Rearrangement and fragmentation of isomeric C4H5O anions upon collision activation: a combined experimental and ab initio study

The 3,4-epoxybutoxide anion on collisional activation in the gas phase form s an intermediate ion complex (vinyl ethylene oxide, HO-) which eliminates water by two competitive pathways, viz, (i) by the HO- ion deprotonating th e neutral to produce a vinylethylene oxide anion deprotonated on the ring a djacent to the vinyl substituent: this anion then ring opens to give -CH2-C O-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 contras t, 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.