A photoionization mass spectrometric investigation of CH3CN and CD3CN

Abstract

A photoionization mass spectrometric investigation of CH3CN and CD3CN from the onset of ionization to ∼20 eV is reported. Photoionization efficiency (PIE) curves of the acetonitrile parent ions and of the fragments C2H2N+, C2HN+, CH2N+, CH+3, CH+2 and their deuterated analogs have been measured. The parent ion curves display isotope-dependent direct ionization as well as autoionization structure. Excitation of one quantum of the doubly degenerate CCN bending mode (ν8) in the ground state of CH3CN+ is observed, indicating the presence of Jahn–Teller coupling. The measured ionization potentials are 12.194±0.005 eV (CH3CN) and 12.235±0.005 eV (CD3CN). Autoionizing Rydberg series converging to the first and second excited electronic states have been observed and assigned. At their thresholds, C2H2N+ and C2D2N+ are produced exclusively from parent ion states which are populated by autoionization. Appearance potentials of the fragment ions for which PIE curves were obtained are reported, and their heats of formation have been calculated where more reliable literature values are not available. The observed fragment ion appearance potentials and relative ion yields indicate that H-atom migrations in the molecular ion are important in the fragmentation mechanisms.