Isotopic pattern halogens

Partial mass spectra showing the isotope patterns in the molecular ion regions for ions containing carbon and (a) only one chlorine atom, (b) only one bromine atom, and (c) one chlorine and one bromine atom. The isotope patterns are quite different from each other. Note how the halogen isotope ratios appear very clearly as 3 1 for chlorine in (a), 1 1 for bromine in (b), and 3 4 1 for chlorine and bromine in (c). If the numbers of halogens were not known, the pattern could be used in a reverse sense to decide their number. 

So far we have treated the X-f1 and the X-f2 elements separately, which is not how they are encountered in most analytes. The combination of C, H, N and O with the halogens F, Cl, Br and I covers a large fraction of the molecules one usually has to deal with. When regarding H, O and N as X elements, which is a valid approximation for not too large molecules, the construction of isotopic patterns can be conveniently accomplished. By use of the isotopic abundance tables of the elements or of tables of frequent combinations of these as provided in this chapter or... 

Example Selective activation of C-H bonds is rarely observed in saturated alkyl groups, but the iridium complex 1 does react by C-H insertion of the metal into a ligand bond upon treatment with LiBr in solution. The reaction can be tracked by LT-FAB-MS (Fig. 9.17). A decreasing intensity of the molecular ion of 1, m/z 812.4, and increasing of 2, m/z 856.4, indicate the progress of this reaction. Furthermore, the halogen exchange is indicated by the isotopic pattern. 

The mass spectra ofbromacil and terbacil and their A-methyl derivatives show only weak molecular ions, but they have the isotope patterns expected from bromine and chlorine compounds, respectively. The major ions arise from [M—55] and [M—56] and also reflect the halogen substituent. Less abundant, but highly characteristic ions, corresponding to [M-99Y and [M-98], are formed by retro Diels-Alder fragmentations. 

Some halogens. Cl and Br, and metals, e.g., Sn, Pt, and Pd, significantly alter the isotope patterns of small molecules... 

Identification of chloro and bromo compounds is a relatively simple matter because of the unique isotopic pattern. The presence of fluoro and iodo compounds, although not easy, can be inferred from the conspicuously low [M - -1]/[M] ratio, which is due to the fact that F and I are monoisotopic. The molecular ion in aliphatic chlorides is visible only in lower monochlorides. With an increase in the number of chlorine atoms, the abundance of the molecular ion decreases further. The t-cleavage to expel a halogen atom often produces an abundant ion (e.g., the base peak in the mass spectrum of r-butyl chloride is C4H9+). The a-cleavage is of low consequence in alkyl chlorides, but the loss of an alkyl radical can be prominent when the alkyl chain is longer than four carbons the product is a flve-membered ring halonium ion ... 

What, then, are the options currently available to the scientist, and is aeeurate mass measurement a help The latter question is largely rhetorieal, especially for anyone who has tried to make sense of product ion spectra obtained on instrumentation capable only of generating nominal mass data. Trivial neutral losses can usually be assigned on the latter instruments, but not always, for example, distinguishing between a neutral loss of methane from that of an oxygen atom is not possible. The presence or absence of a halogen isotope pattern can still be used to limit possible explanations for a product ion. On the other hand, the benefit of accurate mass measurement is dramatic, as noted earlier, in that it affords a dramatic reduction in elemental compositions that are possible explanations of the measured w/z value—and thus limits the number of possibilities that must be considered by the analyst. 

Molecular ion Fragments m/z 69 [M-501+- or [Frag-50]+ For saturated aliphatic halogen compounds often weak, for polyhalogenated compounds often absent Characteristic isotope patterns for Cl and Br CF3 CF2 ... 

Some elements, in particular the halogens chlorine and bromine, which are contained in many active substances, plastics and other technical products, can be recognized by the typical isotope patterns. These easily recognized patterns are shown in Figures 3.37-3.43. The intensities shown are scaled down to a unit ion stream of the isotope pattern. The lowering of the specific response of the compound as a function of, for example, the degree of chlorination, is shown. The simple occurrence of the elements shown as a series is used as a reference in each case. 

Mass spectrometry is particularly useftil in the detection of isotopes. Due to the relatively high percentages of the heavy isotopes, the halogens chlorine and bromine are readily identified by the pattern of peaks in the molecular ion region of their mass spectra (Fig. 10.3). 

A few years ago, the determination of the shape of an isotope pattern of a multi-halogenated ion was a classic university mass spectrometry exercise. Now, many free programs available online allow the instant determination of the shape of an isotope pattern of any ion from its raw formula. The earlier exercise is no longer of interest but a spectrometrist should understand the main principles. The calculations above remain applicable to ions carrying large numbers of halogens but the next section will demonstrate that not all isotopomers may be observed. 

Halogens. The patterns of isotope peaks should indicate the nature and number of halogen atoms in the molecule. This is especially useful for aromatic halogen compounds, but may be less valuable for aliphatic compounds which often exhibit a weaker molecular ion peak. 

Unfortunately, it is not always possible to take advantage of these characteristic patterns to identify halogen compounds. Frequently the molecular ion peaks are too weak to permit accurate measurement of the ratio of the intensities of the molecular ion and isotopic peaks. However, it is often possible to make such a comparison on certain fragment ion peaks in the mass spectrum of a halogen compound. The mass spectrum of 1-bromohexane (Fig. 8.47) may be used to illustrate this method. The presence of bromine can be determined using the fragment ion peaks at m/e values of 135 and 137. 

---