H₂CN⁺ and H₂CNH⁺: New insight into the structure and dynamics from mass-selected threshold photoelectron spectra

In this paper, we reinvestigate the photoionization of nitrogen containing reactive intermediates of the composition H₂CN and H₂CNH, molecules of importance in astrochemistry and biofuel combustion. In particular, H₂CN is also of considerable interest to theory, because of its complicated potential energy surface. The species were generated by flash pyrolysis, ionized with vacuum ultraviolet synchrotron radiation, and studied by mass-selected threshold photoelectron (TPE) spectroscopy. In the mass-selected TPE-spectrum of m/z = 28, contributions of all four isomers of H₂CN were identified. The excitation energy to the triplet cation of the methylene amidogen radical H₂CN was determined to be 12.32 eV. Considerable activity in the C–N mode of the cation is visible. Furthermore, we derived values for excitation into the triplet cations of 11.72 eV for cis-HCNH, 12.65 eV for trans-HCNH, and 11.21 eV for H₂NC. The latter values are probably accurate to within one vibrational quantum. The spectrum features an additional peak at 10.43 eV that corresponds to excitation into the C_2v-symmetric H₂CN⁺. As this structure constitutes a saddle point, the peak is assigned to an activated complex on the singlet potential energy surface of the cation, corresponding to a hydrogen atom migration. For methanimine, H₂CNH, the adiabatic ionization energy IE_ad was determined to be 9.99 eV and the vibrational structure of the spectrum was analyzed in detail. The uncertainty of earlier values that simply assigned the signal onset to the IE_ad is thus considerably reduced. The spectrum is dominated by the H–N–C bending mode ν₁⁺ and the rocking mode ν₃⁺. All experimental data were supported by calculations and Franck-Condon simulations.