3.4. NBO Analysis
Natural Bond Orbital (NBO) analysis is useful tools to assess the intramolecular interactions (second-order perturbation energies, E2) with high precision. The most important second-order perturbation energies (in kcal/mol) for interaction of CO molecule with CNT and DNTs extracted from NBO analysis are shown in Table 4. The values demonstrate that in the complexes of CNT and DNTs the σ- orbital of C-C, N-C, B-C and C-O bonds and the lone pair orbitals of O, S, N and C act as an electron donor, and the antibonding orbitals of CO molecule and X-C bond (X= S, N, B and C) act as an electron acceptor. In the most significant acceptor-donor interactions, the electrons in complexes of NCNTs were transferred from the σ- orbital of the N-C bond (σ N-C) or the lone pair orbital of N (LP N) as an electron donor to the antibonding orbitals of CO molecule (σ* C-O) as an electron acceptor with the energy in the range of 6.08-8.78 kcal/mol. In addition, the electrons could be transferred from the lone pair orbital of O (LP O) and the lone pair orbital of CO molecule (LP C(CO)) to the antibonding orbitals of N-C bond (σ* N-C) with the energy of 7.05 and 0.75 kcal/mol respectively. In the complexes of SCNTs, the electrons were partially transferred from the lone pair orbital of S (LP S) to the antibonding orbitals of CO molecule (σ* C-O) with the energy in the range of 0.48-1.61 kcal/mol. Moreover, the electrons were donated from the lone pair orbital of CO molecules (LP C(CO)) to antibonding orbitals of S-C bond (σ* S-C) with the energy in the range of 0.17-0.22 kcal/mol. In the complexes of BCNTs, the electrons were donated from the σ- orbital of the B-C bond (σ B-C) to the antibonding orbitals of CO molecules (σ* C-O) with the energy in the range of 4.51-17.11 kcal/mol.