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Monday, January 23, 2017

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.

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