Discussion
The 19F-NMR spectrum of CF2Br - CBr2CN at - 98°C shows :
Answer: C
The spectrum shows two doublets and a singlet.
At -98°C rotation about the C-C bond is slow on the NMR time scale, so each staggered rotamer gives its own spectrum. Looking along the C-C bond, the CF2Br carbon carries F, F and Br, and the CBr2CN carbon carries Br, Br and CN.
In the rotamer where the Br of CF2Br is anti to the CN group, a mirror plane makes the two fluorines equivalent, giving a singlet. In the other two rotamers that Br is anti to one of the bromines; these two are mirror images with identical spectra, and in them the two fluorines are non-equivalent and couple with each other, giving an AB pattern of two doublets. Two singlets or two doublets alone would leave out one kind of rotamer.
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