The use of fluorinated probes in Dynamic NMR spectroscopy

Variable temperature NMR spectroscopy, often referred to as Dynamic NMR (DNMR), is a powerful tool for investigating stereodynamic processes. Separate (anisochronous) signals observed at appropriate temperatures, when the motion is slow, broadens on warming and eventually coalesce when the motion becomes fast on the NMR timescale. By computer simulation of the band shape the rate constants (k values) can be derived in the temperature range where this feature is detectable. From the k values the free energy of activation (G≠ in kcal mol-1) can be extracted by means of the Eyring equation (1). The biphenyl scaffold can be used for the evaluation of steric parameters (“B value”) of various groups, by monitoring the rotational baddiers of appropriate biphenyls by dynamic DNMR. To detect the formation of conformational enantiomers the use of a diastereotopicity probe, containing two geminal identical group bonded to a tetrahedral carbon, is required. When the motion slow down, the NMR signal splits into two equivalent signals and the barreir for the formation of the enantiomers can be determined. In this framework, a fluorinated probe can take advantage of the very large chemical shift scale of 19F spectra. By making use of a novel diastereotopicity probe, i.e. C(CF3)2OH, it has been possible to measure by very low temperature 19F NMR spectroscopy the elusive aryl-aryl rotation barriers of biphenyls bearing the OH and F groups in one ortho position.