Vicinal fluorine-proton coupling constants
The angular dependence and the effect of individual substituents upon the NMR vicinal fluorine-proton couplings 3JFH have been studied using data sets of experimental and calculated couplings. Coupling constants for a series of fluoroethane derivatives, CHXF-CH3 and CH2F-CH2X (X = CH3, NH2, OH, and F), were calculated by means of the SCF ab initio and semiempirical INDO/FPT methods. The calculated couplings reproduce correctly the main experimental trends in spite of the limitation in the calculation because of lack of electronic correlation and the use of medium size basis set. The individual substituent effects DeltaKXi ni are described by quadratic expressions on the relative electronegativities of substituents DeltachiXi (DeltaKXi ni = k0 ni + kniDeltachiXi + kniiDeltachi2 Xi). A selected data set of 58 experimental couplings, ranging from 1.9 to 44.4 Hz, has been collected from the literature. An extended Karplus equation with 16 coefficients that includes the electronegativity substituent effects has been derived from the experimental data set with a root-mean-square deviation of 1.2 Hz. Copyright 1998 Academic Press.