Effects of hydration on the molecular structure of metal ion-atrazine dimer complexes: a MOPAC (PM3) study

A semi-empirical (PM3) study of atrazine dimer formation and atrazine monom er and dimer complexes containing Mg2+ Ca2+, Cd2+, Pb2+, or Zn2+ is reporte d. The two low energy atrazine dimers include distorted V (form A) or L sha pe (form B) dimers that provide a cleft or pocket for the insertion of a me tal ion. Insertion of a divalent metal ion in this pocket results in a sepa ration of the dimer atrazine molecules. The atrazine molecules in metal ion -atrazine dimer complexes provide ring and side-chain nitrogens that bind t o Mg2+, Ca2+, Cd2+, Pb2+, or Zn2+. The majority of metal ion-sidechain nitr ogen interactions involve the -N-i-propyl rather than the -N-ethyl side-cha in nitrogen. The metal ion-nitrogen (side-chain or ring) distances are gene rally in the order of their ionic radii. Addition of either one or two wate rs of hydration to the metal ion-atrazine dimes complexes lowers the enthal pies of formation and reaction considerably. The computed enthalpies of for mation and reaction for the hydrated metal ion-dimer complexes are ordered Ca2+ < Mg2+ < Pb2+ < Zn2+ < Cd2+. Density-functional theory (DFT) and PM3 s tructures of Zn(atrazine)(H2O)(2)(2+) are virtually identical, supporting t he use of the PM3 method for these complexes. (C) 2000 Elsevier Science B.V . All rights reserved.