Vicinal fluorine-proton coupling constants. Ab initio calculations of angular dependence and substituent effects Original Research Article

A data set of vicinal fluorine-proton coupling constants has been calculated by means of the SCF ab initio and semiempirical INDO/FPT methods. The angular dependence, the effect of individual substituents, and the effect of interaction between two substituents upon the 3JFH couplings have been studied for the molecules CH2FCH3, CHF2CH3, CH2FCH2F, CF3CH3, and CHF2CH2F. The four contributions to 3JFH (JFC, JSD, JOD and JOP) have been computed using the standard basis sets 6-31G, 6-31G∗, 6-31G∗∗ and 6-311G∗∗ and a double zeta basis set [4s2p1d/2s1p] with additional tight s functions on the H and F. The agreement with the experimental data is better for the last basis set but the trends of the angular dependence and substituent effects are also reproduced by the remaining basis sets. The major contribution arises from the FC term and the remaining contributions are much smaller being the OP the most important. The individual effect of an electronegative substituent depends on the carbon to which is bonded, being more important when the substituent is bonded to the carbon with the coupled hydrogen. The effect of interaction between two substituents seems to be not negligible, reaching values up to 6 Hz. The most important calculated interaction effects are the geminal δC012FF, δC034FF and δC134FF as well as the vicinal δC213FF and δC214FF.