A Molecular Orbital Study of the Selective Adsorption of Simple Hydrocarbon Molecules on Ag⁺- and Cu⁺-Exchanged Resins and Cuprous Halides

An extended Hückel molecular orbital (EHMO) study is undertaken for the adsorption of C₂ and C₃ hydrocarbon molecules on highly olefin-selective adsorbents, Ag⁺- and Cu⁺-exchanged sulfonic acid resins and copper(I) halides, by using a resin model and different crystal faces of halides. Consideration of the HOMO and LUMO energy levels of the adsorbate and adsorbent can give a qualitative indication of both the direction of electron transfer and the relative bond strength. Quantitative EHMO results show that for π complexation of olefins on resins, the strength of the bond follows the order Ag⁺ > Cu⁺ > H⁺. The bond strengths of olefins on Cu⁺ exposed on different crystal faces of the halides follow the order (111) > (001) > (111), favoring Cu⁺ with a lower coordination number. A strong anion effect on π-complexation is predicted by EHMO. For C₂H₄ on the (111) face of the copper(I) halides, the bond strength order is CuCl > CuBr > Cul > CUSO₃C₆H₅. The order is reversed for the (001) and (111) faces of the halides. From an analysis of the atomic orbital occupations and the net charges, a quantitative understanding of the π-complexation bond is obtained. The a donation to the bond is substantially higher than the d–π∗ back-donation. For the example of C₂H₄ on AgSO₃C₆H₅, the contribution from σ donation accounts for 84% of the bond, whereas that by the d–π∗ back-donation accounts for only 16%. The EHMO results are in general agreement with the available experimental data.