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 ...

Primary fragmentations of benzaldoxime include loss of H', OH', HCN, CO and HCNO and this paper discusses the first three losses. The previously reported losses of H 2 0 and 0 were proven to be thermal in origin. Information about the investigated fragmentations was obtained from the deuterated analogues and their metastable characteristics. The OH' elimination is complex. The loss of HCN exhibits a metastable with a composite structure. This fragmentation may occur both via a 4-centred and 5-centred mechanism. A fluorine atom at the ortho position of the phenyl ring favours the 5-centred mechanism.

The effect of a phenyl group o'n the mechanisms of isomerization in the ionized methyl esters of simple a,P-unsaturated acids (methyl acrylate and related compounds) has been investigated with the aid of deuterium labelling as well as inlformation from mass analysed ion kinetic energy spectra and first field free region metastable peak shapes. Substitution of a hydrogen atom of the 0-CH, group by a phenyl group (benzyl acryiate and homologues) greatly enhances the rate of [ester]' 4 [acid]? isomerhations (loss of H,O and COOH'). It is inferred that this is due to an accelerating effect of the phenyl group on the first and the third steps (ring opening of the key intermediate ion which has the structure of the ionized enol form of ybutyrolactone) of the reaction whose mechanism is basically the same as that in methyl acrylate. A phenyl group present at the a-or &position of the vinylic double bond appears to suppress the [ester]?-+[acid]? isomerization in some compounds by promoting other reactions or opening up a new reaction pathway, i.e. the loss of CH,O from ionized methyl atropate for which mechanisms are proposed.

Isomerization of oxime molecular ions into dtrone molecular ions upon electron impact does not generallyoccur, but it was established with the aid of deuterium labelling tbat it is essential for loss of CH; from o-metbylbenzaldoxime.

It is shown that the elemental composition of a product ion generated from a multiply charged ion in a charge separation reaction can be determined with a double focusing instrument under conditions of high mass and energy resolution. Results for a number of charge Separation reaction$ in benzylamine are briefly discussed.Useful structural information can be obtained from the estimation of the intercharge distance in doubl charged ions decomposing according to the reactionThis type of reaction is generally studied in compounds containing one or more. hetero atoms which facilitates the accommodation of the two positive charges.' A problem frequently encountered in this connection is the determination of the elemental composition of the ions [m2]+ and [m3]+ generated in these charge separation reactions.We wish to report that these elemental compositions can be established unambiguously from the presence of satellite peaks in an accelerating voltage scan under conditions of high energy resolution.Due to the mutual repulsion of the two positive charges, ions [m2]' and [m3]' generated from a doubly charged ion will have a somewhat higher kinetic energy than those generated from a singly charged precursor. In spectra taken with a single focusing instrument this results in the formation of satellite peaks to the high mass side of the normal peak^;^,^ a well known example is the [CH3]+ ion the spectrum of benzene, which is accompanied by an intense satellite peak due to the reaction: [7812+ -+ [63]++[15]'.3 These satellites are not observed in spectra obtained with a double focusing instrument because of its energy f ocusine properties.They can, however, be detected easily by setting the magnet current to collect the main beam of ions at a selected mle value and performing a narrow scan of the accelerating voltage under conditions of high energy resolution. Ions of the same elemental composition generated in tbe ion source from the decomposition of a doubly charged ion are then observed as satellite peaks at the low voltage side of the main beam of ions. This is shown in Fig. 1