Chapter 13: Spectroscopy

Isotope patterns for -Cl and -Br

• Isotopes are different types of the atoms that have the same atomic number (i.e. same number of protons in the nulceus) but different mass numbers (because there are a different number of neutrons in the nucleas). Hence they are the same element but the isotopes have dfferent masses.
• Mass spectrometers are capable of separating and detecting individual ions even those that differ only by a single atomic mass unit (note in reality mass spectrometers are far more sensitive than that !)
  
• As a result, molecules containing different isotopes can be distinguished.
• This is most apparent (at this level) when atoms such as bromine or chlorine are present in a molecule because those elements naturally exist with a significant % of the heavier isotope.
• For example, while C has 2 common isotopes, ¹²C and ¹³C, ¹³C represents only about 1% of natural carbon. In contrast, Cl has 2 common isotopes, ³⁵Cl and ³⁷Cl, with about 25% being ³⁷Cl.
• Typically, one looks at the molecular ion peak, "M" (since this is being identified and used to determine the MW).
• When working with MW from the molecular ion in MS, the best approach is to always use the lighter ion (i.e. M) and the mass of the lighter isotope (i.e. for Cl use 35 not 35.5, or, for Br use 79 and not 80).
• Note that it is not a good idea to look for Cl at m/z = 35 and 37 or Br at m/z = 79 and 81 as these peaks are typically very small.

• ³⁵Cl : ³⁷Cl exists naturally in an almost 3:1 ratio, so we observe peaks at "M" (molecules with an atom of ³⁵Cl) and "M+2" (molecules an atom of ³⁷Cl) are obtained with relative intensity 3:1
  
  

• ⁷⁹Br : ⁸¹Br exists naturally in an almost 1:1 ratio, so we observe peaks at "M" (molecules with an atom of ⁷⁹Br) and "M+2" (molecules an atom of ⁸¹Br) are obtained with relative intensity 1:1
  

• Note that since the relative natural abundances of the isotopes are different, you can tell the difference between the presence of Cl and Br. The patterns are different, they look different.
• "M+1" peaks are usually seen due to the presence of ¹³C in the sample but because 13C is only about 1% of natural carbon, the peaks tend to be small (unless there is a large number of C atoms present). Note that you can see the small peaks due to the presence of ¹³C in the figures shown for Cl and Br, they look like little shadows on the right of the other peaks.

The following two examples of mono-haloalkanes mass spectra show the characteristic isotope patterns of monohalogenated molecules. The patterns are highlighted in the green boxes:

Example 1 :

This MS is of 2-chloropropane, C₃H₇Cl.

Note the characteristic Cl isotope pattern at 78 (M) and 80 (M+2) in a 3:1 ratio. Careful inspection also shows very small peaks at 79 and 81 that are due to the presence of 13C isotopes.
Loss of ³⁵Cl from M = 78 or ³⁷Cl from 80 gives the base peak a m/z = 43 (M - 35 = M+2 - 37 = 43) corresponding to the secondary propyl cation. Since the Cl has been lost, the M = 43 peak doesn't show a Cl isotope pattern.
Note that the peaks at m/z = 63 and 65 represent fragment ions (M - 15 = loss of CH₃) that still contain Cl and therefore also show the 3:1 isotope pattern.
The very small peak at 79 represents M+1, the small number of molecules that contain ³⁵Cl and an atom of ¹³C rather than ¹²C.
The even smaller peak at 81 presents M+2+1 = M+3, a very small number of molecules that contain ³⁷Cl and an atom of ¹³C rather than ¹²C.
The peak at M = 44 is related to the M = 43 peak in that it represents the secondary propyl cation but those with an atom of ¹³C rather than ¹²C.

Example 2 :

This MS is of 1-bromopropane, C₃H₇Br.

Note the isotope pattern at 122 and 124 represents the M and M+2 in a 1:1 ratio. Careful inspection also shows very small peaks at 123 and 125 that are due to the presence of 13C isotopes.
Loss of ⁷⁹Br from 122 or ⁸¹Br from 124 gives the base peak a m/z = 43, corresponding to the propyl cation.
Note that other peaks, such as those at m/z = 107 and 109 (yes, they are small) still contain Br and therefore still show the 1:1 isotope pattern.

Note: the isotope patterns for polyhalogenated molecules (such as having both -Cl and -Br or with multiple -Cl or -Br) give different (but still characteristic isotope patterns).

© Dr. Ian Hunt, Department of Chemistry