Probing (1)J(C-F) and (n)J(F-F) spin-spin coupling constants for fluoroazines: an ab initio theoretical investigation.

Ab initio equation-of-motion coupled cluster singles and doubles calculations have been carried out to evaluate one-bond C-F coupling constants (1)J(C-F) and three-, four-, and five-bond F-F coupling constants (n)J(F-F) for a series of mono-, di-, and trifluoroazines. The computed (1)J(C-F) and (n)J(F-F) values for these are in good agreement with available experimental coupling constants. The values of (1)J(C-F) vary as the number and positions of N atoms and the number and relative positions of C-F bonds change, but it is difficult to discern general patterns for these changes due to opposing effects of the Fermi contact and paramagnetic spin-orbit terms. The majority of (1)J(C-F) values lie in a range that includes the three monosubstituted pyridines. For trifluoroazines, (1)J(C-F) for a C-F bond that is ortho to two other C-F bonds is greater than (1)J(C-F) for the other two bonds. F-F coupling constants arise in these molecules when the two C-F bonds are ortho, meta, or para. Values of (3)J(F-F) are relatively large and negative, whereas values of (5)J(F-F) are relatively large and positive. (4)J(F-F) may be positive or negative and large or small. The value of this coupling constant depends on the nature of the atom that links the two C-F bonds and the number and positions of N atoms in the ring. The calculations carried out in this study at a reliable level of theory give values for one-bond C-F and n-bond F-F spin-spin coupling constants for the fluoroazines that are not available experimentally. In addition, the patterns that describe the changes that occur in these molecules provide a basis for predicting their values in larger, related systems in the absence of experimental data and direct calculations.