Proton, Carbom-13 and FLuorine-19 NMR Studies of N-Alkyl-8-fluoroquinolinium Halides: Relative Signs of Through-space 1H-19F and 13C-19F Coupling Constants

Article abstract of DOI:10.1002/mrc.1270200206

Because of experimental and theoretical interest in long-range 1H-19F and 13C-19F coupling constants between proximate atoms, a series of N-alkylfluoroquinolinium compounds was synthesized and the NMR parameters were accurately measured.By means of spin tickling and selective population inversion difference spectroscopy, the 5J(HF) and 4J(CF), which are associated with the proximate fluorine and mrthyl groups, were shown to be of the same (positive) sign as the directly bonded 13C-1H and vicinal 1H-1H coupling constants.In N-methyl-8-fluoroquinolinium iodide (1a) and chloride (1b) 5J(HF) = +8.8 and 4J(CF) = +16.3 Hz, and in N-ethyl-8-fluoroquinolinium iodide 5J(HF) = +3.0, 6J(HF) = +1.5, 4J(CF) = (+)15.8 and 5J(CF) = +/= 5.2 Hz. calculated molecular orbital results, which were based on the INDO-FPT method and a molecular geometry from the x-ray diffraction data for 1b, were in reasonable agreement with the 4J(CF) values, but the 5J(HF) values were of the wrong sign and magnitude.