HOW SENSITIVE ARE CLUSTER COMPOSITIONS TO ENERGETICS - A JOINT BEAM EXPANSION THERMOCHEMICAL STUDY OF WATER-METHANOL TRIMETHYLAMINE CLUSTERS

We report the results of joint beam expansion and thermochemical studies of clusters containing ionic hydrogen bonds between water (W), trimethylamine (T), and methanol (M). The cluster distributions H+WnTm show magic numbers for (n, m) = (1, 3), (2, 4), and (3, 5), where the solvent shells are completed by blocking groups around a H3O4 core ion. Larger clusters with n > 3 show deviation from the n + 2 rule, suggesting structural changes. In H+MnTm clusters, the H+MnT2 sequence is always more predominant than the H+Mn+1T series independent of expansion or ionization conditions. In the three component clusters, the predominant series is H+WnMmTn+2. The cluster distributions from the beam expansion are quite sensitive to the clusters' binding energies as measured by high-pressure mass spectrometry. Even though ionization involves large excess energies, the observed cluster distributions show sensitivity to thermochemical differences as small as 1-3 kcal/mol.