Chlorine natural abundance

The experiments we have so far described have been used to study nuclei with spin I = Vi ( ll, 13C, 31P). Our model compounds 1 and 2 contain two further atoms (oxygen and chlorine), which have no NMR-active isotope with spin Vi. Oxygen does however have an NMR-active isotope with spin I = 5/2 but very low natural abundance (0.037%) this is 170. Chlorine has two NMR-active isotopes 35C1 (I = 3/2,75.53%) and 37C1 (I = 3/2,24.47%). 

ISOTOPES There are a total of 25 isotopes of chlorine. Of these, only two are stable and contribute to the natural abundance on Earth as follows Cl-35 = 75.77% and Cl-37 = 24.23%. All the other 23 isotopes are produced artificially, are radioactive, and have half-lives ranging from 20 nanoseconds to 3.01 x 10+ years. 

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. 

Fig. 14 Natural abundance carbon-13 spectrum of 1 (50 MHz) with expansion where necessary to show doublet structure. The assignments are as follows (from left to right), aromatic C bonded to oxygen (doublet) aromatic C bonded to chlorine (singlet) aromatic CH (singlet) methine (doublet) CDC13 OCH2 (doublet) CH3 (doublet). Multiplet splittings are due to coupling with phosphorus and are (except for Jpc) small...

This example illustrates how m/e values of ions that differ only in isotopic composition can be used to determine elemental compositions. The important isotopes for this purpose in addition to those of chlorine are the stable isotopes of natural abundance, 13C (1.1%), 15N (0.37%), 170 (0.04%), lsO (0.20%). As a further example, suppose that we have isolated a hydrocarbon and have determined from its mass spectrum that M + = 86 mass units. In the absence of any combination reactions there will be an (M + 1)+ ion corresponding to the same molecular ion but with one 13C in place of 12C. The intensity ratio (M + 1 )+/M+ will depend on the number of carbon atoms present, because the more carbons there are the greater the probability will be that one of them is 13C. The greater the probability, the larger the (M + 1 )+/M+ ratio. For n carbons, we expect... 

Chlorine has two naturally occurring isotopes f 7CI with a natural abundance of 75.77% and an isotopic mass of 34.969 amu, and 17CI with a natural abundance of 24.23% and an isotopic mass of 36.966 amu. What is the atomic mass of chlorine ... 

Now if a molecule contains more than one chlorine atom, the appearance of isotope clusters can be calculated by the probabilities of isotope distributions and the natural abundances of die isotopes. For example, if a molecule contains two chlorine atoms such as o-dichlorobenzene, dien diere will be peaks at M, M + 2, and M + 4 for molecules which have two 35C1, one 35C1 and one 37C1, and two 37C1 (Figure 11.48). 

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. 

Chlorine is abundantly available in NaCl and in saltwater. Hence, the quantity of chlorine combined in these natural sources is enormous. The Great Salt Lake contains 23% salt, and the Dead Sea contains about 30%. Chlorine also occurs in a few minerals, but the abundance of naturally occurring salt water makes these of little importance. 

EXAMPLE 2.2 Natural chlorine (Z = 17) is composed of two isotopes, 35C1 and 37C1. The atomic mass listed for chlorine in the periodic table is 35.5. (a) What is the composition of each nucleus (b) What is the natural abundance of each isotope (You may assume for the purposes of this question that the exact mass of each isotope is exactly equal to its mass number, though in general this is not the case.)... 

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+2 peak is one-third the height of the molecular ion peak, the compound contains a chlorine atom because the natural abundance of Cl is one-third that of the Cl. 

FIGURE 12.12 Electrospray ionization mass spectra of bupropion (solid line) and [ Hgjhydroxy-bupropion (dotted line). Note that the protonated molecular ions (MH+, respectively, at mjz 256 and 262) exhibit characteristic chlorine isotope peaks that have 25% of the molecular ion intensity at m/z 258 and 264, due to the relative natural abundance of Cl and Cl. This is reflected also in the MH+-H2O ions at mjz 238 and 244. Data provided by R.L. Walsky and R.S. Obach, Pfizer, New York, NY. 

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. 

The number of protons in an atom is sort of like the characteristic that distinguishes a human as male. If a person has this characteristic, then he is male. If an atom has a certain number of protons, then that decides what element it is. But different males can have different masses. We could easily find the average mass of a sample of males by adding all their individual masses and dividing by the number in the sample. The same procedure is followed for elements on the periodic table. As can be seen in the appendix, the mass for chlorine is given as 35.45 amu, which is not the mass of either isotope. It is an average over the natural abundance of isotopes. However, it is the number of protons that determines the element, so even though isotopes may... 

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. 

The number of elements that are known to be biologically important comprises a relatively small fraction of the 109 known elements. Natural abundance limits the availability of the elements for such use. Molybdenum (Z = 42) is the heaviest metal, and iodine (Z = 53) is the heaviest nonmetal of known biological importance. The metals of importance in enzymes are principally those of the first transition series, and the other elements of importance are relatively light sodium, potassium, magnesium, calcium, carbon, nitrogen, phosphorus, oxygen, chlorine, and, of course, hydrogen. 

In nature, 75.771 % of all chlorine atoms, by weight, are Cl-35, and 24.229% are the heavier isotope, Cl-37. The percentage distribution of the isotopes of an element is called the percent natural abundance or just natural abundance of the isotopes. All samples of chlorine, no matter the source, have the same percent natural abundance of the two isotopes. It is a nonchanging mixture of isotopes, but remember that even though they are isotopes of one another, they are still atoms of the same element and have the same chemical properties. For comparison, the isotope composition of two other elements, carbon and magnesium, are given in the following table. 

To determine the weighted-average atomic mass of chlorine, which has two isotopes, Cl-35 and Cl-37, the natural abundance and the atomic mass of each isotope must be known. Both are given in the following table. 

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