Theoretical investigation of the effect of the rare gas matrices on the vibrational spectra of solvated molecular ions: Cu+CO

Vibrational dynamics of adsorbed CO2: Separability of the CO2 asymmetric stretching mode

Separability of the CO2 asymmetric stretching mode is probed theoretically by performing highly accurate vibrational calculations on the CO2 and K+CO2 model systems. The proposed approach is applied to a model case of the vibrational dynamics of the CO2 molecule adsorbed in K-FER zeolite. The CCSD(T) level is fully adequate for quantitative decription of the CO2 vibrational dynamics, and all important effects on the vibrational dynamics of CO2 adsorption complexes can be estimated rather accurately (within 5 cm–1) at the DFT level of theory.

Computational and FTIR spectroscopic studies on carbon monoxide and dinitrogen adsorption on a high-silica H-FER zeolite

Adsorption (at a low temperature) of carbon monoxide and dinitrogen on a high-silica ferrierite-type zeolite (H-FER, Si : Al = 27.5 : 1) was investigated by means of variable temperature infrared spectroscopy and theoretical calculations at the periodic DFT level. This combined experimental and computational approach led to detailed characterization of several types of hydrogen-bonded OHCO and OHN(2) complexes, formed by interaction between the adsorbed molecules and the Brønsted acid OH groups of the zeolite. CO or N(2), forming linear complexes with OH groups pointing towards a sufficiently ample void space, show the largest adsorption enthalpy which was found to be in the (approximate) range of -25 to -29 kJ mol(-1) for CO and -15 to -19 kJ mol(-1) for N(2). Less stable OHCO or OHN(2) complexes can be formed when either the Brønsted acid OH group is involved in intra-zeolite hydrogen bonding or when the free space available is too small to allow formation of linear complexes without previous re-location of the proton of the OH group involved. The details of experimental IR spectra in the O-H, C-O, and N-N stretching regions could be interpreted on the basis of good agreement between experimental and calculated results.

Theoretical studies of Cu(I) sites in faujasite and their interaction with carbon monoxide

Sitting, coordination, and properties of Cu(I) cations in zeolite faujasite are investigated using a combined quantum mechanics-interatomic potential function method. The coordination of Cu(I) ions depends on their location within the zeolite lattice. Cu(I) located inside the hexagonal prisms (site I') and in the plane of six-membered aluminosilicate rings on the walls of sodalite units (site II) is threefold coordinated, whereas Cu(I) located in the supercages (site III) is twofold coordinated. In agreement with available experimental data Cu(I) appears to be more strongly bound in sites I' and II than in site III. The binding energy of site II Cu(I) ions increases with the number of Al atoms, but only closest Al atoms have a substantial influence. The CO molecule binds more strongly onto sites with weaker bound cations and lower coordination. We assign the two CO stretching IR bands observed for Cu(I)-Y zeolites to sites II with one Al (2157-2161 cm(-1)) and two Al atoms (2140-2148 cm(-1)) in the six-membered aluminosilicate ring. For Cu(I)-X we tentatively assign the high frequency band to site III (2156-2168 cm(-1)) and the low-frequency band to site II with three Al atoms in the six-membered ring (2136-2138 cm(-1)).

Single and dual cation sites in zeolites: theoretical calculations and FTIR spectroscopic studies on CO adsorption on K-FER

Interaction of CO with K-FER zeolite was investigated by a combination of variable-temperature IR spectroscopy and computational study. Calculations were performed using omega(CO)/r(CO) correlation method in combination with a periodic density functional theory model. On the basis of agreement between experimental and calculated results, the following carbonyl complexes were identified: (i) mono- and dicarbonyl C-down complexes on single K(+) sites characterized by IR absorption bands at 2163 and 2161 cm(-1), respectively; (ii) complexes formed by CO bridging two K(+) ions separated by about 7-8 A (dual sites) characterized by a band at 2148 cm(-1); and (iii) isocarbonyl (O-down) complexes characterized by a band at 2116 cm(-1). The bridged carbonyl complexes on dual K(+) sites are about 5 kJ/mol more stable than monodentate (monocarbonyl) CO complexes. The C-O stretching frequency of monocarbonyl species in K-FER depends on K(+) location in the zeolite, and not on K(+) coordination to the framework. A combination of theoretical calculations using a periodic density functional model and experimental results showed formation of two types of monocarbonyls. The most abundant type appears at 2163 cm(-1), and the less abundant one at 2172 cm(-1). These experimentally determined wavenumber values coincide, within +/-2 cm(-1), with those derived from theoretical calculations.

