Comparison with Other Anionic H-bonds

A systematic comparison of various symmetric anionic complexes provides a solid basis for comparison, The data listed in Table 6,8 illustrate the rapid decline in binding energy as the electronegativity of the atoms diminishes, or as one passes from first to second-row atoms. Concomitant with this weakening of the interaction is the lengthening of the H-bond. In a number of cases, there is a fine distinction as to whether the H-bond is centrosymmet- 

Recent NMR measurements of a series of complexes in the solid state indicate that many of the same principles apply to H-bonds of the FH F type. Centrosymmetric H-bonds occur only for short interfluorine distances, about 2.3 A or less. The gas-phase bond length of 2.278 A is therefore consistent with the ab initio finding of a centrosymmetric H-bond in (P H F) . The crystalline environment can alter the proton transfer potential to a noncentrosymmetric type by just a small H-bond elongation . 

Methyl substitution apparently induces little change in this behavior. MP2 calculations with a 6-31+G basis set yield a binding enthalpy of 26.3 kcal/mol for 

The binding energy (-AE, ) of this structure is calculated to be 28.8 kcal/mol at the SCF level, increasing to 33.0 when MP2 correlation is added. These results are smaller than the gas-phase measurement of AH° of 36.8 kcal/mol The disagreement is amplified by the fact that BSSE and ZPE corrections have not been added to obtain a theoretical enthalpy. Like several of the systems mentioned above, the noncentrosymmetric H-bond illustrated in Fig. 6.10a, changes to a more symmetric structure, and the proton transfer potential converts from double to single-well character when correlation effects are accounted for. At the 

For purposes of comparison, the anti-anti and syn-syn geometries were also studied by the authors . The configurations pictured in Fig. 6.10b and 6.10c were optimized. The anti-anti geometry was found to be more stable than syn-syn by about 3 kcal/mol. At the SCF level, the anti-anti is 4 kcal/mol higher in energy than anti-syn geometry, but this difference is reduced to only 1 kcal/mol at MP2. 

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