Diversity of protonated mixed pyrene–water clusters investigated by collision induced dissociation

Abstract

Protonated mixed pyrene–water clusters, (Py)_m(H₂O)_nH⁺, where m = [1–3] and n = [1–10], are generated using a cryogenic molecular cluster source. Subsequently, the mass-selected mixed clusters undergo controlled collisions with rare gases, and the resulting fragmentation mass spectra are meticulously analyzed to discern distinct fragmentation channels. Notably, protonated water cluster fragments emerge for n ≥ 3, whereas they are absent for n = 1 and 2. The experimental results are complemented by theoretical calculations of structures and energetics for (Py)(H₂O)_nH⁺ with n = [1–4]. These calculations reveal a shift in proton localization, transitioning from the pyrene molecule for n = 1 and 2 to water molecules for n ≥ 3. The results support a formation scenario wherein water molecules attach to protonated pyrene PyH⁺ seeds, and, by extension, to (Py)₂H⁺ and (Py)₃H⁺ seeds. Various isomers are identified, corresponding to potential protonation sites on the pyrene molecule. Protonated polycyclic aromatic hydrocarbons are likely to be formed in cold, dense interstellar clouds and protoplanetary disks due to the high proton affinity of these species. Our findings show that the presence of protonated PAHs in these environments could lead to the formation of water clusters and mixed carbon–water nanograins, having a potential impact on the water cycle in regions of planet formation.