Characterization of Hydrogen Bonds in the End-Functionalized Single-Wall Carbon Nanotubes: A DFT Study

A systematic computational study is carried out to shed some light on the structure of semi-conducting armchair single-wall carbon nanotubes (n,n) SWCNTs, n = 4, 5 and 6, functionalized at the end with carboxyl (-COOH) and amide (-CONH2) from the viewpoint of characterizing the intramolecular hydrogen bondings at the B3LYP/6-31++G(d, p) level. Geometry parameters display different types of intramolecular hydrogen bonding possibilities in the considered functionalized SWCNTs. All of the hydrogen bondings are confirmed by natural bonding orbitals (NBO) analysis as well as nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) parameters. Based on NBO analysis, the calculated n(0) -> sigma(CH)* delocalization energies E(2), 1.15 kcal/mol to 7.04 kcal/mol, are in direct relation with the hydrogen bonding strengths. Differences in the chemical shielding principal components of C-13 and O-17 nuclei correlate well with the strengths of the hydrogen bondings. Participating in stronger hydrogen bondings, a larger SWCNT has a decreasing effect on C-13(-O) and O-17 isotropic chemical shieldings, sigma(iso), consistent with the NBO analysis. The considerable changes of C-13/O-17 sigma(iso) can be interpreted as a result of shielding tensor component orientation. The C-13(-O) and O-17 quadrupole coupling constants C-Q decrease under the effect of hydrogen bonding while asymmetry parameters eta(Q) significantly increase, indicating that O-17 eta(Q) is more sensitive to hydrogen bondings.