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How do I know if my unknown contains a fluorine atom(s)? … Part 2

April 24, 2008

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.

Cfdeterminationstr_apr222008 Cfdetermination13c_apr222008

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.

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