Fragmentation mechanisms

Finally, accurate mass measurement can be used to help unravel fragmentation mechanisms. A very simple example is given in Figure 38.2. If it is supposed that accurate mass measurements were made on the two ions at 203.94381 and 77.03915, then their difference in mass (126.90466) corresponds exactly to the atomic mass of iodine, showing that this atom must have been eliminated in the fragmentation reaction. 

Some transfer agents react by addition-fragmentation (Section 6.2.3) or abstraction-fragmentation mechanisms. Both of these processes involve the formation of a short-lived intermediate. The reaction scheme for addition-fragmentation can be summarized schematically as follows (Scheme 6.3). 

Compounds with a thiocarbonyl a to the S-S bond such as the dithiuram (e.g. 8f2Al and xanthogen disulfides (e.g. 9)M have transfer constants that are much higher than other disulfides. In part, this may be due to the availability of another mechanism for induced decomposition (Scheme 6.9) involving addition to the C S double bond and subsequent fragmentation. Thiocarbonyl double bonds are very reactive towards addition and an addition-fragmentation mechanism has been demonstrated for related compounds (Section 6.2.3.5). 

Most monosulfides generally have very low transfer constants. Exceptions to this rule are allyl sulfides (Section 6,2.3.2) and thiocarbonylthio compounds such as the trithiocarbonatcs and dithiocstcrs (Section 9.5.3) that react by an addition-fragmentation mechanism. 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Other transfer agents which react with propagating species by an addition-fragmentation mechanism include the thionc derivatives (81-83) and RAFT... 

For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

Mass Spectra and Chemical Structure While there are a number of books (Refs 16, 30, 49 64) already referred to, which deal with details of the instrumentation and techniques of mass spectrometry, there are several concise introductory texts (Refs 10, 21 52) on the interpretation of mass spectra. Still other recent books deal comprehensively with organic structural investigation by mass spectrometry. One of these (Ref 63) discusses fundamentals of ion fragmentation mechanisms, while the others (Refs 7, 15, 20, 28 29) describe mass spectra of various classes of organic compounds. In the alloted space for this article methods of interpretation of mass spectra and structural identification can not be described in depth. An attempt is, therefore, made only to briefly outline the procedures used in this interpretation... 

Comprehensive correlation tables of various ion series, characteristic fragment ion and neutral fragments from different classes of compounds are available in various sources (Refs 10, 11 63). A number of general rules in understanding fragmentation mechanisms and for prediction of prominent peaks in a spectrum are summarized in Refs 10 63... 

Snyder AP, Harden CS. 1990. Determination of the fragmentation mechanisms of organophosphorus ions by water and deuterium oxide atmospheric-pressure ionization tandem mass spectrometry. II. Dialkylphosphonate ions. Org Mass Spectrom 25(6) 301-308. 

Thermolytic processes are so far known to play only a minor role in the generation of the PO ion. An isolated case is the thermolysis of the monosodium salt of acetonylphosphonic acid at 150 °C, which leads to acetone and sodium polymetaphosphate 107). A cyclic fragmentation mechanism, as known for P-ketocarboxylic acids, could lead to PO in this case. 

Figure 4.2 Mass spectrum of palmitic acid and main fragmentation mechanisms. This spec trum was obtained by DE MS with a Polaris Q ion trap external ionisation mass spectrometer. Reproduced from Talanta, 74, Ribechini et al., 2008a, Copyright 2008 with permission from Elsevier...

Addition-cured model systems, of silicone networks, 22 569 Addition-curing silicones, 22 35 Addition-fragmentation mechanism, of chain- transfer (CT) agents, 23 383-384... 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

Brief discussions of some topics of this chapter may also be found in physical chemistry textbooks however, much better introductions are given in the specialized literature. [4-11] Detailed compound-specific fragmentation mechanisms, ion-molecule complexes, and more are dealt with later (Chap. 6.). 

Great efforts have been made to generate accurate and reliable ion thermochemistry data (Chap. 1.2.8). Once such data is available, it can be employed to elucidate fragmentation mechanisms and in addition, it is useful for obtaining some background on the energetic dimensions in mass spectrometry. 

D.R. Manrice, T.H. Courtney, Modeling of mechanical alloying Part 1. Deformation, coalescence, and fragmentation mechanisms, Metall. Met. Trans. A 25(1) (1994) 147-148. 

As is illustrated in Figure 9.7, homolytic a-cleavage is the major fragmentation mechanism for chains containing hetero atoms. 

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