Observation of Magic Numbers for (ROH)_nH₃O⁺ Heteroclusters (R = CH₃, CH₃CH₂, (CH₃)₂CH, and CH₃CH₂CH₂): Implications for Cluster Ion Structure

We report in this paper nearly identical ion intensity distribution curves for ROH/water heterocluster ions for a variety of alcohols. The cluster ions (ROH)₉(H₂O)H⁺ and (ROH)₁₀(H₂O)₂H⁺ were found to display enhanced stability for all of the alcohols studied. Heteroclusters with n < 7 could not be observed upon ionization of neat alcohol clusters and were generally found with low intensity from the mixed alcohol/water clusters. We will introduce in this paper a structural model which can account for both the stability of the (ROH)₉(H₂O)H⁺ ions and the loss of water from clusters with n < 7. This model implies a “proton switch” at a specific cluster size, leading to what may be thought of as a central H₃O⁺ ion completely solvated by a ring (or chain) of hydrogen-bonded alcohols. The stable structure formed for(ROH)₉(H₂O)H⁺ consists of three fused five-membered rings, each consisting of 4 ROH's and a H₃O⁺ hydrogen-bonded together. This structure is closely related to that proposed by Castleman and co-workers for the dodecahedral water cluster ((H₂O)₂₁H⁺).