Unimolecular reactions of ionized methyl butenyl ethers

Abstract

The reactions of metastable C 4H 7OCH ·+ 3 radical cations generated by ionization of methyl ethers derived from stable butenols have been investigated by means of 2H- and 13C-labelling experiments, kinetic energy release measurements and analysis of collision-induced dissociation spectra. The reactions shown by the ionized allylic ethers, CH 3CHCHCH 2OCH ·+ 3 and CH 2C(CH 3)CH 2OCH ·+ 3, are almost identical, especially at low internal energy. However, the behaviour of these isomers of C 4H 7OCH ·+ 3 differs subtly from that of CH 2CHCH(CH 3)OCH ·+ 3. All these species undergo mainly loss of a methyl radical is slow dissociations, usually with formation of CH 2CHCHO +CH 3 as the product ion. The specificity of hydrogen atom selection shown in methyl radical elimination from 2H-labelled analogues of CH 2CHCH(CH 3)OCH ·+ 3 is distinct from that displayed by the two other allylic isomers of C 4H 7OCH ·+ 3. In contrast to the three ionized allylic ethers, metastable CH 2CHCH 2CH 2OCH ·+ 3 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 C 4H 7O + product ion is mainly CH 2CH 2CH 2CHO +, rather than CH 2CHCHO +CH 3.