A scaling method based on the linear correlation between the CO bond length and the CO stretching frequency has been applied to the CO molecule adsorbed on the Cu-exchanged MFI zeolite. Effects of anharmonicity, cluster size, unit cell size, and the Madelung potential were investigated. Interaction of CO with zeolite framework was described at the combined RI−BLYP/IPF level. The inner part of the combined model (RI−BLYP description) consisted of up to 23 TO4 tetrahedra. The effect of the Madelung potential on CO stretching frequencies was negligible. All Cu+ sites on the channel intersection and on the wall of the main channel are characterized by the CO stretching frequencies in the narrow range of 2159−2164 cm-1 in excellent agreement with experimental data. The Cu+ sites on the wall of the zigzag channel show slightly higher frequencies (3−6 cm-1). These sites are populated, however, only when the framework Al atoms are at T4 or T10 positions. The proposed computational scheme provides essentially the same level of accuracy as obtained from CCSD(T) calculations for small copper carbonyl species with overall error smaller than 5 cm-1.

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.

The effect of the rare-gas matrices on the vibrational spectra of solvated molecular ions was investigated. An approach based on the full-dimensional coupled-clusters description of the gas phase ion augmented with the Monte Carlo averaging of frequency shift due to the interaction with the matrix is presented. The calculated νR(CO) of the Cu+CO ion in the Ne matrix at 2229 cm−1 agrees within 5 cm−1 with the experimentally determined value. The remaining small discrepancy between theory and experiment is due to the error in harmonic frequency calculations of the gas phase Cu+CO ion at the coupled-clusters level of theory. Calculated frequency shift due to the effect of Ne matrix allows us to predict the gas-phase harmonic ω=2272 cm−1 and anharmonic ν=2243 cm−1 vibrational frequencies for Cu+CO. A large difference (−31 cm−1) in ΔνR(CO) stretching frequencies between Ne and Ar matrices was found. The ΔνR(CO) does not depend on the cavity size and on the temperature. Major contribution to ΔνR(CO) comes from five Ar atoms. Together with the CO ligand these five atoms are octahedrally coordinated to the Cu+ ion. The effect of the basis set superposition error was found to be important for the calculations of ΔνR(CO) in the rare-gas matrix.

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