Aggregation-induced effects on the ion-molecule reactivity of protonated acetic acid clusters toward methylamine

Non-covalent interactions within molecular aggregates are expected to influence ion-molecule reactivity, yet systematic experimental investigations remain challenging because of the difficulty of producing, selecting, and reacting isolated clusters of well-defined size. Here, we combine molecular-beam cluster production, synchrotron vacuum-ultraviolet (VUV) photoionization, tandem mass spectrometry, and density functional theory calculations to investigate the reactions of size-selected protonated acetic acid clusters (Ac_xH⁺, x = 2-5) with methylamine. Protonated clusters are formed by dissociative photoionization of neutral aggregates and reacted with methylamine under single-collision conditions. For all cluster sizes investigated, the dominant reaction proceeds through ion-molecule capture followed by proton transfer and fragmentation, leading mainly to the incorporation of one methylamine molecule with the loss of two acetic acid molecules. The product distribution shows little dependence on the photon energy used for ionization, indicating that the observed chemistry is largely insensitive to the internal energy deposited during cluster formation. Density functional theory calculations show that proton transfer to methylamine and formation of non-covalent adducts are highly exothermic, while the large excess energy released upon association accounts for the rapid fragmentation of the initially formed complexes and the absence of stable adducts in the mass spectra. Beyond the present acetic acid system, this work demonstrates the feasibility of investigating the intrinsic ion-molecule reactivity of mass-selected molecular clusters by combining synchrotron-based photoionization experiments with quantum-chemical calculations.