Potential Energy Surfaces and Spectroscopic Parameters

The observed vibration frequencies of a molecule depend on two features of the molecular structure the masses and equilibrium geometry of the molecule and the potential eneigy surface, or force field, governing displacements from equilibrium. These are described as kinetic and potential effects, respectively for a polyatomic molecule the form and the frequency of each of the 3N—6 normal vibrations depend on the two effects in a complicated way. The object of a force field calculation is to separate these effects. More specifically, if the kinetic parameters are known and the vibration frequencies are observed spectroscopically, the object is to deduce the potential eneigy surface. A major difficulty in this calculation is that the observed frequencies are often insufficient to determine uniquely the form of the potential energy surface, and it is necessary to use data on the frequency shifts observed in isotopically substituted molecules or data on vibration/rotation interaction constants observed in high resolution spectra in order to obtain a unique solution. 

In summary, a high diversity of potential models for molecular mechanics calculations of zeolites hitherto exists. From the theoretical point of view, an appropriate force field should be able to predict structures and vibrations with similar accuracy. On the other hand, the structure of a system under study is determined by the energy minimum, whereas normal modes are dependent on the curvature (second derivative) of the potential energy surface. Consequently, force fields obviously successful in predicting structural features might not automatically be appropriate for simulating vibrational spectra. The only way to overcome this difficulty is to include experimental spectroscopic data into the parametrization process [60]. Alternatively, besides structures and energies a matrix of force constants obtained in quantum mechanical calculations can be included into the quantum mechanical data base used to tune the parameters of the potential function [51]. 

The observation of an equilibrium between molecular and ionic hydrogen-bonded complexes evidences the two minima on the potential energy surface [34], We have not only observed such equilibria spectroscopically but have also been able to obtain the thermodynamic parameters of each step. These data show that the stronger the hydrogen bond, the more favorable... 

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