O. Zakharieva-Pencheva

What kind of support for the interpretation of infrared spectra can be expected from cluster MO calculations on zeolite systems?: Application of the SCC-Xα method to the sorption complex of carbon monoxide on a S1 calcium ion in zeolite A

Abstract In order to get more detailed information on the adsorbed state of carbon monoxide in zeolites A, quantum chemical SCC-Xα calculations have been performed on a M[Si3Al3O6(OH)12] cluster (M  Ca2+, Na+), modelling the favoured sorption site in front of S1 cations. Evaluation of the electronic structure gave several well-separated energy bands. CO bonding to the cluster is predicted for linear MCO as well as MOC arrangements. From the total electronic energies an equilibrium distance CaC of 2.7 A can be inferred. Both findings are in agreement with results from classical calculations of the interaction of CO with zeolite Na4Ca4A and are able to explain the observed IR band pattern. Ascribing the partial charges from Xα calculations to the atoms of the cluster in the classical model leads to improvement of the electric field strength values inside the zeolite skeleton from which frequency shifts more consistent with the observed spectrum are expected.

Molecular orbital calculations and normal coordinate analysis of CO2 and N2O in zeolites

Abstract SCC-Xα MO calculations and normal coordinate analysis were performed on CO 2 and N 2 O in the free and adsorbed state. For approaching S-I cations a linear arrangement with the molecular axis perpendicular to the plane of the oxygen six-ring is confirmed. Electronic charge density is withdrawn from the antibonding HOMO of the adsorbed molecules towards the zeolite cluster, connected with an internal charge redistribution within the molecule. The changes of the respective force elements are consistent with the quantum chemical results. The results assist the assignment of the IR bands of the sorption complexes, proving the formation of both O-bound and N-bound species in case of N 2 O, the latter being energetically more favourable.

Application of semiempirical MO calculations to the interaction of hydrocarbons with zeolites

Abstract In order to support previous spectral studies and normal coordinate analysis data, the semiempirical SCC-Xα method has been applied to the interaction of propene and cyclopropane with Ca 2+ ions in front of the sixford windows connecting the sodalite units and the supercages in faujasites and A-type zeolites. From preliminary results, the ion can be approached by propene in a geometry intermediate between parallel or perpendicular orientation of its molecular plane normal to the double bond, while in case of cyclopropane an edge-on interaction seems to be favoured.

IR and Mössbauer spectra of hexacyano complexes

Abstract IR and Mossbauer spectra of MK(Fe(CN) 6 ).4H 2 O complexes where M=La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb reveal the character of the outer rare earth cation coordination. The interaction of the post lanthanide elements with the nitrogen end of CN − shifts significantly the bands in mid infrared region, provokes an increase in the integral intensity of the band near 450 cm −1 and influences the isomer shift values.

Vibrational analysis of propene adsorbed in zeolites Y

Abstract Normal coordinate analysis was performed for propene adsorbed in faujasites NaY, CaY and MgY applying a valence force model. The force fields obtained were compared with that of the free molecule. Only small shifts of the fundamentals are observed due to the weak interaction with the zeolites. Although in general the results are comparable to those of propene adsorbed in different zeolites A, some distinct features such as a stronger influence on the CC stretching vibration, a smaller hindrance of the CH bending vibration and a higher reduction of the CH (CH 3 ) bond strength are detectable, which must be attributed to both the different composition and structure of the zeolite framework. The frequency shifts and the corresponding changes of the force constants are related to the geometry of the sorption complex of propene on the zeolite surface.

Study of the geometry of cyclopropane adsorbed on a calcium ion in zeolites a by SCC-Xα calculations

Abstract On the basis of SCC-Xα calculations the geometry of the sorption complex of cyclopropane adsorbed near a Ca ion in a zeolite six-ring was found to be an edge-on configuration with a distance of 317 pm between the Ca ion and the center of the CC bond, the plane of cyclopropane coinciding with the mirror plane of the six-ring cluster. The electronic structure of this sorption system has been explored.

Profitability of the combined application of Mo calculations and vibrational analysis on intrazeolitic sorption complexes

Abstract The adsorption of CO in Ca- and Ni-exchanged zeolite A is studied by IR-spectroscopy, quantum chemical calculations and vibrational analysis. A similar experimental frequency shift of the CO fundamental vibration is shown to arise from different mechanisms. In case of Ca-bonded CO the main contribution comes from direct strengthening of the CO bond while in case of Ni-bonded CO vibrational coupling turns out to be the dominant mechanism.

Calculation of IR- and Raman-active vibrations of hexacyano complexes of FeII, CoIII, MnII, CrIII VII

Abstract A complete vibrational analysis of the hexacyano complexes of Fe II , Co II , Mn II , Cr III , V II is presented on the basis of the valence force field method. The calculated frequencies of the complexes are in good agreement with the experimental values. The direct and inverse vibrational problems are solved for all the complexes and the correct force fields obtained. In the case of [Fe(CN) 6 ] 4− the direct and inverse electrooptical problem is also solved. The calculated electrooptical parameters satisfactorily describe the experimental intensities of the bands in the mid-IR region.

Quantum chemical studies on the sorption state of hydrocarbons in zeolites. SCC-Xα calculations on propene adsorbed to a calcium occupied six-ring cluster

Abstract Using the self-consistent charge-Xα method, a stable sorption complex was found for propene approaching the S1 centre of zeolite A or the S2 centre of zeolites X and Y. The following conditions were used: the C1-C2 bond was parallel to the Si1-Si2 line, 320pm distant from the calcium ion; the propene carbon backbone was parallel to the plane of the six-ring cluster, the centre of the double bond being displaced by 26.5 pm with respect to the calcium ion in the direction of the Si2 atom so that the terminal carbon atom (C1) was closer to the calcium ion than C2.

Normal coordinate analysis of molecules adsorbed on zeolite surfaces: Part II. Propene adsorbed in zeolites A

Abstract Vibrational analysis has been performed for propene in the gas phase and in zeolites A adsorbed near Li + , Na + , Mg 2+ , Ca 2+ amd Zn 2+ ions, using a valence force model. Withdrawal of electron density from the π bond to the unoccupied s orbitals of the cations represents the main contribution to the interaction, giving rise to a monotonous decrease of the CC force constant in the sequence of growing polarizing power of the cations. From this, an approach of the cations by the double bond must be assumed in accord with the weakening of the adjacent CH bonds. From further analysis there are indications for the hindrance of the deformation motion due to adsorption, while the methyl group remains rather unperturbed. Normal coordinate analysis of adsorbed species supports well the predictions of the geometrical arrangement and electronic structure from quantum chemical calculations