PSYCHE to Evaluate 1H-19F Coupling Constants

Even small molecules can yield very complex ¹H NMR spectra as the result of spin - spin coupling.  This is particularly true for small molecules that contain fluorine.  It can sometimes be challenging to determine which splittings are due to ¹H-¹H coupling and which are due to ¹H-¹⁹F coupling.  One can collect a ¹H spectrum with ¹⁹F decoupling to give a spectrum with only ¹H-¹H coupling present.  Even with this data, it may be difficult to evaluate the ¹H-¹⁹F coupling constants by comparing the ¹H[¹⁹F] spectrum to the ¹H spectrum due to the complexity of the multiplets.  The ¹H-¹⁹F coupling constants can however be read directly from a ¹H PSYCHE spectrum.  The PSYCHE spectrum provides a ¹H decoupled ¹H spectrum, leaving only the ¹H-¹⁹F coupling behind.  The bottom trace of the figure below shows the 300 MHz ¹H NMR spectrum of 2,3-difluoro pyridine.  The spectrum is quite complex, making it difficult to assign ¹H-¹H and ¹H-¹⁹F couplings.  The middle trace shows the ¹H[¹⁹F] spectrum which allows the evaluation of all of the ¹H-¹H coupling constants (³J_H5-H4 = 4.8 Hz, ⁴J_H5-H3 = 1.6 Hz and ³J_H4-H3 = 8.0 Hz.  The top trace shows the ¹H PSYCHE spectrum which allows one to evaluate all of the ¹H-¹⁹F coupling constants.  For this compound, ⁴J_H3-F1 = ³J_H3-F2 = 9.8 Hz, ⁴J_H4-F2 = 3.2 Hz and ⁴J_H5-F1 = 1.8 Hz.