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 (3JH5-H4 = 4.8 Hz, 4JH5-H3 = 1.6 Hz and 3JH4-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, 4JH3-F1 = 3JH3-F2 = 9.8 Hz, 4JH4-F2 = 3.2 Hz and 4JH5-F1 = 1.8 Hz.
Source:
University of Ottawa NMR Facility Blog