Loss of methyl from [H₂CC(OH)CH₃]^+˙ ions prepared by electron impact ionization of unstable 2‐hydroxypropene

Abstract

Abstract Unstable 2‐hydroxpropene was prepared by retro‐Diels‐Alder decomposition of 5‐ exo ‐methyl‐5‐norbornenol at 800°C/2 × 10 −6 Torr. The ionization energy of 2‐hydroxypropene was measured as 8.67±0.05 eV. Formation of [C 2 H 3 O] + and [CH 3 ] + ions originating from different parts of the parent ion was examined by means of 13 C and deuterium labelling. Threshold‐energy [H 2 CC(OH)CH 3 ] +˙ ions decompose to CH 3 CO + +CH 3 ˙ with appearance energy AE (CH 3 CO + ) = 11.03 ± 0.03 eV. Higher energy ions also form CH 2 COH + + CH 3 with appearance energy AE (CH 2 COH + ) = 12.2–12.3 eV. The fragmentation competes with hydrogen migration between C(1) and C(3) in the parent ion. [C 2 H 3 O] + ions containing the original methyl group and [CH 3 ] + ions incorporating the former methylene and the hydroxyl hydrogen atom are formed preferentially, compared with their corresponding counterparts. This behaviour is due to rate‐determining isomerization [H 2 CC(OH)CH 3 ] +˙ →[CH 3 COCH 3 ] +˙ , followed by asymmetrical fragmentation of the latter ions. Effects of internal energy and isotope substitution are discussed.