Hat tip to a recent talk by Chris Braddock.

79/81Bromine and 35/37chlorine are easy to see in a mass spectrum, as their isotopic abundance leads to characteristic M/M+2 molecular ion ratios (1:1 for bromine, 3:1 for chlorine).  Still, it can be tough to determine the exact location of said bromine/chlorine by NMR, especially if the compound contains a mix of halides.

13C functional group shifts. The 0-50ppm region is a little crowded.

13C functional group shifts. The 0-50ppm region is a little crowded.


However, with the varied atomic mass the bromine/chlorine isotopes have slightly different C-X bond lengths.  This means the carbons experience ever so slightly different environments, with a corresponding change in shift [1].   Alkyl chlorides experience a change of about 0.1hz, enough to see peak doubling on modern spectrometers (turn off window functions/apodization).

1-Chloro-3-Iodopropane NMR (close-up)Unfortunately bromine causes less than half the same shift, a difference that’s invisible absent extremely high fields or custom pulse sequences.  Under normal conditions the bromine-induced doublet instead shows up as a single peak, about 50% broader than the other carbons in my survey of 1-bromopentane.



[1] The clearest explanation I’ve heard is that the shorter C-X bond length slightly shields the carbons, by bringing electrons from the halide closer to the carbon’s nucleus.  Potentially an oversimplification.