A New Generalized Karplus-Type Equation Relating Vicinal Proton-Fluorine Coupling Constants to H−C−C−F Torsion Angles

A new seven-parameter Karplus-type relation between vicinal proton−fluorine coupling constants and the corresponding H−C−C−F torsion angles has been developed. The optimum values of the seven parameters were determined using a data set consisting of 57 3JHF values and the corresponding ΦHF torsion angles (ranging from cis, gauche, and trans regions) based upon ab initio structures as well as on the conformational analysis of temperature-dependent 3JHH of monofluorinated nucleosides 1−11, which were also complemented with the data from conformationally fixed compounds 12−22. The best fit generalized Karplus-type equation shows the difference between input and back-calculated 3JHF below 2.9 Hz with the overall rms deviation of 1.38 Hz, amounting to a total of 4% error for transoid coupling of ≈40−46 Hz, which compares very well with about 5% error encountered earlier in the experimental and the back-calculated 3JHH of 8−9 Hz in the latest Karplus−Altona equation (ref 14). This is the first use of a new Karp...