Torsional analyses of trans-2-butene and propene cations: A comparative investigation of two prototypical ions with different degrees of symmetry

The mass analyzed threshold ionization spectra of trans-2-butene and propene have been obtained using single-step vacuum ultraviolet excitation. The ionization potential for trans-2-butene is $73 605±4 cm−1,$ while for propene it is $78 587±4 cm−1.$ Both species have progressions of the low frequency, torsional, normal modes. Using the torsional normal mode and first overtone from a trans-2-butene cation the torsional barrier is determined to be approximately 453 cm⁻¹, assuming a sinusoidal potential. Normal mode analysis indicates that all low frequency normal modes of the propene cation involve substantial internal motion of the vinyl component, and the spectrum shows a very anharmonic torsional mode progression. These factors complicate direct torsional barrier analysis from the experimental lines, but through the use of various ab initio methods the propene torsional barrier is determined to be approximately 429 cm⁻¹. Due to the anharmonicity found in propene, the correlation corrected vibrational self-consistent field method in GAMESS was used to verify the assignments of the experimental lines. The torsional barriers for both cations are found to lie approximately 275–300 cm⁻¹ below the barrier heights of the neutral species.