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+).