Dissociation reactions of low-energy pentenyl methyl ether radical cationsC5H9OCH3 center dot+

The dissociation chemistry of the low-energy C5H9OCH3.+ ions generated from the 13 isomeric pentenyl methyl ethers derived from stable alkenols has be en studied. This was done by examining their metastable ion characteristics , in conjunction with H-2 and C-13-labelling as well as collision-induced d issociation and neutralisation-reionisation experiments. The influence of t he position and substitution pattern of the double bond on the chemistry of these C6H12O.+ species is considered. The closely similar reactions of C2H 5CH=CHCH(2)OCH3(.+), CH2=CH-CH(C2H5)OCH3.+, 4(.+), and CH2=C(C2H5)CH2OCH3., 13(.+), point to a common chemistry, which is rationalised in terms of fa cile 1,2-H and 1,2-C2H5 shifts via distonic ions. Each of the other isomers displays a distinct, though often related, chemistry. The eight allylic io nised ethers easily lose CH; to produce C5H9O+ oxonium ions, whose structur e was established by CID experiments; ions 3(.+)/4(.+)/13(.+) also readily expel C2H5. to give C4H7O+ ions of structure CH2=CH-C+(H)OCH3. Elimination of CH3OH is also significant for 3(.+)/4(.+)/13(.+) and for (CH3)(2)C=CHCH2 OCH3.+, 8(.+), and CH3CH=C(CH3)CH2OCH3.+, 11(.+). Besides expelling CH3. an d/or C2H5. and CH3OH, the three homoallylic isomers undergo dissociations w hich are (almost) absent for their allylic counterparts: thus, both CH3CH=C H(CH2)(2)OCH3.+; 2(.+), and CH2=CH-CH(CH3)CH2OCH3.+, 10(.+), lose H-. and H 2O, whereas CH2=C(CH3)CH2CH2OCH3.+; 7(.+), is unique in predominantly losin g CH2O. For the losses of CH2O and H2O mechanisms are proposed in which ion -neutral complexes of the type [C5H10.+/CH2O] and [C6H10.+/CH2O] are key in termediates. The behaviour of the non(homo)allylic isomer, CH2=CH(CH2)(3)OC H3.+, 1(.+), is similar to that of 2(.+) but the reactions occur in differe nt proportions. A mechanism for the facile loss of an alkyl radical from 1( +) is proposed in which 1,4-H shifts and distonic ions as well as communica tion with ionised cyclopentyl methyl ether, 14(.+), play an important role.