13C NMR and quantum-chemical investigation of cation/ethene interaction

Abstract

Abstract 13 C NMR spectra of ethene molecules adsorbed in cation-exchanged zeolites of X and Y type were measured as a function of pore filling. Typical chemical shifts to higher magnetic fields of ethene in silver-exchanged zeolites can be explained by the formation of Ag + /ethene complexes. In contrast to other olefine molecules no specific interaction between ethene and sodium cations could be found. For this reason the complex Na + /ethene has been investigated by several methods with respect to the energetical behaviour and the 13 C NMR shielding tensors. Molecular electrostatic potential energy, the semi-empirical PCILO, CNDO/2 and ab initio SCF calculations confirm the localization of the sodium ion above the double bond. On the basis of this stable arrangement the 13 C NMR shielding tensors have been estimated within the Pople approximation and PCILO, CNDO/2 and CNDO/S wavefunctions. Only small variations = −2 ppm could be detected in comparison with the isolated ethene molecule. The result which has been found with coupled Hartree-Fock calculations with large gaussian basis sets is somewhat greater (−8.8 ppm) and the resonance line of the calculated complexes is shifted to higher magnetic fields compared to the isolated molecule. This result confirms the assumption that the interaction with sodium ions plays no dominant role in the observed chemical shifts.