Theoretical studies of amidation reaction at carbon nanotube tips by means of the ONIOM technique: expanding the higher level.
In order to study how the choice of model chemistry can affect the results for amidation reaction of carboxylated SWNTs with methylamine, we attempted to expand the higher ONIOM level to six fused aromatic rings at the SWNT edge. The higher theoretical level was described with HF/STO-3G quantum mechanics, and the lower level with UFF universal force field. Introducing additional carboxylic, hydroxyl and quinone groups at the higher ONIOM level gives rise to the formation of new hydrogen bonds. The latter sometimes drastically change the overall geometry of reaction complexes, transition states and products, as well as their energies. In two of the three sets of SWNT models employed in the present study, the energetic preference for amide formation on armchair SWNT tips versus zigzag SWNT tips can be seen, although in all cases, the reaction turns endothermic. For the third set of models, including three hydroxyl and three carboxylic groups at the higher level, the amidation reaction is much more unfavorable for armchair SWNT.