Cycloadditions to Control Bond Breaking in Naphthalenes, Fullerenes, and Carbon Nanotubes:  A First-Principles Study

Covalent functionalizations represent a very promising avenue to engineer or manipulate carbon nanotubes. However, in metallic tubes the electrical conductance can drop by several orders of magnitude following functionalization, due to sp³ rehybridization of the sidewall carbons that strongly disrupts the conjugated π-network. First-principles calculations have predicted that some divalent functional groups, carbenes or nitrenes, can instead recover the original sp² hybridization and perfect metallic conductance of the pristine tubes. In these cycloaddition reactions, the extra bond added by the functional group with each of the bridgehead carbons is compensated by a breaking of the sidewall bond between them, restoring in the process the original sp² environment. We characterize this bond-breaking chemistry with extensive first-principles calculations and highlight its sensitivity to the orientation of the π-electron system of the chosen addend. Using dinitrocarbene as a model case, we show that the bridgehead carbon atoms can reversibly rehybridize from sp² to sp³ in response to the π orientation of the addends. These results suggest a novel route to modulating the electronic properties of carbon nanotubes that is based on orbital rehybridization and that can be directed with optical or electrochemical means.