Combined DFT with NBO and QTAIM studies on the hydrogen bonds in (CH3OH) n (n=2-8) clusters

The (CH3OH) (n) (n = 2-8) clusters formed via hydrogen bond (H-bonds) interactions have been studied systemically by density functional theory (DFT). The relevant geometries, energies, and IR characteristics of the intermolecular OH center dot center dot center dot O H-bonds have been investigated. The quantum theory of atoms in molecule (QTAIM) and natural bond orbital (NBO) analysis have also been applied to understand the nature of the hydrogen bonding interactions in clusters. The results show that both the strength of H-bonds and the deformation are important factors for the stability of (CH3OH) (n) clusters. The weakest H-bond was found in the dimer. The strengths of H-bonds in clusters increase from n = 2 to 8, moreover, the strengths of H-bonds in (CH3OH) (n) (n = 4-8) clusters are remarkably stronger than those in (CH3OH) (n) (n = 2, 3) clusters. The small differences of the strengths of H-bonds among (CH3OH) (n) (n = 6-8) clusters indicate that a partial covalent character is attributed to the H-bonds in these clusters. The linear relationships between the electron density of BCP (rho(b)) and the H center dot center dot center dot O bond length of H-bonds as well as the second-perturbation energies E(2) have also been investigated and used to study the nature of H-bonds, respectively.