Pesticide interaction with environmentally important cations: A theoretical study of atrazine

The 2-chloro-1,3,5-triazine-4,6-diamine denoted AtraMod thereafter was used as a model of the atrazine pesticide molecule. AtraMod complexes with Na+ and Ca2+, two soil abundant cations, have been studied by means of Hartree-Fock, density functional theory (B3LYP), Moller-Plesset perturbation theory (MP2) and coupled cluster theory (CCSD(T)). The order of stability of the three types of complexes is the same whatever the computational level but depends on the cation. Finally, the complexation energy is strongly overestimated with B3LYP using a large basis set, all the more in the case of Ca2+. The MP2 results, in good agreement with the CCSD(T) values, will be considered as the reference. A study of atrazine isomers with Na+ and Ca2+ inspired from the three types of AtraMod complex is also presented. AtraMod is a good model for atrazine with Na+ as it allows to determine unambiguously the relative stability of the three families of isomers. For atrazine-Ca2+, the situation is less obvious as two families of isomers overlap. However, the study of AtraMod-Ca2+ enables to describe correctly the binding site in atrazine isomers. Finally, thermodynamic values (at 298.15 K and 1 atm) have been computed for AtraMod and atrazine complexes at various levels. For a better understanding of the interaction, a Reduced Variational Space Self Consistent Field (RVS SCF) energy decomposition has been performed for each complex with AtraMod and for a selection of isomers for atrazine. The electrostatic contribution is the predominant term of the interaction energy, however the preparation energy (to promote AtraMod from its isolated geometry to the one in the complex) must be considered to recover the right order in complexation energy. In none of the complexes the cation interacts along the dipole moment direction. A multipolar expansion of the electrostatic interaction between a charge (cation) and the AtraMod molecule (dipole, quadrupole and octupole...) has to be used to model the electrostatic contribution. (C) 2011 Elsevier B.V. All rights reserved.