April 24, 2008
by Arvin Moser, Team Manager, Application Scientists, ACD/Labs
Without a set routine to acquire a 19F NMR spectrum for every single sample, an elucidator must resort to routine NMR experiments for clues to the presence or absence of fluorine.
For the elucidator who routinely acquires a proton decoupled 13C NMR, the spectrum can provide some clues into the presence of fluorine. Carbons in the vicinity of a fluorine atom(s) will show up as split resonances due to 13C-19F couplings.
The segmented 13C NMR spectrum below illustrates four carbons resonances with splitting patterns. The couplings constants are as follows: CF (101 ppm) at 175.4 Hz, CH (70 ppm) at 37.2 Hz, C (48 ppm) at 22.9 Hz, CH (33 ppm) at 19.2 Hz, and CH3 (23 ppm not shown) at 6.0 Hz. The coupling constants themselves are clues to the proximity of the carbon atom to fluorine.
TIP: On a first pass, there may appear to be extra carbons present in the spectrum. It is important to look for patterns such a ‘doubling up’ of carbons. Secondly, it is wise to compare the spectral carbon count to the molecular weight and see if anything is amiss.