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

摘要

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH 3 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 Fa-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 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 distance and partial proton transfer from Fb to Fa across the F b-Hb-Fa hydrogen bond. In contrast, 2hJFa-N has a smaller absolute value and 2hJFb-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 Fa-Ha-N and Fb-H b-Fa 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 Fa-Ha-N hydrogen bond, and from Fb to Fa across the two equivalent F b-Hb-Fa 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 Fa-Ha distance of 1.80 Å, 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. © 2006 American Chemical Society.