Two-, three-, and four-bond N–F spin–spin coupling constants in fluoroazines

Abstract

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.

Keywords:

• fluoroazines
• NMR coupling constants
• EOM-CCSD calculations

Acknowledgements

The authors thank the Ohio Supercomputer Center for continuing computational support. This work was carried out with financial support from the Spanish Ministerio de Educación y Ciencia (Project No. CTQ2009-13129-C02-02) and Comunidad Autónoma de Madrid (project MADRISOLAR, ref S-0505/PPQ/0225).