Vibrational Effects on the F−F Spin−Spin Coupling Constant (2h J F - F) in FHF- and FDF

Calculating F−F spin−spin coupling constants across hydrogen bonds has represented a significant challenge to theory. In this study, ab initio calculations have been carried out to evaluate vibrational effects on the F−F spin−spin coupling constant (2h J F - F) for FHF-. The coupling-constant surface 2h J F - F was generated at EOM-CCSD/(qzp,qz2p), and two-dimensional wave functions for the symmetric and asymmetric stretching vibrations were obtained from the CCSD(T)/aug‘-cc-pVTZ potential surface. The effect of the FHF- bending mode was examined using one-dimensional calculations along the normal coordinate for the bending motion. Although 2h J F - F is dominated by the Fermi-contact term in the region of the surface surrounding the equilibrium structure, the paramagnetic spin−orbit and spin dipole terms are important in determining the absolute value of 2h J F - F. In the ground vibrational state, the expectation value of the F−F distance increases, and the expectation value of 2h J F - F decreases to 212.7 Hz, significantly less than the equilibrium value of 254.4 Hz. This decrease is due primarily to a decrease in the expectation value of the Fermi-contact term. The ground-state expectation value of the F−F coupling constant is consistent with an experimental estimate of 220 Hz, obtained by extrapolation of experimental values of 2h J F - F for larger clusters of [F(HF) n ]−. For FDF-, the expectation value of 2h J F - F in the ground vibrational state is 223.1 Hz. Thermal vibrational averaging at 298 K over lower-energy excited vibrational states has essentially no effect on 2h J F - F.