Hydrogen bond, directional influence

When the two wells are of similar energies, and the crystal structure allows, the above will no longer be the situation. We may then expect a number of consequences There may be a measurable displacement of the hydrogen between the two sites induced by such factors as a change in temperature, application of an electric field, and irradiation with light both tautomers may be present at symmetry-independent sites in the crystal different tautomers may be present in different crystal modifications and the presence of molecular substituents that do not directly affect the properties of the hydrogen bond may influence the tautomerism via the crystal structure. 

Examples are shown in Figures 23,24, and 25. Quantum chemical calculations for the example in Figure 23 suggested that the two hydrogen bonds do not (or only marginally) influence each other [69]. This might be because the approach is from opposite directions so that the two donors are not in steric conflict with each other. Somewhat different are two hydrogen bonds directed to a carbonyl acceptor. 

The solution phase is modeled explicitly by the sequential addition of solution molecules in order to completely fill the vacuum region that separates repeated metal slabs (Fig. 4.2a) up to the known density of the solution. The inclusion of explicit solvent molecules allow us to directly follow the influence of specific intermolecular interactions (e.g., hydrogen bonding in aqueous systems or electron polarization of the metal surface) that influence the binding energies of different intermediates and the reaction energies and activation barriers for specific elementary steps. 

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