| Abstract
| - Calculating F−F spin−spin coupling constants across hydrogen bonds has represented a significant challengeto theory. In this study, ab initio calculations have been carried out to evaluate vibrational effects on the F−Fspin−spin coupling constant (2hJF-F) for FHF-. The coupling-constant surface 2hJF-F was generated at EOM-CCSD/(qzp,qz2p), and two-dimensional wave functions for the symmetric and asymmetric stretching vibrationswere obtained from the CCSD(T)/aug‘-cc-pVTZ potential surface. The effect of the FHF- bending mode wasexamined using one-dimensional calculations along the normal coordinate for the bending motion. Although2hJF-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 2hJF-F.In the ground vibrational state, the expectation value of the F−F distance increases, and the expectationvalue of 2hJF-F decreases to 212.7 Hz, significantly less than the equilibrium value of 254.4 Hz. This decreaseis due primarily to a decrease in the expectation value of the Fermi-contact term. The ground-state expectationvalue of the F−F coupling constant is consistent with an experimental estimate of 220 Hz, obtained byextrapolation of experimental values of 2hJF-F for larger clusters of [F(HF)n]−. For FDF-, the expectationvalue of 2hJF-F in the ground vibrational state is 223.1 Hz. Thermal vibrational averaging at 298 K overlower-energy excited vibrational states has essentially no effect on 2hJF-F.
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