I am observing four extraneous peaks on 19F, each at around 0.5% of the main signal at +11Hz, +88Hz, -17Hz, and -185Hz (+ is downfield) away from the main signal. Due to legal issues, I am not able to divulge the structure of the main compound, but can tell you that the fluorine signal comes from three isochronous benzylic flourines (i.e. a (trifluoromethyl)benzene constituent of a larger compound isolated as a salt). These four peaks do not appear to be impurity related since chiral and achiral HPLC methods reveal the material to be immaculate, while C13 NMR did not give enough sensitivity to see at 0.5% (although I might be able to analyze concentrated sample for an extended amount of time) and 1H NMR is very complicated (could C13 decouple and not spin).. Further, different lots as well as recrystallizations of the material show the same relative intensity of the four peaks, which should, theoretically, lower the impurity level. The peaks are not spin-side bands as I had analyzed without spinning and was able to exclude spin-side bands when I did spin the material. These peaks do not appear to be C13 satellites as these should show up as symmetrical peaks equidistant from the main signal (?). What other artifacts could possibly explain the appearance of these four signals or what other tests could I do to evaluate the signals as real or artifacts?
asked Oct 04 '11 at 11:24
I wouldn't rule out 13C satellite peaks. The intensity seems right. The 13C-19F isotope shift can be fairly large, so the satellites wouldn't be symmetric. The isotope shift would be constant in ppm, but in Hz it would scale with field. The inner pair would probably be two-bond (maybe three bond???) and the outer pair would be one bond. Does this make any sense? The sure check would be if you had the facility to do 13C decoupling.
Just ran a trifluorotoluene sample on our AV 300. The one-bond J(FC) that I see (273 Hz) is exactly the difference between your outer lines, and the longer range is the same as your inner lines. To compare the isotope shift, I would have to know what field your spectrometer is.