Spectroscopy
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Mass spectra - how do I calculate the intensity ratio of the 84/86/88 peaks of CH2Cl2 (dichloromethane)?
Corrie writes
As you probably know, the two isotopes of chlorine, 35Cl and 37Cl, occur naturally in an approximately 3:1 ratio.
Imagine picking Cl atoms out of a (very big!) model kit to build your dichloromethane molecule. There is a 3 in 4 chance of picking a 35Cl atom as the first one. The second atom has a 3 in 4 chance of being the same isotope. So there is a 9 in 16 chance of both Cl atoms being the lighter isotope, giving a mass peak of 84 for the molecule.
Now imagine picking a 35Cl atom first, as before - a 3 in 4 chance. The probability of picking a second Cl atom as 37Cl instead this time is 1 in 4. This combination could happen the other way around, first picking the heavier isotope, then the lighter one. So there is a (2 x 3) in 16, i.e. a 6 in 16 chance of a mixture of the two Cl isotopes occurring in your molecule.
By now you should be able to work out what the probability of two 37Cl atoms occurring in the molecule is, given that each has a 1 in 4 chance of being picked.
So the 84/86/88 mass spectrum peaks should be in the ratio 9:6:? Hope you can complete the answer?
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updated: 20 November 2006
