Dissociation Reactions of Low-Energy Pentenyl Methyl Ether Radical Cations C₅H₉OCH₃^•+

Abstract

The dissociation chemistry of the low-energy C 5 H 9 OCH 3 •+ ions generated from the 13 isomeric pentenyl methyl ethers derived from stable alkenols has been studied. This was done by examining their metastable ion characteristics, in conjunction with 2 H and 13 Clabelling as well as collision-induced dissociation and neutralisation-reionisation experiments. The influence of the position and substitution pattern of the double bond on the chemistry of these C 6 H 12 O •+ species is considered. The closely similar reactions of C 2 H 5 CH=CHCH 2 OCH 3 •+ , 3 •+ , CH 2 =CH-CH(C 2 H 5)OCH 3 •+ , 4 •+ , and CH 2 =C(C 2 H 5)CH 2 OCH 3 •+ , 13 •+ , point to a common chemistry, which is rationalised in terms of facile 1,2-H and 1,2-C 2 H 5 shifts via distonic ions. Each of the other isomers displays a distinct, though often related, chemistry. The eight allylic ionised ethers easily lose CH 3 • to produce C 5 H 9 O + oxonium ions, whose structure was established by CID experiments; ions 3 •+ / 4 •+ / 13 •+ also readily expel C 2 H 5 • to give C 4 H 7 O + ions of structure CH 2 =CH-C + (H)OCH 3. Elimination of CH 3 OH is also significant for 3 •+ / 4 •+ / 13 •+ and for (CH 3) 2 C=CHCH 2 OCH 3 •+ , 8 •+ , and CH 3 CH=C(CH 3)CH 2 OCH 3 •+ , 11 •+. Besides expelling CH 3 • and/or C 2 H 5 • and CH 3 OH, the three homoallylic isomers undergo dissociations which are (almost) absent for their allylic counterparts: thus, both CH 3 CH=CH(CH 2) 2 OCH 3 •+ , 2 •+ , and CH 2 =CH-CH(CH 3)CH 2 OCH 3 •+ , 10 •+ , lose H • and H 2 O, whereas CH 2 =C(CH 3)CH 2 CH 2 OCH 3 •+ , 7 •+ , is unique in predominantly losing CH 2 O. For the losses of CH 2 O and H 2 O mechanisms are proposed in which ion-neutral complexes of the type [C 5 H 10 •+ / CH 2 O] and [C 6 H 10 •+ / H 2 O] are key intermediates. The behaviour of the non-(homo)allylic isomer, CH 2 =CH(CH 2) 3 OCH 3 •+ , 1 •+ , is similar to that of 2 •+ but the reactions occur in different 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 communication with ionised cyclopentyl methyl ether, 14 •+ , play an important role.

Diastereomeric differentiation of peptides with CuII and FeII complexation in an ion trap mass spectrometer

“…This proton transfer is particularly favored by the Cu II coordination increasing the NH bond acidity 55. Secondly, the produced deprotonated N ‐terminal amine induces the Cu II reduction into Cu I and a concomitant homolytic cleavage of the benzylic bond with a 1–2 H radical shift,56–59 allowing the [CH 3 C 6 H 4 O] • radical loss. Interestingly, the loss of CH 2 C 6 H 4 O (106 u) from [(M − H) + Cu II ] + was not observed.…”