Pesticide interaction with environmentally important cations: A theoretical study of atrazine in interaction with two Ca2+ cations

The atrazine, 2-chloro-N-4-ethyl-N-6-isopropyl-1,3,5-triazine-4,6-diamine, is a pesticide molecule with an herbicide effect. In order to evaluate the behavior of pesticides in soils, we intend to model the soil surface by its mineral part. We then have to complex the atrazine molecule on a surface substituted by two Ca2+ cations. This atrazine-(Ca2+)(2) study is the starting point of a challenging work on atrazine in soils. Lots of different isomers has been investigated on the potential energy surface for atrazine-(Ca2+)(2) complexes at the B3LYP/6-31G* level. The order of stability of these different structures has been rationalized analyzing the interaction with both alkyl side chains and the pyramidalization of the nitrogen bearing those chains. Moreover for a better understanding of the interaction, a Reduced Variational Space Self Consistent Field (RVS SCF) interaction energy decomposition has been performed at the HF/6-31G*//B3LYP/6-31G* level for four complexes, three of them have the lowest relative energies of all calculated isomers. For these isomers, the percentages of electrostatic, polarization and charge transfer contributions are of the same order of magnitude as in atrazine-Ca2+ isomers. However, the Ca2+-Ca2+ energy is important to consider and leads to repulsive complexation energy. In order to go further on the analysis, thermodynamic values (at 298.15 K and 1 atm) have been computed for atrazine-(Ca2+)(2) complexes at B3LYP level with two different bases: 6-31G* and 6-311+G(2d,2p). The complexation enthalpy is of the same order of magnitude as complexation energy at both levels. The complexation free energy order is slightly different and can be explained based on vibrational entropic considerations. Finally, the interaction of atrazine-(Ca2+)(2) complexes with clay surfaces should lead to attractive complexation energy due to large dispersive effects. (C) 2013 Published by Elsevier B.V.