Metastable ion decomposition

Similar results were obtained for the cleavage of the (S—C) bond in ionized 122Si). For metastable ion decompositions it is observed that the loss of C2D is favoured over C2Hj elimination by a factor of 2.8 1. This value reflects directly the operation... 

Fig. 2.14. Influence of the reverse activation energy on KER and thus, on peak shapes in metastable ion decompositions, suitable experimental setup as prerequisite. From left no or small reverse barrier causes Gaussian peak shape, whereas medium or yields flat-topped peaks and large For causes dish-shaped peaks.

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

The lifetime of every ion will depend on its internal energy. Ion decomposition is a statistical phenomenon that, therefore, depends on internal energy. Thus, metastable ion decomposition can assist in spectral interpretation. 

Peaks which arise from metastable ion decomposition are normally broad and of low intensity. They arise from the fragmentation of ions which have already been accelerated out of the ion source but have not yet reached the magnetic field. They are thus displaced from the position in the spectrum which would correspond to their true mass. The position of the metastable peak (m ) is related to the mass of the precursor ion (m and the mass of the product ion ( m2) by the equation. 

The other situation in which energetic factors predominate is when ions have low internal energies and hence long reaction times. The competition among metastable ion decompositions is, for example, very sensitive to their relative critical energies, E0. This fact underlies recent rationalisations of decomposition pathways of metastable ions [101,111]. 

The collection efficiencies Gx and Gn appearing in eqns. (25) and (26) are almost invariably neglected in determinations of isotope effects, i.e. it is implicitly assumed that Gz = Gu. The product ions m and mJi will have different masses and sometimes different translational energies (e.g. when products of metastable ion decompositions are considered). Neglecting collection efficiencies is, therefore, justifiable only if the instrument has insignificant mass and energy discrimination. For the purposes of the discussion which follows, it will be assumed that these conditions have been met and that Gt and Gn are indeed equal. 

Intermolecular isotope effects have been studied in the El mass spectra (at low energies of 11—35eV) of variously deuterated methanols [197]. Studies of metastable ion decompositions in deuterated methanols revealed small intermolecular isotope effects for H atom loss [76, 536]. 

The intramolecular isotope effect, IH //D, on metastable ion decomposition of benzene to lose a hydrogen atom has been reported as 1.9 [59]. A tandem magnetic deflection/ion cyclotron resonance (ICR) instrument has been used to study isotope effects on metastable ion decompositions of benzene, toluene and anisole in some detail [779]. 

Metastable ion decomposition of hydrogen sulphide to give sulphur ions and molecular hydrogen has been investigated [247]. The metastable peak abundances were in the ratios 16 2 1 for H2S HDS D2S. 

In the 20 eV El mass spectrum of CHD = CHD and CD2CDH, the isotope effects IhJIhd and IHd/Id2 were found to be 1.76 and 1.53, respectively [331]. With the metastable ion decompositions, isotope effects of about 30 were obtained [331, 645]. Isotope effects on loss of molecular hydrogen from all ethylene isomers have been recently... 

Intermolecular isotope effects observed [534] in metastable ion decompositions of (C3H8)t and (C3D8)t again evidence the fact that intermolecular isotope effects on ion abundances are not a reliable guide to kinetic isotope effects. The isotope effect on methane loss (lcH4/fcD ) is 7 cf. 87 for (ICH4 //ch,d) with CH3CD2CHD2 (above). 

Intermolecular isotope effects /c2h4 //c2d4 in the range 1.3 -1.9 have been reported for metastable ion decompositions effecting loss of ethylene from various triazole molecular ions with 7V-ethyl side chains [568, 573]. Subsequent elimination of N2 from the benztriazole fragment ions occurs with an isotope effect of 1.5 (unlabelled vs. perdeuterated ions), perhaps indicating that a hydrogen transfer is involved in the decomposition [573],... 

The o-substituted benzoic acid methyl ester of formula (CH3)2NC6H4C02CH3 has been found to lose both the ester methyl and the amine methyl in metastable ion decompositions. It has been proposed that the bond cleavage to eliminate the ester methyl is accompanied by a hydrogen transfer from one of the other methyl groups to the carbonyl oxygen in a 7-membered cyclic transition state [83]. The mechanistic proposal rests upon isotope effects observed on substituting deuterium in methyl groups. 

No metastable peak has been observed for loss of a methyl group following El of acetaldehyde, CH3CHO however, the metastable ion decomposition to lose CD3 has been seen with CD3CDO [695], This intermolecular isotope effect has been interpreted in terms of the isomerisation (CH3CHO)f - (CH2=CHOH)t occurring in the unlabelled molecule and precluding methyl loss [695]. 

The McLafferty rearrangement in certain carboxylic acids initially forms the ion (CH2 = C(OH)2)t, which then undergoes metastable ion decomposition to lose a hydroxyl radical. With the labelled ions [CD2 = C(OH)(OD)]t and [CH2=C(OH)(OD)]t, the isotope effect 7oh/ od has been measured as 0.38 [526, 753], 0.56 [342] and 0.42 [407], i.e. OD- loss was more probable than OH - loss. This observation has been interpreted [407, 526] in terms of the rate-determining step... 

Isotope effects /Hio/ hdo of 4 .57 [814] and 1.35 [604] have been observed for metastable ion decompositions of [CH3(CD3)CHCH2OH]t and [CH3(CD3)CHCH=OH]+, respectively. Both eliminations have been shown to involve 1, 4 hydrogen transfer. 

Metastable 1- and 2-tetralol ions have been found to lose water by specific 1, 4 and 1, 3 eliminations, respectively. Using a type of internal reference method, an isotope effect of 2.0 on loss of water relative to total metastable ion decompositions was obtained for 1-tetralol [345]. The acetoxy derivatives of 1- and 2-tetralol lose acetic acid by specific mechanisms and no isotope effect was observed in either case [923]. 

From a study of metastable ion decompositions of [C2(H, D)sS] + ions, the average isotope effect, i, for acetylene loss was reported as 1.6 [137]. Isotope effects on metastable ion decompositions of (C3H7S)+ ions have proved difficult to study, because of hydrogen randomisation and facile isomerisation of ion structures. Nevertheless, the metastable ion abundances for H2S and HDS loss from [CH3(CD3)C = SH]+ have been shown to be in the ratio 2.2 1 [136]. 

For ions of initial structure (CH3CH2CH=SH)+ and (CH3CH=SCH3)+, average deuterium isotope effects, i, for metastable ion decompositions effecting loss of hydrogen sulphide of 2.3 and 1.8, respectively, were determined [136]. 

The loss of a methyl radical from the 4- -butylpyridine ion has been shown to exhibit an isotope effect of 1.1 on decomposition in the El source and an isotope effect in the range 1.3—1.6 for metastable ion decomposition [642] [see discussion of t-butylbenzene in Sect. 7.5.4(a)]. 

Theories of translational energy release in unimolecular decompositions are discussed in Sect. 8.1. Qualitative lines of explanation are discussed, in conjunction with the experimental results to which they relate, in Sects. 8.2—8.4. The extensive data on translational energy releases in source reactions, including PIPECO, and in metastable ion decompositions are collected together in tables (Sect. 8.5). The emphasis is on decompositions of polyatomic ions, although many triatomics are included in the tables. The coverage includes both fundamental and mechanistic studies. 

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