Quantum Chemical Analysis of the Molecular and Fragment Ion Formation of Butyrophenone by High-Electric Field Tunnel Ionization at Atmospheric Pressure.

The molecular ion M^+· was observed when the liquid sample of butyrophenone was supplied using atmospheric pressure corona discharge (APCD). In contrast, the vapor supply resulted in the formation of the protonated molecule [M+H]⁺. The mass spectrum obtained with the liquid supply showed two distinctive fragment ions at m/z 105 and 120, resulting from α-cleavage and McLafferty rearrangement (McLR), respectively. The APCD spectrum showed peaks of M^+· and the characteristic two fragment ions that were the same as the field ionization mass spectra of butyrophenone as reported by Chait et al. and Beckey et al. The formation of the molecular and fragment ions strongly indicated that high-electric field tunnel ionization (HEFTI) occurs by the HEF strength exceeding 10⁸ V/m at the tip of the corona needle in APCD. The charge and spin density distributions of the molecular and fragment ions were analyzed by quantum chemical calculations using time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) analysis.