Two-, three-, and four-bond N-F spin-spin coupling constants in fluoroazines
Ab initio EOM-CCSD calculations have been performed to investigate 2-, 3- and 4-bond 15N–19F coupling constants in mono-, di-, and trifluoroazines. 2J(N–F) values are negative and are dominated by the Fermi-contact (FC) term. Absolute values of 2J(N–F) tend to decrease as the number of N atoms in the ring increases, and may also be influenced by the number and positions of C–F bonds. 3J(N–F) values are positive with three exceptions, are usually dominated by the FC term, and also tend to decrease as the number of N atoms increases. The three molecules which have negative values of 3J(N–F) have dominant negative paramagnetic-spin orbit (PSO) terms, and are structurally similar insofar as they have an intervening C–F bond between the N and the coupled F. 4J(N–F) values are negative because the PSO, FC, and spin-dipole (SD) terms are negative, with only one exception. Four molecules have significantly greater values of 4J(N–F). These are structurally similar with the coupled N bonded to two other N atoms. The computed EOM-CCSD nJ(N–F) coupling constants are in good agreement with the few experimental values that are available.