Interpreting 2hJ(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 19F,1H, 19F,15N and 1H,15N 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 1J(F,H) decreases and that of 1hJ(H,N) increases rapidly along the proton-transfer coordinate, even in the region of the proton-shared F—H—N hydrogen bond. In contrast, 2hJ(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 1hJ(H,N) increases and 1J(F,H) decreases, while 2hJ(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 © 2002 John Wiley & Sons, Ltd.