Equilibrium constants of charge-transfer complexes determined from 19F nuclear magnetic resonance absorption measurements

Abstract

Measurements of the line position of 19F nuclear magnetic resonance absorptions of 2,3,5,6-tetrafluoro-p-benzoquinone (fluoranil) and of 1,4-dicyano-2,3,5,6-tetrafluorobenzene in the presence of various concentrations of methylbenzenes have been made. These observations have enabled the equilibrium constants for the appropriate electron-donor–acceptor complexes to be calculated. The results show the expected increase in stability in the two series of complexes with increasing methylation of the electron-donor molecule. Fluoranil forms stronger complexes than 1,4-dicyano-2,3,5,6-tetrafluorobenzene. The complexing ability of the latter is about equal to that of 1,3,5-trinitrobenzene. For a given complex, the association constant decreases as the solvent is altered in the series carbon tetrachloride, chloroform, 1,2-dichloroethane-dichloromethane. For the series of methylbenzene–fluoranil complexes in carbon tetrachloride, the chemical shift of the fluorine nuclear magnetic resonance in the complex, relative to the chemical shift in uncomplexed fluoranil, increases as the stability of the complex increases. These values of equilibrium constant have been used to recalculate the molar extinction coefficients of some complexes.