Ab initio Modeling of methanol interaction with single-walled carbon nanotubes

Equilibrium geometries and binding energies with respect to the interaction of a methanol molecule (MeOH) with single-walled carbon nanotubes (SWCNTs) of various diameter are calculated by means of density-functional theory including an empirical energy correction for dispersion (DFT-D). This theory is validated by comparing DFT-D results for the model systems benzene-MeOH and coronene-MeOH with corresponding results from high-level wave function-based theory. DFT-D potential energy surface (PES) scans along the intermolecular distance using different functionals are compared with spin-component scaled second-order Moller-Plesset perturbation theory (SCS-MP2) energies and reveal a consistent mutual agreement between the two approaches. Hydrogen-terminated tube sections are used to represent the armchair (4,4), (5,5), (6,6), (8,8), and (10,10) SWCNTs. Similar binding energies are found for the armchair (4,4) and the zigzag (7,0) tubes with similar diameter, but the methanol-SWCNT distance is strongly dependent on the tube type (armchair or zigzag). The interaction energy is found to be diameter-dependent ranging from -15.0 KJ mol(-1) for the smallest diameter tube to -20.0 KJ mol(-1) for graphene, which represents the limit of a tube with infinite diameter. Calculating the binding energies for differently curved coronene-MeOH models that could be used in a QM/MM approach shows that >90% of the methanol-tube interaction can be captured with this small model. Furthermore, the electronic properties of zigzag tube sections are examined. Because of the tertmination of the dangling bonds with hydrogen atoms, orbitals arise that are localized at the tube ends and that are not present in the infinitely long SWCNTs. Nevertheless, an approach is presented to extrapolate the band gaps of SWCNTs from gaps calculated within the cluster approach using tube sections of increasing length.