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

Abstract

An extended Huckel molecular orbital (EHMO) study is undertaken for the adsorption of C 2 and C 3 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 C 2 H 4 on the (111) face ofthe copper(I) halides, the bond strength order is CuCl > CuBr > CuI > CuSO 3 C 6 H 5 . 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 σ donation to the bond is substantially higher than the d-π * back-donation. For the example of C 2 H 4 on AgSO 3 C 6 H 5 , 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.