Zeolite force fields and experimental siliceous frameworks in a comparative infrared study

Infrared spectra (IR) of a great variety of zeolite frameworks in the limit of pure silica composition are calculated by molecular dynamics and also recorded experimentally. This enables us to study and assess the effect of three flexible force fields from the literature developed for zeolites in reproducing the IR spectra: the force fields by Demontis (J. Phys. Chem.1988, 92, 867), Nicholas (J. Am. Chem. Soc.1991, 113, 4792), and Hill (J. Phys. Chem.1995, 99, 9536). On one side, a qualitative comparison is undertaken; on the other, a similarity index is introduced to perform a quantitative assessment of the similarity of spectra. It is applied to experimental spectra and enables us to arrange the frameworks in three different sets. It can also be applied to study the agreement of the spectra obtained with the three force fields with experimental spectra on a quantitative basis. The force field by Nicholas performs best, followed by the force field by Demontis. Frameworks are therefore analyzed purely theoretically with the Nicholas force field to investigate the dependency on frameworks. This yields a new classification in sets, which is found to be related to the topology of the frameworks. Surprisingly, these sets do not agree with the sets obtained with experimental spectra. As a consequence, it is found that none of the force fields is good enough to enable the identification of frameworks based on their experimental spectra. In a comparison of spectra generated by different force fields, it is found that the Nicholas and Hill force fields generate the most similar IR spectra.