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Using different computational methods (isolated cluster, periodic models) and levels of DFT theory (hybrid B3LYP, GGA PW91 and PBE, LDA-CA), one obtained an accurate approximation function for the characterization of the band splitting (BS) of symmetric and asymmetric vibrations of the CO32- anion in cationic zeolite forms. The correlation obtained between the carbonate geometry and BS does not involve the O-C-O angles as found throughout the series of cluster or periodic models considered. The parameter values of the correlation function slightly depend on the method applied for the energy computation of the models. The "BS-geometry" function suits well for the BS values computed at both the cluster and periodic levels with exception of very few deviations presented herein. The accuracy of the evaluation of the geometry is estimated versus spectroscopic data. The relevance of carbonates for the explanation of the unusually high separation of CO mixtures with various gases is also discussed.