Vibrational effects on the F-F spin-spin coupling constant ((2h)J(F-F)) inFHF- and FDF-

Calculating F-F spin-spin coupling constants across hydrogen bonds has repr esented a significant challenge to theory. In this study, ab initio calcula tions 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 surfac e 2hJF-F was generated at EOM-CCSD/(qzp,qz2p), and two-dimensional wave fun ctions 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 norm al coordinate for the bending motion. Although (2h)J(F-F) is dominated by t he Fermi-contact term in the region of the surface surrounding the equilibr ium structure, the paramagnetic spin-orbit and spin dipole terms are import ant in determining the absolute value of (2h)J(F-F). In the ground vibratio nal state, the expectation value of the F-F distance increases, and the exp ectation 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 dec rease in the expectation value of the Fermi-contact term. The ground-state expectation value of the F-F coupling constant is consistent with an experi mental 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 expectat ion value of (2h)J(F-F) in the ground vibrational state is 223.1 Hz. Therma l vibrational averaging at 298 K over lower-energy excited vibrational stat es leas essentially no effect on (2h)J(F-F).