Armchair sites

Two basic issues can be distinguished here (i) the physico-chemical nature of zigzag and armchair sites at the edges of fused-benzene-ring structures, including their differences and similarities with respect to polycyclic aromatic hydrocarbon (PAH) molecules, and (ii) the reconstruction of these edges to potentially more stable structures. 

Figure 3.3 Mechanism for methane formation on (a) zigzag and (b) armchair sites. (Adapted from ref. 57.)...

FIGU RE 13.2 Reaction of molecular oxygen with graphite layer, (a) Physisorption of oxygen on the basal plane, (b) Chemisorption of oxygen at defective sites vacancies (site A), armchair (site B), and zigzag edges (site C). 

The basic building block of carbon is a planar sheet of carbon atoms arranged in a honeycomb structure (called graphene or basal plane). These carbon sheets are stacked in an ordered or disordered manner to form crystallites. Each crystallite has two different edge sites (Fig. 2) the armchair and zig-zag sites. In graphite and other ordered carbons, these edge sites are actually the crystallite planes, while in disordered soft and hard carbons these sites, as a result of turbostratic disorder, may not... 

The issue of the chemical nature of graphene edges, and thus of (re)active sites, was addressed head-on by Radovic and Bockrath [29]. Density functional theory was used to minimize problems that truly ab initio approaches such as Hartree-Fock are known to have, but careful comparison with experimental results in several seemingly unrelated fields was also performed. The key argument of relevance here is that, rather than being H-terminated, the sites that are most relevant for the chemical (re)activity of sp2 hybridized carbon materials are of carbene- and carbyne-type, as illustrated below for the zigzag (a) and armchair edges (b). 

Fig. 14.8 B3LYP/6-3lG(d) optimized structures of (3, 3), (4,4), (5,5) and (6,6) armchair SWNTs of 9 and 15 carbon layers. The carbon atom sites for H-atom chemisorption are labeled. The structures are reproduced with permission from [21], copyright 2007 American Chemical Society...

Stability of, for example, (10,10)-nanotxibes, but kinetic reasons contribute as well to the effect. In achiral nanotubes, and especially in those of the armchair type, the replacement of metal atoms by carbon is much easier due to the orientation of the lattice structure (see below). What is more, several SWNTs at a time wiU emerge from very reactive sites. With the prevaiUng temperature being constant for all tubes nucleating in these zones, their respective diameters wiU also be more or less the same. Consequently, they may form a symmetric packing, which is why the most stable bundles are observed in these cases. 

Figure 3.2 Three states of graphite edge sites if the third valence is not nsed (a) remain as before (zigzag), (b) pair with a neighboring edge atom to form a partially triple bond (armchair), or (c) revert to a carbenoid. (Adapted from ref. 49.)...

An attempt was made by Kyotani and Tomita [88] to analyze the reaction of carbon with NO or N2O using an ab initio method. This method allows simulation of the chemisorption process of these gas molecules on zigzag and armchair edge sites of carbon by calculating the thermodynamics and the orbital bond... 

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