Probing 1J(C-F) and nJ(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 1J(C−F) and three-, four-, and five-bond F−F coupling constants nJ(F−F) for a series of mono-, di-, and trifluoroazines. The computed 1J(C−F) and nJ(F−F) values for these are in good agreement with available experimental coupling constants. The values of 1J(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 1J(C−F) values lie in a range that includes the three monosubstituted pyridines. For trifluoroazines, 1J(C−F) for a C−F bond that is ortho to two other C−F bonds is greater than 1J(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 3J(F−F) are relatively large and negative, whereas values of 5J(F−F) are relatively large and positive.4J(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.