I have several molecules, each with 2 chlorine atoms and all of them are showing M-1 value in their respective EI spectrum. Checked with NIST library, there too it is same.
Generally, a molecule containing one Chlorine atom would give molecular ions M+ and M+2 in a 3:1 ratio, because Chlorine contains 3 times as much of the 35Cl isotope as of the 37Cl isotope.
In your case (2 Chlorine atoms), one would expect to get M+, M+2 and also M+4, because now there's more possible combinations of the aforementioned isotopes. I think your M-1 comes from something else. Do you have any aromatic structures with for example ether-, aldehyde- or amino functions or perhaps a benzylic hydroxyl?
Regards,
JR
Chlorine has two isotopes 35Cl and 37Cl. The intensity of M+2 intensity would be 2/3 height of the M. If M is 100% then M+2 is 66.67%.
If you have two atoms you have to choose the first one. There is a 3/4 possibility of choosing a 35Cl
Then you have to choose the second atom. Once again there is 3/4 chance of choosing a 35Cl
So the overall probability of choosing a 35Cl35Cl molecule is 3/4 x 3/4 = 9/16 (M)
Likewise the probability of getting a 37Cl37Cl is 1/4 x 1/4 = 1/16 (M+4)
Hence the probability of 37Cl35Cl = 6/16 (M+2)
So the ratio is 9 : 6 : 1
In mass spectrum, M-1 at atomic mass 34 is due to either HCl, H2S, etc......
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