[vhpk]

Hi everyone,
I've got a concern with mass spectroscopy and hope get the help from everyone. In the book " Organic structure from the spectra", it is stated that

Any molecular ion or fragment which contains 2 bromine atoms will have a pattern of M:M+2:M+4 with the signal fraction in the ration 1:3:1

( assume that the proportions of 2 isotopes are equal)
I don't know how to get the result above as for my calculation, I get 1:2:1.
Any help would be appreciated.
Thank you.

[bromidewind]

I believe this is a typo in your book. You're right, it should be 1:2:1. Take a look at some of these sample mass specs of dibromo compounds. These are two different isomers of the formula C₂H₄Br₂.

<a href="http://webbook.nist.gov/cgi/cbook.cgi?ID=C557915&Units=SI&Mask=200">1,1-dibromoethane</a>
<a href="http://webbook.nist.gov/cgi/cbook.cgi?ID=C106934&Units=SI&Mask=200">1,2-dibromoethane</a>

Zoom in on the three peaks at the end and you can clearly see that the relative abundance of M+, M+2, and M+4 are in a ratio of 1:2:1.

I know that chlorine gives a ratio of 3:1, but I'm not sure what a dichloro compound would give.