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

Abstract

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.