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

Abstract

Abstract 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 3 J FH couplings have been studied for the molecules CH 2 FCH 3 , CHF 2 CH 3 , CH 2 FCH 2 F, CF 3 CH 3 , and CHF 2 CH 2 F. The four contributions to 3 J FH ( J FC , J SD , J OD and J OP ) 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 δC 012 FF , δC 034 FF and δC 134 FF as well as the vicinal δC 213 FF and δC 214 FF .