Theoretical study of the adsorption of 2-nitro-1-propanol on smectite surface models

Because their layered structure, high specific surface and natural occurrence, specifically in soils, clay minerals play an important role in the adsorption of pollutants, especially organic compounds. Different nitroorganic compounds are adsorbed in soils coming from pesticides, agrochemical and other different sources. Adsorption of nitrobenzene derivatives on phyllosilicates has been studied by experimental and theoretical techniques. In this work, the adsorption of 2-nitro-1-propanol on Na-smectite surface cluster models was theoretically studied at HF/6-31+G** and B3LYP/6-31+G** levels of theory. Three cluster models were designed in order to elucidate the different contributions of the mineral part of the system to the adsorption process, finding a very localized interaction between the adsorbate and the surface, mainly through an interaction between the NO2 group with the surface cation and an hydrogen bond of the hydroxyl group with the basal oxygens; in addition, some weak CH…O and O…O interactions are also found. The adsorption energies have been estimated between −10 and −14 kcal/mol, depending on the model and level of theory. With Atoms In Molecules analysis the bonding structure of the Na+ with the surface was characterized, finding the surface cation show four or five (3, −1) bond critical points with the oxygens of the surface (four for the interaction with the basal oxygens and one with the oxygen of the hydroxyl in the ditrigonal cavity). Adsorption process is able to produce some modifications in the geometry of the surface, increasing the rotation of the siloxane tetrahedral units, together with the vertical distance Na-surface, and the weakening of the Na⋯Ohydroxyl interaction. All these facts suggest that the deformation of the surface is in part related with the bonding cation-surface.