Penning ionization and ion fragmentation of formamide HCONH2 by He*, Ne*, and Ar* in molecular beams

Mass spectra from Penning ionization by metastable atom bombardment (MAB) in the title system at kinetic energies near 1 kcal/mol are reported. The experiments employ a supersonic excited noble gas beam crossing an effusive beam of formamide vapor. Product ions are extracted perpendicular to the plane of the beams, analyzed by a quadrupole mass filter, and counted by a scintillation-type ion counter. Relative to 70 eV electron impact, the He(*) and Ne(*) spectra show more extensive breakage of C-N and C-H bonds despite the smaller available energy, while the Ar(*) spectrum shows only the molecular ion (m/z 45), H atom elimination (44), and the decarbonylation products CO+NH(3)(+) (17). Fragmentation in the latter system has been analyzed using a combination of ab initio calculations and Rice-Ramsperger-Kassel-Marcus theory with tunneling correction; good agreement with the experimental 45/44/17 intensity ratio 100/6.8+/-0.7/6.2+/-1.7 is obtained. 15% of m/z 17 and 50% of m/z 44 is attributed to tunneling. The ab initio decarbonylation reaction path yields a hydrogen bonded H(2)N-HCO(+) transition state, which transfers a proton while proceeding downhill to the observed products, while both the path and the energetics support the earlier conclusion that the lowest lying electronically excited state of the ion (2pi or 2a(")) crosses the ground state early along the reaction path, thereby dominating the dynamics of decarbonylation.