19F– 19F and 19F– 1H spin–spin coupling constants in cyclic FH polymers (FH) n, n=2–6

Abstract

Spin–spin coupling constants 2h J(F–F), 1 J(F–H), and 1h J(H–F) have been obtained for cyclic complexes (FH) n , with n=2–6, from ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations. Although both the Fermi-contact (FC) term and 2h J(F–F) increase and become positive as the cluster size increases, the FC term is not a good quantitative approximation to 2h J(F–F). The paramagnetic spin–orbit (PSO) and spin–dipole (SD) terms which contribute to 2h J(F–F) appear to be sensitive to the orientation of the hydrogen-bonded pair. However, the large increase in the FC term and 2h J(F–F) as the size of the cluster increases is due primarily to the reorganization of σ electron densities in both ground and excited states, and is another manifestation of cooperativity effects in hydrogen-bonded cyclic polymers. The FC term and 1 J(F–H) always increase upon complex formation, but 1 J(F–H) increases only slightly as the size of the cluster increases due to a concurrent decrease in the PSO term. The changes in 1 J(F–H) as a function of polymer size reflect the polarization of electron density away from H and toward F in the ground state, and the electron reorganization which occurs in the excited states which couple to the ground states through the FC and PSO operators. The FC term is a good approximation to 1h J(H–F), and is always negative, indicating that the hydrogen bonds in the FH clusters are traditional hydrogen bonds.