Rearrangement and fragmentation of isomeric C_4H_5O anions upon collision activation: a combined experimental and ab initio study

摘要

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 natural to produce a vinylethylene oxide anion deprotonated on the ring adjacent to the vinyl substituent: this anion then ring opens to give ~-CH_2-CO-CH=CH_2 and (ii) by the HO~- ion deprotonating at the ring methylene group to give a cyclic anion which ring opens to yield ~-CH_2-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 ~-CH_2-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.