Multicomponent cluster ions. 2. Comparative stabilities of cationic and anionic hydrogen-bonded networks. Mixed clusters of water-methanol

Abstract

The thermochemistry of the mixed water-methanol cationic clusters (H/sub 2/O)/sub n/(CH/sub 3/OH)/sub m/H/sup +/ and anionic clusters ((H/sub 2/O)/sub n/(CH/sub 3/OH)/sub m/ - H)/sup -/ (i.e. clusters containing OH/sup -/ or CH/sub 3/O/sup -/) was measured. The stability of the total hydrogen-bonded network in each positive cluster is greater by 3-6 kcal/mol than in the corresponding negative cluster. The variation of the stabilities of the cationic and anionic clusters with composition shows remarkable similarities. (1) Both H/sub 3/O/sup +/ x nH/sub 2/O and OH/sup -/ x nH/sub 2/O show effects of solvent shell filling at n = 3. (2) Both for anions and cations, neat methanol clusters are more stable than neat water clusters. (3) Both CH/sub 3/OH/sub 2//sup +/ and CH/sub 3/O/sup -/ are solvated more strongly by methanol than by water. (4) Stepwise ion solvation compresses the differences between the gas-phase acidities of H/sub 2/O and CH/sub 3/OH and also between the gas-phase basicities. The compression effect with increasing solvation is somewhat larger in the positive than in the negative ions. (5) Both for anions and cations, stepwise replacement of H/sub 2/O by CH/sub 3/OH is exoergic for every step from neat water methanol. The results indicate thatmore » in the water-methanol clusters, the favored topology places methanol molecules near the charged centers and water molecules at the periphery. This is in contrast to blocked clusters such as water-acetonitrile, where hydrogen-bonding requirements place water at the protonated center and acetonitrile at the periphery.« less