Bromine natural abundance

Isotope ratios. For some elements (most notably bromine and chlorine), there exists more than one isotope of high natural abundance e.g. bromine has two abundant isotopes - Br 49 % and Br 51 % chlorine also has two abundant isotopes- Cl 25 % and Cl 75% (Table 4.1). The presenee of Br or Cl or other elements that contain significant proportions (> 1%) of minor isotopes is often obvious simply by inspection of ions near the molecular ion. 

Members of the last group, and especially chlorine and bromine, are most easily recognised from the characteristic patterns of the peaks, spaced at intervals of two mass units, which they produce in the spectrum. Typical patterns for combinations of bromine and chlorine atoms are shown in Figs 3.77 and 3.78. It may be difficult to estimate the number of oxygen atoms due to the low natural abundance (0.20%) of lsO. 

Several chlorine isotopes exist with mass numbers ranging between 32 and 40. The two stable isotopes are Cl and Cl with natural abundances of 75.77% and 24.23% respectively, while the others are radioactive. Bromine also has two stable isotopes, Br and Br, with natural abundances of 50.69% and 49.31% respectively, while the others are radioactive. Iodine has only one stable isotope, and numerous radioactive ones are known. Astatine is known only as its radioisotope see Radioactive Decay). 

If the M and M+2 peaks are of about the same height, the compound contains a bromine atom because the natural abundances of r and Br are of about the same. 

The mjz value of the molecular ion is the summation of all the atomic masses in the molecule, including the naturally occurring isotopes. For organic molecules you will find a small peak M + 1) above the apparent molecular ion mass (M ) value due to the presence of C. The importance of isotope peaks is the detection of chlorine and bromine in molecules since these two elements have large natural abundances of isotopes, e.g. Cli Cl = 3 1 and Br Br = 1 1. The mass spectra produced by molecules containing these atoms are very distinctive with peaks at M + 2 and even M + 4 and M + 6 depending on how many chlorine or bromine atoms are present. The identification of the number and type of halogen atoms is illustrated in Box 30.1. 

Both the chlorine and the bromine data were obtained with die microwave discharge. High concentrations of chlorine and bromine atoms were observed 20 cm. from the discharge after 90 msec, of travel at a pressure of 0.4 mm. of Hg. The chlorine and bromine were found to be almost 100% dissociated when compared to nitrogen dioxide as a quantitative reference. The first derivatives of the absorption curves of plus are shown in Figure 2. The spectra of the two isotopes are easily sorted out from the known natural abundances of CF and The task of sorting out the Br" and Br i spectra was not attempted, since both isotopes are present to about the same extent in bromine and their hyperfine splitting constants are fairly similar. 

For example, the presence of bromine can be determined easily, because bromine causes a pattern of molecular ion peaks and isotope peaks that is easily identified. If we identify the mass of the molecular ion peak as M and the mass of the isotope peak that is two mass units heavier than the molecular ion as M -t- 2, then the ratio of the intensities of the M and M+2 peaks will be approximately one to one when bromine is present (see Chapter 8, Section 8.5, for more details). When chlorine is present, the ratio of the intensities of the M and M + 2 peaks will be approximately three to one. These ratios reflect the natural abundances of the common isotopes of these elements. Thus, isotope ratio studies in mass spectrometry can be used to determine the molecular formula of a substance. 

Although the fragmentation patterns we have described are well characterized, the most interesting feature of the mass spectra of chlorine- and bromine-containing compounds is the presence of two molecular ion peaks. As Section 8.4 indicated, chlorine occurs naturally in two isotopic forms. The natural abundance of chlorine of mass 37 is 32.5% that of chlorine of mass 35. The natural abundance of bromine of mass 81 is 98.0% that of Br. Therefore, the intensity of the M -I- 2 peak in a chlorine-containing compound should be 32.5% of the intensity of the molecular ion peak, and the intensity of the M -I- 2 peak in a bromine-containing compound should be almost equal to the intensity of the molecular ion peak. These pairs of molecular ion peaks (sometimes called doublets) appear in the mass spectra of ethyl chloride (Fig. 8.49) and ethyl bromide (Fig. 8.50). 

