Proton, carbon‐13 and fluorine‐19 NMR studies of <i>N</i>‐alkyl‐8‐fluoroquinolinium halides: Relative signs of through‐space ¹H¹⁹F and ¹³C¹⁹F coupling constants

Abstract

Abstract Because of experimental and theoretical interest in long‐range 1 H 19 F and 13 C 19 F 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 5 J (HF) and 4 J (CF), which are associated with the proximate fluorine and methyl groups, were shown to be of the same (positive) sign as the directly bonded 13 C 1 H and vicinal 1 H 1 H coupling constants. In N ‐methyl‐8‐fluoroquinolinium iodide (1a) and chloride (1b) 5 J (HF)=+8.8 and 4 J (CF)=+16.3 Hz, and in N ‐ethyl‐8‐fluoroquinolinium iodide 5 J (HF)=+3.0, 6 J (HF)=+1.5, 4 J (CF)=(+)15.8 and 5 J (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 4 J (CF) values, but the 5 J (HF) values were of the wrong sign and magnitude.