Observation of “Intramolecular” Ion-molecule Reactions Within Ionized Clusters: Hetero Systems Involving Methyl Halides and Oxygen-containing Compounds

“Intramolecular” ion-molecule reactions are observed following electron impact ionization of heterocluster beams of methyl halides with oxy compounds. The resulting fragmentation patterns reveal the mechanisms of these reactions: extensive intramolecular rearrangements occur within the excited, solvated cation. The major reactive processes can be rationalized in terms of intramolecular analogues of known gas-phase, bimolecular ion-molecule reactions, such as the protonation reaction M_nA_m→M_n-1A_mH⁺+ CH₂X (where M = methyl halide, X = Cl or I, and A = oxy compound). Here A includes (CH₃)₂CO, CH₃OH, H₂0 (and their perdeuteriated analogues), and (CH₃)₂O. Several novel reactive pathways have been inferred from the cluster ion mass spectra. From observation of the dependence of reaction yields and relative rate constants upon cluster size it is evident that even a few “solvent” molecules on the cation suffice to alter the course of the reaction. For neat clusters the protonation rates decrease as cluster size increases. This can be explained by the increased stability of the ion (with increasing cluster size n), hence an increasing barrier to reaction as a function of n. In the case of the heterocluster ions this trend is reversed: the rates of analogous ion-molecule reactions are faster than those for the neat cluster ions, and these rates increase with n. These results can also be explained by energetic considerations.