Ab initio study of hydrogen bonding and proton transfer in 3:1 FH:NH3 and FH:collidine complexes:: Structures and one- and two-bond coupling constants across hydrogen bonds

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH3 complexes at their own optimized MP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions of corresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has an open structure with a proton-shared F-a-H-a-N hydrogen bond, while the isolated equilibrium 3:1 FH:collidine complex has a perpendicular structure with an Fa-Ha-N hydrogen bond that is on the ion-pair side of proton-shared. The F-a-N coupling constant ((2h)J(Fa-N)) for the equilibrium 3:1 FH:NH3 complex is large and negative, consistent with a proton-shared F-a-H-a-N hydrogen bond; (2h)J(Fa-N). is positive, reflecting a short F-b-F-a distance and partial proton transfer from F-b to F-a across the F-b-H-b-F-a hydrogen bond. In contrast, (2h)J(Fa) has a smaller absolute value and (2h)J(Fb-Fa) is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidine geometry, consistent with the structural characteristics of the F-a-H-a-N and F-b-H-b-F-a, hydrogen bonds. Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistent with experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the role of the solvent is to promote further proton transfer from Fa to N across the F-a-H-a-N hydrogen bond, and from F-b to F-a across the two equivalent F-b-H-b-F-a hydrogen bonds. The best correlations between experimental and computed coupling constants are found for complexes with perpendicular proton-transferred structures, one having the optimized geometry of a 3:1 FH:collidine complex at an F-a-H-a distance of 1.80 angstrom, and the other at the optimized 3:1 FH:collidine geometry with distances derived from the experimental coupling constants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidine complex as the structure which exists in solution.