Nonsymmetric Vibronic Activity in the Planar Propargyl Cation Revealed by High-Resolution VUV-MATI Spectroscopy

Abstract High-resolution vacuum ultraviolet mass-analyzed threshold ionization spectroscopy was used to probe the low-frequency vibronic structure of the propargyl cation following threshold ionization. Rotational-contour analysis combined with zero-field extrapolation yielded a field-free adiabatic ionization energy of 70,170 ± 3 cm–1 and constrained the cationic rotational constant to A+ = 9.39(3) cm–1, both consistent with previous high-resolution measurements. Most notably, the ν8 fundamental is directly observed at 263 cm–1 together with its 82 overtone at 523 cm–1, while ν7 activity is also resolved. The ν7 and ν8 fundamentals are not reproduced by the conventional harmonic Franck–Condon (FC) treatment of the well-established planar C2V cation, whereas the totally symmetric 82 overtone is predicted with appreciable intensity and supports the ν8 assignment and progression. Displaced-geometry calculations demonstrate the strong sensitivity of the FC envelope to small structural displacements but do not establish symmetry lowering. The results therefore provide direct experimental constraints on nonsymmetric threshold-photoionization intensity and on possible vibronic, non-Condon, and anharmonic contributions.