Ab Initio Study of Complexes with Two Cations as N−H Donors to F^-:  Structures and Spin−Spin Coupling Constants across N−H−F Hydrogen Bonds

A systematic ab initio study has been carried out to determine the MP2/6-31+G(d,p) structures and EOM-CCSD coupling constants across N−H−F−H−N hydrogen bonds for a series of complexes F(H₃NH)₂⁺, F(HNNH₂)₂⁺, F(H₂CNH₂)₂⁺, F(HCNH)₂⁺, and F(FCNH)₂⁺. These complexes have hydrogen bonds with two equivalent N−H donors to F^-. As the basicity of the nitrogen donor decreases, the N−H distance increases and the N−H−F−H−N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, ^2hJ_F-N increases in absolute value and ^1hJ_H-F changes sign. F(H₃NH)₂⁺ complexes were also optimized as a function of the N−H distance. As this distance increases and the N−H···F hydrogen bonds change from ion-pair to proton-shared to traditional F−H···N hydrogen bonds, ^2hJ_F-N initially increases and then decreases in absolute value, ¹J_N-H decreases in absolute value, and ^1hJ_H-F changes sign. The signs and magnitudes of these coupling constants computed for F(H₃NH)₂⁺ at short N−H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)₂⁺ complex in solution. However, even when the N−H and F−H distances are taken from the optimized structure of F(collidineH)₂⁺, ^2hJ_F-N and ^1hJ_H-F are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N−H−F hydrogen bonds in the isolated gas-phase F(collidineH)₂⁺ complex have too much proton-shared character relative to those that exist in solution.