The sidewall additions of diazomethane to (n, n), n = 3-10 armchair single-walled carbon nanotubes (SWCNTs) on two different orientations of C-C bonds have been studied using the ONIOM(B3LYP/6-31G(d):PM3) approach. The binding energies of SWCNTs complexes with CH2N2, CH2 and their transition-,state structures were computed at the B3LY-P/6-31G(d) level. The effects of diameters of armchair SWCNTs on their binding energies were studied. Relative reactivities of all the SWCNTs and their complexes based on their frontier orbital energies gaps are reported. (c) 2007 Elsevier B.V. All rights reserved.