Valence shell threshold photoelectron spectroscopy of the CH_xCN (x = 0-2) and CNC radicals

We present the photoelectron spectroscopy of four radical species, CH_xCN (x = 0-2) and CNC, formed in a microwave discharge flow-tube reactor by consecutive H abstractions from CH₃CN (CH_xCN + F → CH_x-1CN + HF (x = 1-3)). The spectra were obtained combining tunable vacuum ultraviolet synchrotron radiation with double imaging electron/ion coincidence techniques, which yielded mass-selected threshold photoelectron spectra. The results obtained for H₂CCN complement existing ones while for the other radicals the data represent the first observation of their (single-photon) ionizing transitions. In the case of H₂CCN, Franck-Condon calculations have been performed in order to assign the vibrational structure of the X^{+ 1}A₁←X ²B₁ ionizing transition. A similar treatment for the HCCN, CCN, and CNC radicals appeared to be more complicated mainly because a Renner-Teller effect strongly affects the vibrational levels of the ground electronic state of the HCCN⁺, CCN, and CNC species. Nevertheless, the first adiabatic ionization energies of these radicals are reported and compared to our ab initio calculated values, leading to new values for enthalpies of formation (Δ_fH₂₉₈⁰(HCCN⁺(X²A^'))=1517±12kJmol^-1,Δ_fH₂₉₈⁰(CCN(X²Π))=682±13kJmol^-1, and Δ_fH₂₉₈⁰(CNC(X²Πg))=676±12kJmol^-1), which are of fundamental importance for astrochemistry.