Thermodynamics of reversible gas adsorption on alkali-metal exchanged zeolites—the interplay of infrared spectroscopy and theoretical calculations

Detailed understanding of weak solid-gas interactions giving rise to reversible gas adsorption on zeolites and related materials is relevant to both, fundamental studies on gas adsorption and potential improvement on a number of (adsorption based) technological processes. Combination of variable-temperature infrared spectroscopy with theoretical calculations constitutes a fruitful approach towards both of these aims. Such an approach is demonstrated here (mainly) by reviewing recent studies on hydrogen and carbon monoxide adsorption (at a low temperature) on alkali-metal exchanged ferrierite. However, the methodology discussed, which involves the interplay of experimental measurements and theoretical calculations at the periodic DFT level, should be equally valid for many other gas-solid systems. Specific aspects considered are the identification of gas adsorption complexes and thermodynamic studies related to standard adsorption enthalpy and entropy.

The vibrational dynamics of carbon monoxide in a confined space-CO in zeolites

Based on theoretical calculations, and a survey of infrared spectra of CO adsorbed on different cation exchanged zeolites, a model is proposed to explain the influence of the zeolite framework on the vibrational behaviour of CO confined into small void spaces (zeolite channels and cavities). The concepts developed should help to understand a number of details relevant to both, precise interpretation of IR spectra and a better understanding of the vibrational dynamics of small molecules in a confined space.

On the site-specificity of polycarbonyl complexes in Cu/zeolites: combined experimental and DFT study

The preferred Cu(+) sites and formation of mono-, di-, and tricarbonyl complexes in the Cu-FER were investigated at the periodic density functional theory level and by means of FTIR spectroscopy. The site-specificity of adsorption enthalpies of CO on Cu-FER and of vibrational frequencies of polycarbonyl complexes were investigated for various Cu(+) sites in Cu-FER. Large changes in the Cu(+) interaction with the zeolite framework were observed upon the formation of carbonyl complexes. The dicarbonyl complexes formed on Cu(+) in the main channel or on the intersection of the main and perpendicular channels are stable and both, adsorption enthalpies and CO stretching frequencies are not site-specific. The fraction of Cu(+) ions in the FER cage, that cannot form dicarbonyl can be determined from IR spectra (about 7% for the Cu-FER with Si/Al = 27.5 investigated here). The tricarbonyl complexes can be formed at the Cu(+) ions located at the 8-member ring window at the intersection of main and perpendicular channel. The stability of tricarbonyl complexes is very low (DeltaH degrees (0 K)>or=-4 kJ mol(-1)).

Theoretical Investigation of CO Interaction with Copper Sites in Zeolites: Periodic DFT and Hybrid Quantum Mechanical/Interatomic Potential Function Study

Periodic DFT and combined quantum mechanics/interatomic potential function (QM-pot) models were used to describe the interaction of CO with the Cu+ sites in FER. The CO stretching frequencies were calculated using omega(CO)(CCSD(T))/r(CO)(DFT) scaling method relating frequencies determined using a high-level quantum-chemical (coupled clusters) method for simple model carbonyls to CO bond lengths calculated using periodic DFT and QM-pot methods for the Cu+-zeolite system. Both periodic DFT and QM-pot models together with omega(CO)/r(CO) scaling describe the CO stretching dynamics with the "near spectroscopic accuracy", giving nu(CO) = 2156 cm(-1) in excellent agreement with experimental data. Calculations for various Cu+ sites in FER show that both types of Cu+ sites in FER (channel-wall sites and intersection sites) have the same CO stretching frequencies. Thus, the CO stretching frequencies are not site-specific in the CO/Cu+/FER system. The convergence of the results with respect to the model size was analyzed. When the same exchange-correlation functional is used the adsorption energies from periodic DFT and QM-pot are in good agreement (about 2 kcal/mol difference) but substantially larger than those of the experiment. The adsorption energy calculated with the B3LYP functional agrees with available experimental data. The overestimation of the adsorption energy in DFT calculations (periodic or QM-pot) is related to a red-shift of the CO stretching mode, both result from an underestimation of the HOMO(5sigma)-LUMO(2pi) gap of CO and the consequent overestimation of the Cu(+)(d)-CO(2pi*) back-donation. For the adsorption energy, this can be overcome by the use of hybrid B3LYP exchange-correlation functional. For the frequency calculations, the DFT problem can be overcome by the use of the omega(CO)(CCSD(T))/r(CO)(DFT) correlation.