| Abstract
| - 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 twoequivalent N−H donors to F-. As the basicity of the nitrogen donor decreases, the N−H distance increasesand the N−H−F−H−N arrangement changes from linear to bent. As these changes occur and the hydrogenbonds between the ion pairs acquire increased proton-shared character, 2hJF-N increases in absolute value and1hJH-F changes sign. F(H3NH)2+ complexes were also optimized as a function of the N−H distance. As thisdistance increases and the N−H···F hydrogen bonds change from ion-pair to proton-shared to traditionalF−H···N hydrogen bonds, 2hJF-N initially increases and then decreases in absolute value, 1JN-H decreases inabsolute value, and 1hJH-F changes sign. The signs and magnitudes of these coupling constants computed forF(H3NH)2+ at short N−H distances are in agreement with the experimental signs and magnitudes determinedfor the F(collidineH)2+ complex in solution. However, even when the N−H and F−H distances are takenfrom the optimized structure of F(collidineH)2+, 2hJF-N and 1hJH-F are still too large relative to experiment.When the distances extracted from the experimental NMR data are used, there is excellent agreement betweencomputed and experimental coupling constants. This suggests that the N−H−F hydrogen bonds in the isolatedgas-phase F(collidineH)2+ complex have too much proton-shared character relative to those that exist in solution.
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