AIM and NBO analyses on hydrogen bonds formation in sugar-based surfactants (α/β-d-mannose and n-octyl-α/β-d-mannopyranoside): a density functional theory study

Density functional theory calculations on α/β-d-mannose (α/β-d-Man) and the corresponding glycosides of n-octyl-α/β-d-mannopyranoside (C₈O-α/β-d-Man) were carried out for geometrical optimisation and stability predictions at the B3LYP/6-31G level of theory. These compounds are related anomerically, since they differ by only the orientation of the hydroxyl group at the C1 position. The aim of this study is to investigate the effect of the hydroxyl group's orientations (axial vs. equatorial) at the C1 position on the intra-molecular interactions and the conformational stability of these isomers. The structural parameters of X-H???Y intra-molecular hydrogen bonds were analysed, while the nature of these bonds was considered using the atoms-in-molecules (AIM) approach. Natural bond orbital (NBO) analysis was used to determine bond orders and the effective non-bonding interactions. We have also reported thermodynamic properties and the electronic properties, such as the highest occupied molecular orbital, lowest unoccupied molecular orbital, ionisation energy, electron affinity, electronic chemical potential, chemical hardness, softness and electrophilicity index in the gas phase for all compounds. These results showed that while α-anomers possess only one intra-molecular hydrogen bond, β-anomers possess two intra-molecular hydrogen bonds, which further confirms the anomalous stability of the latter in the self-assembly phenomena.