Intracluster reaction, fragmentation, and structure of monomethylamine, dimethylamine, and trimethylamine cluster ions

Ammonia, monomethylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) clusters, formed in a supersonic expansion, were investigated using a multiphoton ionization time-of-flight mass spectrometer. The observed major product ions resulting from prompt fragmentation following ionization are (NH₃)_nH⁺, (CH₃NH₂)_nH⁺, [(CH₃)₂NH]_nH⁺, and [(CH₃)₃N]_nH⁺. Detection of stable CH₂=NH₂⁺ and (CH₂=NH-CH₃)⁺ immonium ions and other fragments provides evidence for molecular rearrangement and fragmentation within our observable time windows. Anomalously large relative intensities observed for (NH₃)₅H⁺, (CH₃NH₂)₄H⁺, [(CH₃)₂NH]₃H⁺, and [(CH₃)₃N]₂H⁺ are attributed to enhanced stability of the cluster ions, due to the complete filling of hydrogen-bonding sites on the "central ions". Fully optimized geometries and stabilization energies of these cluster ions have been predicted using ab initio molecular orbital methods. The experimental findings are in very good agreement with the calculated results.