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Does my unknown structure contain Bromine?

June 3, 2008

MS and NMR are complementary elucidation tools. Knowing when to apply the correct tool can facilitate the elucidation process.

Compounds with bromine atoms exhibit a distinct ion pattern on a mass spectrum. The A+2 peak for a monobrominated compound appears at almost identical intensity to the 79Br peak due to the presence of 81Br (~49.3% natural isotope abundance). A compound with two bromines shows a distinct A+4 peak with an approximate ratio of 1:2:1.

The following EI mass spectra are for 3-bromopropanenitrile and (1Z)-1,2-dibromobut-1-ene with nominal masses at 133 and 212 Da, respectively. The top MS shows 3 identified fragments containing bromine (m/z 79/81, 93/95 and 133/135) as noted by the ~1:1 intensity of the ion clusters. The ion peak at m/z 54 does not show a 1:1 ratio since the fragment does not contain a bromine atom.


The MS for (1Z)-1,2-dibromobut-1-ene shows 4 identifiable fragments containing bromine (m/z 117/119, 133/135, 197/199/201 and 212/214/216). In particular, ion clusters at m/z 197/199/201 and 212/214/216 show the distinct ~1:2:1 intensity. This is a result of the varying amounts of the isotopes 79Br and 81Br, that is, the 3 peaks represent 79Br/79Br : 79Br/81Br : 81Br/81Br.


TIP: Check the fragment ion peaks too for the distinct pattern especially when the molecular ion peak is not visible.


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