Metastable ion studies. XII—Molecular and fragment ion structures for isomeric C₄H₆O₂ acids

Abstract

Metastable peak characteristics, ionization and appearance energy data and isotopic labelling experiments have been applied to a study of the fragmentation behavioUr of the molecnlar ions of the isomeric C A O , acids, cis and trans-uotonic acids, methacrylic acid, butenoic acid and cydopropane carboxylic acid. Prior to the losses of H,O and CH,, all the metastable molecular ions rearrange to [cis-crotonic acid]+' ions. Loss of H,O, which generates a composite metastable peak, is proposed to yield vhylketene and/or cydobutenone molecular ions. Detailed mechanisms are presented for the isomerbations of the various molecular ions and for the above fragmentations. Ionized 3-butenoic and cydopropane carboxylic acids display a major loss of CO from their metastable ions, a minor prows in the other isomers. The metastable peaks consist of two components and these are ascribed to the formation of propen-1-01 and aUyl alcohol as daughter ions. Some comparative data are presented for the isomeric C&O, aads, tiglk acid, angelic acid and seneaoic acid. INTRODUCllONIt has long been known that fragmentations of many simple ionized olefins are preceded by rearrangements sufficiently extensive to lead to the random statistical loss of labelled and unlabelled groups of carbon and hydrogen atoms, and it has been widely concluded that the molecular ions decompose via a single structure or a mixture of common i0ns.l The more complex isomeric 1, l-di-and tri-alkyl substituted ethylenes have closely similar mass spectra, but in these larger molecules it is believed that the unrearranged molecular ions fragment over different potential surfaces to yield a common daughter ion from which all further reactions pr0ceed.2.~ In general (although not invariably) the effect of introducing a heteroatom into such molecules is to reduce the interconvertibility of their molecular ions, thus rendering more certain the identification of ion structures. Mass spectra1 studies of ethylenes having a single carboxyl substituent are few, although some dibasic ethylenic acids have received detailed attent i~n .~ The higher homologues of some P,yunsaturated acids have been investigated' as have the methyl esters of some hexenoic acids.6Our interest in the simple unsaturated carboxylic acids was aroused by a brief survey of their normal mass spectra and the complex shapes of the metastable peaks associated with their primary fragmentations. In this paper we report observations on metastable peaks accompanying the losses of H,O, CH,' and CO from all acids of formula C4H60, and we discuss the isomerizations of the molecular ions which precede these fragmentations. The analogous reactions of the corresponding methyl and ethyl esters are discussed elsewhere.' ~~ RESULTS AND DISCUSSIONNormal mass spectra and heats of formation of the molecular ionsThe 70 eV mass spectra of cis-crotonic acid (l), wunscrotonic acid (2), methacrylic acid (3), 3-butenoic acid The ionization energies (I(M)) of the five acids were obtained from their photoelectron (PE) spectra; these ...

Secondary metabolites from Ganoderma lucidum and Spongiporus leucomallellus

“…In the EI-spectrum, the fragment ion at m/z 478 shows the loss of water ([M À H 2 O] + ) described before in systems with a double bond in b-c-position to a carboxy group suggesting this fragmentation to take place at position C-28 (Lauwers et al, 1973;Herrmann and Schwarz, 1976;Holmes et al, 1979). The fragment ions m/z 423, 327 and 169 represent the loss of the chain C-1 to C-3 ([M À C 3 H 5 O 2 ] + ), the loss of the complete side chain C-20 to C-26 ([M À C 9 H 13 O 3 ] + ), and the loss of the triterpene skeleton at position 20 ([M À C 22 H 31 O 2 ] + ), respectively, due to r-fragmentation.…”

Isomeric and tautomeric [C4H4O]+• ions; their thermochemistry and collisionally induced fragmentation characteristics

“…The above results permit further analysis of observations of the elimination of H 2 0 from ionised methacrylic acid (13) This proposal cannot be demonstrated by labelling experiments, but the CA mass spectrum of the ion-source generated [C4H40]+' ions from this reaction can most satisfactorily be reproduced from a 2:3 mixture of the CA mass spectra of [vinylketene]+' and the branched ketene.…”

“…In an earlier study (13), it was concluded that [vinylketene]+' was the predominant tnlz 68 ion generated by the loss of H 2 0 from cis and trans crotonic acids and cyclopropanecarboxylic acid. However, the metastable peaks for these reactions were of composite shape and it was considered that the [C4H40]+' isomer ionised cyclobutenone might be cogenerated.…”

EI fragmentation of E,Z-3-iodo-2-alkenoic acids and methyl E,Z-3-iodo-2-alkenoates

“…the e-fission and the water loss [19][20][21][22][23] are absent in the EI mass spectra (Table 1). All compounds give molecular ion peaks.…”