Investigation of the mechanism of [M–H] ⁺ ion formation in photoionized <i>N</i> ‐alkyl‐substituted thieno[3,4‐ <i>c</i> ]‐pyrrole‐4,6‐dione derivatives during higher order MS ^<i>n</i> high‐resolution mass spectrometry

The proposed chemical structure of the N-alkyl TPD ion species, following the loss of a single hydrogen atom, was revealed during APPI-HR-CID-MSⁿ (n = 3) experiments on the [M-H]⁺ species. Hydrogen radical (H^•) abstraction from the alkyl side chain (e.g., hexyl, heptyl, octyl, 2-ethylhexyl, and nonyl) triggered a rearrangement in the radical cation structure of the N-alkyl-TPD derivatives, initiating cyclization and forming a six-membered ring that connects the oxygen atom to the third carbon atom in the alkyl chain. In addition, theoretical calculations supported the APPI-HR-CID-MSⁿ (n = 3) experiments by demonstrating that the proposed chemical structure, resulting from the intramolecular cyclization of the N-alkyl-TPD ion species, was stable in the presence of chlorobenzene. These findings will aid the structural determination and elucidation of molecules with similar core structures.