Loss of DNC from ionized 4-hydroxypyridine-OD: An intriguing reaction unravelled by theory and experiment

Low energy 4-hydroxypyridine ions (HP-1) decarbonylate but also readily lose hydrogen (iso)cyanide. Surprisingly, this reaction leads to a specific loss of the label from the OD-labelled isotopomer. Our tandem mass spectrometry experiments show that ionized vinylketene, CH2CHCHCO+ (1), is the product ion. This ion is also generated by the decarbonylation of 4-cyclopentene-1,3-dione ions (CP-1) and a computational analysis using the CBS-QB3 model chemistry indicates that these seemingly unrelated ionic systems share similar dissociation mechanisms.In this study, a mechanism is presented for the decarbonylation of metastable ions CP-1 that satisfies the energy requirement dictated by its appearance energy. Our computational analysis further shows that ions HP-1 may isomerize into the (iso)imino analogues of CP-1, by consecutive H-shift, ring-opening and cyclization steps. The CP-1 analogues serve as the immediate precursors for the specific loss of DNC (rather than DCN) from OD-labelled HP-1 and also its decarbonylation into the vinyl(iso)ketenimine ions CH2CHCHNCH+ (3) and CH2CHCHCNH+ (4).