Mass spectrometry of hydrogen bonded clusters of heterocyclic molecules: Electron ionization vs. photoionization

Clusters of small heteroaromatic ring molecules pyrrole, imidazole and pyrazole were studied in a molecular beam experiment. Neutral cluster size distributions under various expansion conditions were probed by a scattering experiment with a He-atom beam. The clusters were ionized by electrons and their mass spectra were measured by a quadrupole mass spectrometer. Alternatively, the ionization was achieved by a non-resonant subsequent multiphoton process with 193 nm photons and the time-of-flight mass spectra were recorded. (1) The differences in the methods of cluster ionization, namely subsequent multiphoton and single photon ionization, and electron ionization are discussed. (2) More importantly, the differences between the ionization of pyrrole clusters on the one side, and imidazole and pyrazole clusters on the other side are studied. The existence of larger protonated fragments in the latter spectra points to a higher photostability in these N–H⋯ N hydrogen bonded systems as opposed to the more weakly N–H⋯π⋯π bound pyrrole clusters. The experiments are accompanied by ab initio calculations supporting and interpreting the experimental results.

Ionization and photochemistry of clusters of small heteroaromatic ring molecules pyrrole, imidazole and pyrazole were studied in a molecular beam experiment.Figure optionsDownload high-quality image (113 K)Download as PowerPoint slide