Migration of a proton as a Function of solvation within {ROH}(n){H2O}H+ cluster ions: Experiment and theory

Metastable and collision-induced decompositions of mass-selected {ROH}(n){H 2O}H+ cluster ions (where R drop CH3-, CH3CH2-, CH3CH2CH2-, and (CH3)(2)CH- ) were observed to exhibit distinct size-dependent behavior. We observe tha t loss of a water molecule is dominant for n less than or equal to 8, where as loss of multiple ROH molecules is the favored decomposition channel for II greater than or equal to 9, resulting in the eventual formation of a sta ble {ROH}(9)-{H2O}H+ cluster ion. We believe this is evidence for two disti nct cluster geometries which explicitly depend on the number of ROH molecul es present. That is, below a certain critical size the proton resides on th e molecule with the highest proton affinity, the ROH. However, above that c ritical cluster size the proton will now preferentially reside on the water molecule, if there are sufficient alcohols to completely and symmetrically solvate the central H3O+. The structural implications of these results wil l be discussed in light of new theoretical calculations which have been per formed on this system.