Methods
All calculations such as geometry optimizations and solvent effects have performed using GAMESS program.41,42 For these calculations, density functional theory (DFT) with ONIOM (ωB97X-D/aug-cc-pvdz and ωB97X-D/3-21G) level of theories have been employed.43 The optimizations of all structures were performed without symmetry constraint or pre-defined conformational structures. The absence of imaginary frequency verified that a structure was a true least at its respective level of theory. The results of frequency calculations were used after applying appropriate scaling factor.44 The effects of solvent were studied through SCRF keyword using Tomasi’s polarized continuum (PCM) model.45 Three solvents, two polar (water with ε=78.355 and acetone with ε=20.493) and one nonpolar (cyclohexane with ε=2.017), were selected to study of the solvent effect on the adsorption energies. These values in both gas and solvent phase obtained from the Gibbs free energies (ΔGad) that were calculated for CNT and DNTs according to equation 1.
∆Gad=GCNT-CO − (GCNT+GCO) (1)
NBO calculations of all structures were performed using the NBO.46 Also, QTAIM analyses were employed using the AIMAll 15.09.27 program.47 Density of state (DOS) plots were obtained from the GaussSumm 2.2.1 program.48