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

摘要

Spin-spin coupling constants 2hJ(F-F), 1J(F-H), and 1hJ(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 2hJ(F-F) increase and become positive as the cluster size increases, the FC term is not a good quantitative approximation to 2hJ(F-F). The paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms which contribute to 2hJ(F-F) appear to be sensitive to the orientation of the hydrogen-bonded pair. However, the large increase in the FC term and 2hJ(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 1J(F-H) always increase upon complex formation, but 1J(F-H) increases only slightly as the size of the cluster increases due to a concurrent decrease in the PSO term. The changes in 1J(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 1hJ(H-F), and is always negative, indicating that the hydrogen bonds in the FH clusters are traditional hydrogen bonds. © 2007 Elsevier Inc. All rights reserved.