| Abstract
| - Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH3 complexes at their own optimizedMP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions ofcorresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has anopen structure with a proton-shared Fa−Ha−N hydrogen bond, while the isolated equilibrium 3:1 FH:collidinecomplex has a perpendicular structure with an Fa−Ha−N hydrogen bond that is on the ion-pair side of proton-shared. The Fa−N coupling constant (2hJFa-N) for the equilibrium 3:1 FH:NH3 complex is large and negative,consistent with a proton-shared Fa−Ha−N hydrogen bond; 2hJFb-Fa is positive, reflecting a short Fb−Fa distanceand partial proton transfer from Fb to Fa across the Fb−Hb−Fa hydrogen bond. In contrast, 2hJFa-N has asmaller absolute value and 2hJFb-Fa is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidinegeometry, consistent with the structural characteristics of the Fa−Ha−N and Fb−Hb−Fa hydrogen bonds.Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistentwith experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the roleof the solvent is to promote further proton transfer from Fa to N across the Fa−Ha−N hydrogen bond, andfrom Fb to Fa across the two equivalent Fb−Hb−Fa hydrogen bonds. The best correlations between experimentaland 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 Fa−Ha distance of 1.80 Å, and theother at the optimized 3:1 FH:collidine geometry with distances derived from the experimental couplingconstants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidinecomplex as the structure which exists in solution.
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