Loss of methyl from ionized 2‐hydroxy‐1,3‐butadiene

The loss of methyl from unstable, metastable and collisionally activated [CH 2 CHC(OH)CH 2 ] + ˙ ions (1 + ˙) was examined by means of deuterium and 13 C labelling, appearance energy measurements and product identification. High‐energy, short‐lived 1 + ˙ lose methyl groups incorporating the original enolic methene (C(1)) and the hydroxyl hydrogen atom (H(0)). The eliminations of C(1)H(1)H(1)H(4) and C(4)H(4)H(4)H(0) are less frequent in high‐energy ions. Metastable 1 + ˙ eliminate mainly C(1)H(1)H(1)H(4), the elimination being accompanied by incomplete randomization of the five carbon‐bound hydrogen atoms. The resulting [C 3 H 3 O] + ions have been identified as the most stable CH 2 CHCO + species. The appearance energy for the loss of methyl from 1 was measured as AE [C 3 H 3 O] + = 10.47 ± 0.05 eV. The critical energy for 1 + ˙ → [C 3 H 3 O] + + CH 3 ˙ is assessed as E c ⩽ 173 kJ mol −1 . Reaction mechanisms are proposed and discussed.