The reactions of metastable C4H7OCH3.+ radical cations generated by ionization of methyl ethers derived from stable butenols have been investigated by means of H-2- and C-13-labeling experiments, kinetic energy release measurements and analysis of collision-induced dissociation spectra. The reactions shown by the ionized allylic ethers, CH3CH=CHCH2OCH3.+ and CH2=C(CH3)CH2OCH3.+, are almost identical, especially at low internal energy. However, the behaviour of these isomers of C4H7OCH3.+ differs subtly from that of CH2=CHCH(CH3)OCH3.+. All these species undergo mainly loss of a methyl radical in slow dissociations, usually with formation of CH2=CHCH=O+CH3 as the product ion. The specificity of hydrogen atom selection shown in methyl radical elimination from H-2-labelled analogues of CH2=CHCH(CH3)OCH3.+ is distinct from that displayed by the two other allylic isomers of C4H7OCH3.+. In contrast to the three ionized allylic ethers, metastable CH2=CHCH2CH2OCH3.+ undergoes a sizeable amount of two other reactions-expulsion of a hydrogen atom or a formaldehyde molecule-in addition to undergoing methyl radical elimination. The specificity of methyl radical loss from this ion is different from those found for the analogous fragmentation of the ionized allylic ethers; moreover, the resultant C4H7O+ product ion is mainly CH2CH2CH2CH=O+, rather than CH2=CHCH=O+CH3
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