1 H and 19 F NMR conformational studies of the monofluorostyrenes in solution. Comparison with theory and vapor phase behavior
1990; NRC Research Press; Volume: 68; Issue: 8 Linguagem: Inglês
10.1139/v90-212
ISSN1480-3291
Autores Tópico(s)Analytical Chemistry and Chromatography
ResumoThe parameters for the high resolution 1 H and 19 F NMR spectra of 2-, 3-, and 4-fluorostyrene are reported for solutions in CS 2 and acetone-d 6 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 19 F 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, 1 H and 19 F NMR, conformations, long-range coupling mechanisms, MO calculations of internal barriers.
Referência(s)