Induced infrared absoption bands of ethylene adsorbed on mordenites: Experimental and theoretical studies of electron-donor-acceptor interactions

Abstract

Infrared spectra of ethylene adsorbed on ion-exchanged mordenites (metallic cation = Li +, Na +, Ca 2+, Zn 2+ and Cd 2+) were measured. Weak bands were found at 1612–1614 and at 1338–1340 cm −1 in Li +, Na + and Ca 2+ mordenites. They are assigned to the totally symmetric ν(CC) stretching and symmetric δ(CH 2) bending modes of the ethylene monomer. In Zn 2+ and Cd 2+ mordenites the corresponding bands shift to lower energy and increase in intensity. These forbidden bands become IR active through the electron-donor-acceptor (charge-transfer) interaction between ethylene and the metallic cation. To estimate the IR intensity induced by the charge-transfer interaction, ab initio SCF MO calculations were carried out for ethylene-chlorine monofluoride (C 2H 4-CIF) complex selected as a model complex. The calculations show that only a small amount of charge-transfer modulation of the CC stretching mode can induce its IR activity enough to observe it in the spectra, and that the direction of the transition dipole is perpendicular to the CC bond.