Noncompeting Metastable Losses of Methyl and Ethylene from Gaseous Butanoic Acid Ions due to Isomerization Prior to Methyl Loss

Article abstract of DOI:10.1021/ja00416a003

Metastable C4H8O^+. ions obtained from butanoic acid and ethyl butanoate undergo considerable γ-hydrogen exchange prior to losing ethylene, but little exchange prior to losing methyl.Therefore the two fragmentations are not directly competing, contrary to the general assumption that all reactions of an ion in the gas phase are competitive.It is concluded that metastable butanoic acid ions which lose methyl isomerize essentially irreversibly to CH3CH2C^.HC(OH)2⁺ and/or CH3CH(C^.H2)C(OH)2⁺ before the γ-methyl becomes exchanged.This accounts for the difference between γ-hydrogen exchange prior to the loss of methyl and ethylene without invoking isolated electronic states, as previously proposed.Butanoic acid ions generated by the McLafferty rearrangement of butanoate esters have a much weaker metastable loss of ethylene than directly ionized butanoic acid.Collisional activation experiments demonstrate that this results from more of the butanoic acid ions derived from ethyl butanoate than from butanoic acid isomerizing prior to collision.Variation in internal energy probably causes this difference in degree of isomerization with the source of the ion.