2D 19F/19F NOESY for the assignment of NMR spectra of fluorochemicals

Two-dimensional (2D) NMR is an invaluable technique for the complete analysis and assignment of chemical structures. Although 19F/19F COSY experiments are routinely used for assignments in perfluorochemicals, interpretation can be difficult because four-bond (4JFF) coupling constants are typically 5–10-fold larger than vicinal (3JFF) coupling constants. Furthermore, the dependence of long range coupling constants on stereochemistry is not always known. Fluorine–fluorine NOESY correlations represent an enhancement in the arsenal of 2D 19F NMR experiments. The NOESY and COSY spectra of 2,2,3,3,4,4,4-heptafluorobutanol and a telomeric perfluorochemical iodide show that COSY identifies the 1,4-fluorine interactions whereas NOESY identifies the vicinal fluorine atoms. The combined experiments have been used to unambiguously assign all of the fluorines in a mixture of cis- and trans-perfluoro-1,3-dimethylcyclohexane and in a substituted perfluorotetrahydrofuran.