Substituent Effects on Chemical Shifts and Spin—Spin Coupling Constants in Trifluorovinyl Derivatives

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Abstract
Examination of the NMR spectra of a variety of trifluorovinyl derivatives of the type CF2=CFY has led to the following correlations between the fluorine NMR parameters and the nature of substituent Y: (1) the chemical shift of the fluorine atom trans to Y and the geminal F–F coupling constant both depend upon the conjugating ability of the group Y, and (2) the chemical shift of the fluorine atom attached to the same carbon as is Y and the cis F–F coupling constant both depend primarily upon the electronegativity of the directly attached atom in substituent Y. The chemical shift of the fluorine cis to Y and the trans coupling constant are not simply related to either of these properties of Y. Although the trans coupling constant is temperature‐independent, the cis and gem coupling constants increase with increasing temperature. Long‐range coupling in a trifluorovinyl thioether compared with that in trifluorovinyl ethers is consistent with d‐orbital participation in bonding of the sulfur atom in the thioether.