Collision-induced dissociations of carboxylate negative ions from 2-ethylbutanoic, 2-methylpropanoic, and pivalic acids. An isotopic labelling study

Abstract

On collisional activation this ion forms CO,-', CH,=CH-, and MeCH=CH-. In addition, elimination of H' and Et' yield Et(R)C=CO,-' (R = Et and H, respectively). The elimination of Et' is not a simple cleavage but occurs by loss of H' from a methyl group followed by loss of ethene. The carboxylate ion also rearranges to Et,CCO,H; this species decomposes to HO-, EtCCH,, and also eliminates the elements of C,H, and CH,. All fragmentations have been studied using ,H and 13C labelling: for example it is proposed that loss of CH, from Et,CCO,H occurs by a six-centre stepwise process in which the first step (formation of an incipient methyl anion) is rate determining. The collisional activation mass spectra of Et,CHCO,-, Me,CHCO,-, and Me,CCO,-are different, all showing characteristic decompositions. For example, all three ions eliminate methane; the mechanism is different in each case.Collision-induced dissociations of negative ions may be used to provide structural information concerning unknown comand also to obtain fundamental information about ion b e h a v i ~u r . ' ~ Carboxylate negative ions have been formed by deprotonation of carboxylic acids using a variety of ionizationThe characteristic fragmentation of RC0,-is decarboxylation to form Rwhen Ris a stabilized species (e.g. Ar-, CH2=CH-CH2 -, HC=C-CH, -Little is known of the fragmentations of alkyl carboxylate negative ions RC02-. Alkyl-ions will not generally be observed since electron affinities are close to zer0.t We therefore chose to study the collision-induced fragmentations of the carboxylate ion of 2-ethylbutanoic acid a in order to determine what alternative fragmentations occur. In particular we wished to determine whether the conversion a into b occurs, since we have shown that when carbonyl systems can form two enolate ions (e.g. R1CHCOCH2R2 and R1CH2COCHR2), the ions are interconvertible under the conditions of collisional a~tivation.~,'~ If the conversion a into b does occur, b should be readily identifiable by fragmentations through the carbanion centre. A symmetrical alkyl substituent was chosen so that any fragmentations of b could be studied using intramolecular isotope effects. During the course of t The methyl anion (electron affinity 0.08 _+ 0.03 32) can be observed. The acetate anion shows a peak corresponding to Me-in its collisional activation (c.a.) mass spectrum, m/z (%, structure) 58 (100, 'CH,CO, -), (0.2, HO-), 16 (0.1, O-.), 15 (2, Me-), and 14 (0.8, CHI-'). Me-is also produced in the reaction between MeCDO and 7 eV electrons.33 In contrast, Et-is not observed in the c.a. mass .spectrum of the proprionate anion: m/z (%, composition) 72 (100, MeCHC02-), 71 (41, C3H302-'), 70 (9, C3HzO,-), 58 (9, *CH2CO2-), 55 (36, C3H30-), 44 (1 1, CO,-'), and 27 (7, CzH3-). 57 (2, CHCO2-X 44 (1, COZ-'), 41 (4, HCSO-), 40 (1, C20-'), 17this study it was also necessary to investigate the decompositions of Me2CHC02 -and Me3CC0, -.

Competing fragmentation processes of β‐substituted propanoate ions upon collision‐induced dissociation

“…Further analysis of the contributing mechanisms was undertaken by considering several scenarios, viz., sequential decarboxylation and elimination reactions as a variation of Route C, generation of complementary product anions (i.e., d and e , Scheme ) by proton transfer within intermediate ion‐neutral complexes in Routes A and B, elimination of water from the enolate ion c in Route B, and variation of reaction energetics through structural changes at the α and β positions of the propanoate moiety. Other than the ion derived from the β‐substituent, no other ions with abundances greater than 0.2% were detected in the CID spectra of 1a – 3a and 5a – 7a (Figs.…”

“…Although the formation of the enolate ion 4c was indicated by the observed complementary anion, the possible loss of water from the enolate ion [42][43][44][45][46] was not observed (Figs. 2(D) and 2(E)).…”

The influence of structural features on facile McLafferty‐type, even‐electron rearrangements in tandem mass spectra of carboxylate anions

“…In earlier days, negative ion mass spectra were typically generated by chemical ionization or fast‐atom bombardment techniques and many studies on the fragmentation behaviour of organic anions were carried out, notably by Bowie and coworkers. These studies typically used hydroxide ions as a base and high‐energy collision‐induced dissociation (CID) techniques to fragment the ions 5–10. However, EE negative‐ion mass spectra are now readily available, thanks to modern mass analyzers and atmospheric pressure or matrix‐assisted laser desorption/ionization methods.…”

“…The McLafferty rearrangement14 is one the most‐studied processes in the mass spectrometry literature,4, 15 including complex studies as to whether the rearrangement is synchronous or step‐wise 16, 17. Although most examples of the McLafferty rearrangement involve radical cations, cases involving EE ions have been known for a long time 4–10, 12, 15. Zollinger and Seibl have laid out detailed criteria by which to judge whether a rearrangement process observed for an EE ion should be classified as a McLafferty rearrangement 18.…”

Generation and reactions of substituted phenide anions in an electrospray triple quadrupole mass spectrometer

“…In some cases, however, the carboxylate anions abstract intramolecularly a proton, e.g. from the α ‐carbon in aliphatic acids, inhibiting decarboxylation 23. Another problem to overcome is the generation of gas‐phase carboxylate anions.…”