ABSTRACT
Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Increasing interest has been focused on the acetylene-based polymer chemistry relevant to cyclic polyynes — the topic of this chapter [1]. Cyclic polyynes are unique members in the family of sp-carbon allotropes. Like linear polyynes, they are composed of unsaturated carbon atoms covalently
bonded with two neighboring atoms. That they have two-dimensional ringtype structures, thus having no terminable end, makes them constitute another class of molecular polyynes. The ring-shaped all-carbon molecules Cn are also referred to as cyclic carbon clusters or cyclo[n]carbons in relation to cluster science or organic chemistry [1-9]. Concerning the electronic structure of the polyyne ring, two orthogonal p-electron systems are not identical due to the strained bent arrangement of bonds. Incorporating trivalent sp2 or tetravalent sp3 carbons as atomic junctions, the polyyne chains can extend large branching structures forming two-or threedimensional polygons. With such characteristic features in mind, researchers have thought that cyclic polyynes may serve as new building blocks of hybrid sp-sp2-sp3 carbon networks or carbynoid structures, and in some cases such compounds have been successfully constructed and characterized in organic chemistry [1b-d]. More generally, one may realize that the strained bent carbon chains are found commonly within the polymeric systems that form during the aggregation of linear polyynes.
