Interpreting 2h J(F,N), 1hJ(H,N) and 1J(F,H) in the hydrogen-bonded FH-collidine complex

Ab initio EOM-CCSD calculations were performed to determine F-19,H-1, F-19,N-15 and H-1,N-15 spin-spin coupling constants in model complexes FH-NH3 and FH-pyridine as a function of the F-H and F-N distances. The absolute value of (1)J(F,H) decreases and that of (1h)J(H,N) increases rapidly along the proton-transfer coordinate, even in the region of the proton-shared F-H-N hydrogen bond. In contrast, (2h)J(F,N) remains essentially constant in this region. These results are consistent with the recently reported experimental NMR spectra of FH-collidine which show that (1h)J(H,N) increases and (1)J(F,H) decreases, while (2h)J(F,N) remains constant as the temperature of the solution decreases. They suggest that the FH-collidine complex is stabilized by a proton-shared hydrogen bond over the range of experimental temperatures investigated, being on the traditional side of quasi-symmetric at high temperatures, and on the ion-pair side at low temperatures. Copyright (C) 2002 John Wiley Sons, Ltd.