We must remember that molecular species exist that contain less abundant isotopes of carbon, hydrogen, nitrogen, and oxygen which will give rise to isotope peaks at M+1, M+2, etc. In Figure 3, you will notice additional peaks at 122 (M+1) and 123 (M+2). These are peaks due to the presence of isotopes of carbon, hydrogen, nitrogen, and oxygen in benzamide, and are not to be confused with the molecular ion peak. Sometimes, the intensity of the M+1 and M+2 peaks can lead to valuable information about the molecular formula of the compound.
To calculate the intensity of the M+1 peak (with respect to the M+ peak), use the following equation:
To calculate the intensity of the M+2 peak, use the following equation:
The following example shows how the above equations can be used to help confirm the formula for a chemical compound.
The above calculations will only approximate the sizes of the M+1 and M+2 peaks. Also, these formulas are really
only useful if the molecular formula is already known, but they provide a good check on the validity of a proposed
molecular formula.
When bromine or chlorine is present in a compound, the M+2 peak becomes very significant. This is due to the fact that, for bromine, two isotopes (79Br and 81Br) are present in a 1:1 ratio in naturally occurring substances, and, for chlorine, two isotopes (35Cl and 37Cl) are present in a 3:1 ratio in naturally occurring substances. If a compound contains bromine, the M+ and M+2 peaks are present in equal intensities. Additionally, if a compound contains chlorine, the M+ and M+2 peaks will be present in a 3:1 ratio. The presence of these M+ and M+2 peaks are very indicative of brominated and chlorinated compounds. Figure 5 and figure 6 contain the spectrums of chloroethane and bromoethane, respectively, to illustrate the point.
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