Observation of "intramolecular" ion-molecule reactions within ionized clusters: The methyl halide systems

"Intramolecular" ion-molecule reactions are observed following electron impact (EI) ionization of methyl halide cluster beams, (CH₃X)_n (X ≡ F, Cl, Br, I). The resulting fragmentation patterns reveal the mechanisms of these reactions in some detail. EI of (CH₃F)_n yields protonated parent ions plus ionic fragments produced by ejection of closed-shell molecules (e.g., HF, H₂, CH₄, and F₂, alone and in various combinations) from the protonated parent. In contrast, the EI of (CH₃Br)_n and (CH₃I)_n yields only parent ions plus ions corresponding to the loss of CH₃ or X. The case of (CH₃Cl)_n is intermediate: some protonated parent ions and corresponding fragments are formed, as well as parent ions. In all cases, the ions observed result from chemical reaction (extensive intramolecular rearrangement) within the excited, solvated ion. The major reactive processes can be rationalized in terms of "intramolecular" analogues of known gas-phase, bimolecular, ion-molecule reactions. For the fluorides and chlorides, the nascent EI-generated CH₃X⁺ ion formed within the cluster reacts with a neutral CH₃X "neighbor" molecule to form the protonated parent ion, ejecting a CH₂X fragment. For the methyl fluoride clusters, the protonation reaction is complete, within the residence time in the ionizer (ca. 1 μs), in contrast to the methyl chloride clusters where the protonation reaction is incomplete. Thus the precursor, the parent cluster ion, is also observed. The protonation reaction efficiency is found to decrease significantly with increasing cluster size. For the bromides and iodides, the nascent CH₃X⁺ ion does not react to form the protonated parent but instead reacts with a neutral CH₃X "neighbor" (within the cluster ion) to form the dimethylhalonium ion. The influence of electron energy upon the fragmentation patterns for all the methyl halide cluster beams suggests that there is an energy barrier to these intracluster reactions not present in the bimolecular case. In addition, reactive processes are observed that have not previously been detected in bimolecular experiments, suggesting unique ion-molecule chemistry that can occur only in the "solvated" environs of a cluster. Such intramolecular reactions provide an important bridge between bimolecular gas-phase reaction dynamics and solution chemistry.