Formyl torsion and cationic structure of gauche conformer in isobutanal by conformer-specific VUV-MATI spectroscopy and Franck-Condon fitting

We report conformational and vibrational assignments of vacuum ultraviolet mass-analyzed threshold ionization spectrum of the isolated gauche conformer, based on previously determined conformer-specific photoionization and conformational stabilities of isobutanal. The vibrational spectrum of the pure cationic gauche conformer was acquired by removing the trans conformer via conformationally effective cooling with Ar carrier gas. The peaks in the spectrum were assigned by Franck-Condon (FC) fitting by adjusting the cationic geometrical parameters of the gauche conformer at the CAM-B3LYP/cc-pVTZ level. Based on the good agreement between the experimental and calculated results, we were able to determine the precise structure of the cationic gauche conformer of isobutanal with C₁ symmetry. Notably, the unveiled vibrational structure was mainly attributed to a geometrical change along the vibrational motions associated with the formyl torsion and CC stretching upon ionization, resulting in their prominent spectral overtones and combination bands with other fundamental vibrations. On the potential energy curve for the formyl torsion of the cationic gauche conformer determined by FC fitting, the transition barrier at 313 cm^-1 preserved the hindered formyl torsion in the case of a harmonic potential well, which was confirmed by the progression of formyl torsion, namely, 33¹, 33², and 33³ observed at 60, 120, and 180 cm^-1, respectively.