Dear Takuya, thank you. 

Regards,

Amandeep.

Dear Amandeep,

The sign of 19F-19F couplings can be confirmed experimentally using alignment in liquid crystal media ( K. G. Orrell,   J. Chem. Soc., Faraday Trans. 2, 1981, 77, p273-286  ), however, analysis of such NMR spectra is non-trivial.

In the isotropic phase, the relative sign of 19F-19F couplings can be tricky to determine just by looking at the NMR spectrum, especially for highly second-order spin-systems, and this can be further complicated by long-range 19F-19F couplings . Spectral analysis can be used to iteratively refine coupling constants (I use NUMMRIT in SpinWorks).

A useful starting point:  

3JFF in a fluoro-aromatic compound is approximately -20 Hz (negative coupling).

4JFF for AA’ in a fluoro- aromatic can be positive or negative depending on the compound.

5JFF in a fluoro-aromatic compound may need to be considered.

A paper by Ray Abraham will be published soon, which provides a method to calculate the size and sign of 4J and 5J 19F-19F couplings in fluoro-aromatic compounds.

Many years ago, analysis of fluoro-aromatic compounds (C6F5-X) produced the size of the nJFF couplings (I. J. Lawrenson, J. Chem. Soc. 1965, p. 1117), and speculated on the sign, with reference to  Evans, Mol. Phys., 1963, 6, p179.

The size and sign of couplings in perfluorotoluene (octafluorotoluene) were published by  C. W. Haigh, J. Hilton L. H. Sutcliffe, and G. J. T. Tiddy,  J. Mag. Reson.,  18, p 241-247 (1975), and coupling constant values updated in the 2016 paper of Abraham.

Chemical shifts (Hz):

1          A         8554.269

2          A'         8554.269

3          B         1058.785

4          B'         1058.785

5          C         5848.748

6          CF3    39946.600

Scalar coupling constants (Hz):

[A,A'] =            -7.54

[A,B] =            -21.5

[A,B'] =            8.50

[A,C] =            5.52

[A,CF3] =       22.0

[A',B] =            8.50

[A',B'] =           -21.5

[A',C] =           5.52

[A',CF3] =       22.0

[B,B'] =            -0.57

[B,C] =            -20.99

[B,CF3] =       0.59

[B',C] =           -20.99

[B',CF3] =       0.59

[C,CF3] =       1.34

Dear Mark, Thank you very much for your detailed response. I'll work through your suggestions.

Regards,

Amandeep.