1H and 19F NMR conformational studies of the monofluorostyrenes in solution. Comparison with theory and vapor phase behavior

Abstract

The parameters for the high resolution 1H and 19F NMR spectra of 2-, 3-, and 4-fluorostyrene are reported for solutions in CS2 and acetone-d6 at 300 K. The populations of the planar cis and trans conformers of 2- and 3-fluorostyrene are deduced from the long-range coupling constants involving the meta and α protons. These populations are insensitive to solvent and appear to be in reasonable agreement with previous 6-31G MO computations for the free molecule; they are also compared with populations deduced from recent rotational and vibronic spectra. The long-range coupling constants for the protons of 4-fluorostyrene imply an internal barrier to rotation about the exocyclic carbon–carbon bond very similar to that in styrene, in agreement with the 6-31G results. The signs of the coupling constants involving 19F and the protons in the side chain are reported and discussed in terms of coupling mechanisms for the three molecules. An earlier surmise, of a positive a electron contribution to the coupling constant over six bonds in an all-trans arrangement, is confirmed for the meta and trans-β protons in 3-fluorostyrene. Keywords: monofluorostyrenes, 1H and 19F NMR, conformations, long-range coupling mechanisms, MO calculations of internal barriers.