Département de chimie moléculaire

The optimum structure of the OH-(H2O)n (n = 4, 5) anionic clusters was studied in a systematic way to locate all its min. energy conformations. The Becke-Lee-Yang-Parr (BLYP) methodol. and the 6-31+G(2d,2p) basis set was used. Two stable conformers of the OH-(H2O)4 cluster were found, one with three waters solvated to the OH- oxygen and another with four waters solvating the same oxygen. The tri-solvated structure is more stable by 1.24 kcal/mol. The transition state connecting these two conformers lies 2.41 kcal/mol above the tri-solvated structure and 1.16 kcal/mol above the tetra-solvated one. Therefore, both conformers should coexist at room temp. For the OH-(H2O)5 cluster, the authors found tri-, tetra-, and penta-solvated min.-energy conformers, although the latter one is not likely to be found at room temp. due to its much lower stability and the negligible barrier it presents when it distorts into the tetra-solvated conformer. The energetics for attaching a water mol. to the OH- hydrogen of the OH-(H2O)3 and OH-(H2O)4 clusters was explored at the BLYP/6-31+G(2d,2p) level. A water mol. in that position is energetically stable when its two hydrogens point to the OH- hydrogen. However, such a conformation is not a min. energy structure on the potential energy surface, because the water drifts to become attached to one of the first solvation shell waters. The reason is the much higher stability of the second conformer. One can avoid this shift by adding enough water mols. to link the water attached to the OH- hydrogen with those on the first solvation shell of the OH-(H2O)3 cluster. This is successfully accomplished in the OH-(H2O)17 cluster, whose optimum Hartree-Fock structure presents four waters coordinated to the OH- oxygen and one more water coordinated to its hydrogen, thus making the total solvation no. of the OH- in this cluster equal to 5. Structures with solvation nos. equal to four are found. However, penta-coordinated OH- anions are not found in the smaller n = 7 or 11 clusters studied here. [on SciFinder(R)]