Bond transition dependent defect in the structure of molecular materials
A Stone–Wales defect is a crystallographic defect that involves the change of connectivity of two π-bondedcarbon atoms, leading to their rotation by 90° with respect to the midpoint of their bond.[1] The reaction commonly involves conversion between a naphthalene-like structure into a fulvalene-like structure, that is, two rings that share an edge vs two separate rings that have vertices bonded to each other.
The reaction occurs on carbon nanotubes, graphene, and similar carbon frameworks, where the four adjacent six-membered rings of a pyrene-like region are changed into two five-membered rings and two seven-membered rings when the bond uniting two of the adjacent rings rotates. In these materials, the rearrangement is thought to have important implications for the thermal,[3] chemical, electrical, and mechanical properties.[4] The rearrangement is an example of a pyracyclene rearrangement.
The activation energy for the simple atomic motion that gives the bond-rotation apparent in a Stone–Wales defects is fairly high—a barrier of several electronvolts.[4][9] but various processes can create the defects at substantially lower energies than might be expected.[8]
The rearrangement creates a structure with less resonance stabilization among the sp2 atoms involved and higher strain energy in the local structure. As a result, the defect creates a region with greater chemical reactivity, including acting as a nucleophile[citation needed] and creating a preferred site for binding to hydrogen atoms.[10] The high affinity of these defects for hydrogen, coupled with the large surface area of the bulk material, might make these defects an important aspect in the use of carbon nanomaterials for hydrogen storage.[10] Incorporation of defects along a carbon-nanotube network can program a carbon-nanotube circuit to enhance the conductance along a specific path.[citation needed] In this scenario, the defects lead to a charge delocalization, which redirects an incoming electron down a given trajectory.
References
^Brayfindley, Evangelina; Irace, Erica E.; Castro, Claire; Karney, William L. (2015). "Stone–Wales Rearrangements in Polycyclic Aromatic Hydrocarbons: A Computational Study". J. Org. Chem. 80 (8): 3825–3831. doi:10.1021/acs.joc.5b00066. PMID25843555.
^Fowler, Patrick W.; Baker, Jon (1992). "Energetics of the Stone–Wales pyracylene transformation". J. Chem. Soc., Perkin Trans. 2 (10): 1665–1666. doi:10.1039/P29920001665.
^ abLetardi, Sara; Celino, Massimo; Cleri, Fabrizio; Rosato, Vittorio (2002). "Atomic hydrogen adsorption on a Stone–Wales defect in graphite". Surface Science. 496 (1–2): 33–38. Bibcode:2002SurSc.496...33L. doi:10.1016/S0039-6028(01)01437-6.