Similarly, the observed intensities of the two quartet sets in BrF are about equal, also in good agreement with the ratio of the natural abundance of the bromine isotopes 7 Br iBr= 50.57 49.43 = 1.03 1. 

Bourns (1963) investigated the bromination of anisole-4-sulphonic-2,6- 2 acid. In the presence of 0 0 to 2-Om bromide ions the ratio kgjkjy increases from 1-00 to 1-31. As a competitive reaction bromode-sulphonation takes place, bromine substituting the sulphonic group in the 4-position. Using anisole-4-sulphonic acid containing the natural abundance of the sulphur isotopes. Bourns measured sulphur isotope effects k jk is 1-003, 1-013 and 1-017 for initial bromide ion concentrations of 0-0, 0-05 and 0-50m respectively. This confirms, that bromodesulphonation also follows mechanism (6-2). 

The natural abundance of Li is high enough so that any lithium salt can be used provided it is acceptable in other respects, such as solubility and freedom from unwanted nuclide production. Lithium bromide would be a poor choice since recoil-bromine atoms are also produced. Lithium carbonate is ideal in this respect but suffers from solubility difficulties. Choice of a suitable chemical form for lithium has been covered in reviews by Rowland and Wolfgang (1956) and Wolf (1960). A typical irradiation will involve a 10% (by weight) solution or slurry of an appropriate lithium salt in an organic liquid or it may be a 10% admixture with an organic solid. 

The effect of the A + 2 elements is more striking for Cl and Br because the A + 2 isotope is highly abundant (unlike that of O or S). For example, the contribution of one chlorine atom to the abundance of the M + 2 peak is 32.5%, and that of one bromine atom is 98.0%. In other words, for a compound that contains one chlorine atom, its [M + 2] will be approximately one-third of [M], The contribution of more than one A - - 2 element is calculated from expansion of the binomial (a + fc)", where a is the natural abundance of the light isotope, b that of the heavy isotope, and n the number of atoms of that element present. If two different A + 2 elements are present (e.g.. Cl and Br), the isotopic pattern is calculated by the expansion of the product given in... 

Although, normally, bromine is a =1 1 mixture of Br and Br and with the advent of mass spectroscopy, it is possible to use enriched mixtures of one or the other isomer to solve the same problem, at the time, radioactive isomers were a reasonable choice. For example, Br, a P- and y-emitter, has a half-life of 57 h, while Br has a half-life of 35.7 h. Similarly, although the natural abundance of iodine is 100%, I, a y-emitter, has a half-life of 56days (See, Hughes, E. D., et al). 

The residual vinyl groups of all labelled samples were analysed quantitatively from peak areas in the NMR spectra by two methods. First, the area of the 137 ppm vinyl peak was compared with the area of all of the aromatic carbon signals in the spectrum due to styrene and divinylbenzene carbons in natural abundance. Second, the area of the 137 ppm peak was compared with the area of all of the aliphatic carbon signals in the spectrum, which includes signals from polymerised labelled carbons of the DVB and from all other aliphatic carbons at natural abundance. It was assumed that all carbon atoms in the sample are equally detectable in each NMR spectrum (Table 9.6). All of the labelled polystyrene networks were also analysed by bromination of residual vinyl groups. 

From the natural abundance of the isotopes of chlorine and bromine in Table 14.2, we can conclude that if the M-E 2 peak is one-third the height of the molecular ion peak, then the compound contains a chlorine atom because the natural abundance of Cl is one-third that of Cl. If the M and M-E2 peaks are about the same height, then the compound contains a bromine atom because the natural abundances of Br and Br are about the same. 

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