carbon nanotubes (SWNTs). Here, 1,3-dipolar cycloaddition (1,3-DC) of two new pyridinium ylides, generated in-situ via the addition of triethylamine (NEt3) to the Krohnke salts N-(4-methyl sodium benzene sulfonate)-pyridinium bromide and N-(4-nitrobenzyl)-pyridinium bromide, to SWNTs under microwave conditions are assessed both theoretically using PM3 (RHF) type calculations and experimentally. Evidence of covalent surface modification is provided by FTIR, UV-vis-NIR and resonance Raman spectroscopy. Solubility of the modified SWNTs increases when compared to as-received SWNTs. Quantification of surface groups is performed via TGA-MS and XPS. 1,3-DC of pyridinium ylides with smaller HOMO-LUMO energy gap is found to be more selective to large diameter SWNTs. Theoretically predicted smaller energy gaps between HOMOylides and LUMO(8,8) SWNT suggest that the charges are probably transferred from pyridinium ylides to SWNTs indicating HOMOylide-LUMO(8,8) (SWNT) controlled 1,3-DC. Regioselectivity of second ylide addition as addendum to ylide-SWNT adduct is also discussed.