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Intramolecular I3C and 'H isotope effects have been measured for unimolecular losses of ethene (theMcLafferty rearrangement) from metastable molecular ions of 2-ethyI-l-phenylbutan-l-one, kthylpentan-Zone and heptan-4one. Primary and secondary deuterium isotope effects are observed at the y-(terminal) and fl-positions, respectively. Large primary isotope effects occur at fl-positions and for the y positions of 3-ethylpentan-2-one and heptan-4one. The carbon isotope effects in the cases of the doubly isotopically labelled CH,COCH(C,H,)(13CH,CH,) and CD,COCD(C,L),)('3CD,CD,) are 1.17 (AO.01) and 1.04 (+0.01), respectively. All of these isotope effects are consistent with a stepwise mechanism in which more than one step is rate determining.

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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, -.

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