| Abstract
| - The optimum structure of theOH-(H2O)n(n = 4, 5) anionic clusters is studied in a systematic wayto locateall its minimum energy conformations. TheBecke−Lee−Yang−Parr (BLYP) methodology and the6−31+G(2d,2p) basis set is used. Two stable conformers of theOH-(H2O)4 cluster have beenfound, one with threewaters solvated to the OH- oxygen and another with fourwaters solvating the same oxygen. The trisolvatedstructure is more stable by 1.24 kcal/mol. The transition stateconnecting these two conformers lies 2.41kcal/mol above the trisolvated structure and 1.16 kcal/mol above thetetrasolvated one. Therefore, bothconformers should coexist at room temperature. For theOH- (H2O)5 cluster, our study hasfound tri-, tetra-,and pentasolvated minimum energy conformers, although the latter one isnot likely to be found at roomtemperature due to its much lower stability and the negligible barrierit presents when it distorts into thetetrasolvated conformer. The energetics for attaching a watermolecule to the OH- hydrogen of theOH-(H2O)3and OH-(H2O)4 clusters hasalso been explored at the BLYP/6-31+G(2d,2p) level. Itis shown that a waterin that position is energetically stable when its two hydrogens pointto the OH- hydrogen. However, sucha conformation is not a minimum energy structure on the potentialenergy surface, because the water driftsto become attached to one of the first solvation shell waters. Thereason is the much higher stability of thesecond conformer. It is shown that one can avoid this shift byadding enough water molecules to link thewater attached to the OH- hydrogen with those on thefirst solvation shell of theOH-(H2O)3 cluster.Thisis is successfully accomplished in theOH-(H2O)17 cluster, whoseoptimum Hartree−Fock structure presentsfour waters coordinated to the OH- oxygen and one morewater coordinated to its hydrogen, thus making thetotal solvation number of the OH- in this cluster equalto 5. Structures with solvation numbers equal to fourare found. However, pentacoordinated OH- anions arenot found in the smaller n = 7 or 11 clustersstudiedhere.
|
