(19)F-(19)F and (19)F-(1)H spin-spin coupling constants in cyclic FH polymers (FH)(n), n=2-6.
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 sigma 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.
Del Bene
Janet E
JE
Department of Chemistry, Youngstown State University, Youngstown, OH 44555, USA. jedelbene@ysu.edu
Elguero
José
J