Stereochemical effect of covalent chemistry on the electronic structure and properties of the carbon allotropes and graphene surfaces

We consider the covalent chemistry of the carbon allotropes with an emphasis on the newest member graphene. We focus on the effect of such chemistry on the geometric and electronic structure of the functionalized materials and the way in which the conjugation is modified by such processes. We conclude that there are two limiting cases: (a) Conventional addition chemistry leading to the formation of sigma-bonds to the graphitic surface in which there is full rehybridization of the derivatized carbon atoms from sp(2) to sp(3); thus these carbon atoms are effectively removed from conjugation and from the electronic band structure (referred to as destructive rehybridization). (b) Covalent chemisorption with formation of an organometallic hexahapto-metal bond that largely preserves the graphitic band structure (constructive rehybridization) and accompanies the formation of bis-hexahapto-metal bonds such as those in (eta(6)-SWNT)Cr(eta(6)-SWNT) which serve to interconnect adjacent graphitic surfaces and significantly reduces the internanotube junction resistance in SWNT networks. The formation of eta(2) dihapto bonds represent an intermediate case of covalent chemistry and is known to be important in carbon nanotubes and particularly the fullerenes but this situation has been treated in detail in previous publications. (C) 2015 Elsevier B.V. All rights reserved.