Rearrangement and Fragmentation of Isomeric C 4 H 5 O Anions upon Collision Activation: A Combined Experimental and ab Initio Study

The 3,4-epoxybutoxide anion on collisional activation in the gas phase forms an intermediate ion complex (vinyl ethylene oxide, HO⁻) which eliminates water by two competitive pathways, viz. (i) by the HO⁻ ion deprotonating the neutral to produce a vinylethylene oxide anion deprotonated on the ring adjacent to the vinyl substituent: this anion then ring opens to give ⁻CH₂–CO–CH=CH₂ and (ii) by the HO⁻ ion deprotonating at the ring methylene group to give a cyclic anion which ring opens to yield ⁻CH₂–CH=CH–CHO. In contrast, when vinyl ethylene oxide is allowed to react with HO⁻ in the source of the mass spectrometer, deprotonation occurs mainly, if not exclusively, at the ring methylene position (as shown by deuterium labelling) to yield the anion ⁻CH₂–CH=CH–CHO. The two ring-opening processes have been investigated using theoretical studies at the MP2 Fc/6-31+G(d) level of theory.