The dicarboxylate model systems

Several neutral carboxylic acids as well as their acid salts (hydrogencarboxylates) form molecular crystals using H-bonded structural motifs. Principal amongst these are simple linear dimers and cyclic dimers, see Appendix 4. Their importance lies not in the INS results from any individual system but the support that can be obtained from the body of results for the different models available for their interpretation. There are, at least, two approaches to interpreting the INS spectra of dicarboxylates [49]. 

The gross outlines of the proton s vibrational potential are similar in both approaches. This is an asymmetric double well, which is deep enough to explain the OH stretch at about 3000 cm, and that are separated by a more or less substantial barrier. Fig. 9.12. 

What differentiates the models is how the double well potential is linked to other vibrations of the molecule or lattice. The two approaches are, first the phonon assisted tunnelling model and, second the independent proton model. 

In the independent proton model the H-bond potential well is disconnected from all other vibrations [51]. Unlike the case of electrons in molecules, where the Bom-Oppenheimer approximation applies, this disconnection cannot be on the grounds of significantly differing energy scales. The energies of H-bond vibrations are very similar to those of the heavy atoms. Rather the separation of the two sets of dynamics is 

The two models have recently come into sharp contrast since they have both been applied to the analysis of the same INS spectrum of benzoic acid [51,53]. The molecular structure of this system is shown in Fig. 9.13. These are the only INS data on O-H-O bonds to be fully analysed using the phonon assisted tunnelling model. However, it is probable that the INS spectra of most systems could be subjected to a similar analysis. 

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