FTIR measurements and quantum chemical calculations of ethylene adsorbed on CuNaY

Abstract

The interaction of ethylene with vacuum dehydrated CuNaY was studied by transmission and diffuse reflectance FTIR spectroscopy, and by cluster model DFT calculations using the B3LYP exchange–correlation functional. The FTIR spectra reveal the adsorption-induced activation of the ν₁ CH stretching, ν₂ CC stretching and ν₃ CH₂ scissoring modes, at ∼3011, 1545/1535 and 1278/1264 cm⁻¹. The pairs of strongly redshifted band components were interpreted on the basis of two chemically inequivalent adsorption sites interacting with ethylene. For theoretical investigations, two cluster models representing copper ions at two different sites (SII and SIII) were constructed using structural fragments with one aluminum atom. The copper ion at SII is trigonally coordinated by the nearest oxygen atoms, whereas a two-fold coordination dominates at SIII. In the presence of ethylene the coordination geometry of copper at SII changes dramatically towards a four-fold coordination including two oxygen ligands and side-on bound ethylene. The same coordination is found for Cu on SIII. The CC bond lengths are elongated by about 0.05 Å and the angles between the CH₂ planes are reduced to at least 158°. The calculated binding energies of ethylene are overestimated but do correctly describe the experimental trend. Population analyses reveal net charge transfers of 0.12/0.13 e towards ethylene, and energy decomposition methods demonstrate prevailing electrostatic and charge-transfer contributions to the adsorbent–adsorbate interaction energy. Most of the vibrational frequency shifts calculated for the adsorption complexes are in agreement with the experimental values.