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 {H2O}H+ cluster ions (where R ≡ CH3 − , CH3CH2 − , CH3CH2CH2 − , and (CH3)2CH − ) were observed to exhibit distinct size-dependent behavior. We observe that loss of a water molecule is dominant for n ≤ 8, whereas loss of multiple ROH molecules is the favored decomposition channel for n ≥ 9, resulting in the eventual formation of a stable {ROH}9{H2O}H+ cluster ion. We believe this is evidence for two distinct cluster geometries which explicitly depend on the number of ROH molecules present. That is, below a certain critical size the proton resides on the molecule with the highest proton affinity, the ROH. However, above that critical 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 will be discussed in light of new theoretical calculations which have been performed on this system.