Theoretical Insight into Configurational Selectivity of Functionalized Single-Walled Carbon Nanotubes Based on the Clar Sextet Theory

Abstract

Based on Clar’s theory of the aromatic sextet, finite-length models of single-walled carbon nanotubes (SWNTs) have been utilized to study the configurational selectivity of modified SWNTs via covalent functionalization with organic substitutions by density functional theory calculations. After considering near-armchair, near-zigzag, and zigzag SWNTs with distinct chiralities, it is found that the parameter R of semiconducting SWNTs plays a significant role in determining the stability difference of functionalized SWNTs; that is, SWNTs with R = 2 exhibit better configurational selectivity than those with R = 1 when binding the same substituent to the sidewall of SWNTs. Except for the zigzag (7,0) and (10,0) SWNTs with R = 1, the most favorable addition site in the considered SWNTs corresponds to the double bond in their Clar unit cells. The methyl-SWNT radicals with different R values exhibit distinct localizations of the unpaired spin, which is supposed to make a major contribution to the diversity of adducts. The addition sites with the largest spin density lead to the most stable adducts in modifications with small substituents for (6,4), (6,5), (9,1), (8,0), and (11,0) SWNTs, but the correlation tends to disappear for substituents with the increased alkyl chain or bulkiness. Particularly, the bulkiness of alkyl groups is more influential than the alkyl chain length. The degree of bond length change at addition sites is mainly affected by structures of SWNTs, and bulky substituents prefer addition positions in favor of releasing the steric repulsion between substituents. Bond angle changes around addition sites reveal that the extent of sp2 distortion depends on addition sites, and para adducts exhibit much larger sp2 distortion than ortho adducts, resulting in more ideal sp3 carbons. In addition, the increase in the alkyl chain and bulkiness of substituents can weaken the formed C–C bonds between the substituent and SWNT; especially, bulky substituents make the strength of covalent C–C bonds much weaker.