AN AB-INITIO STUDY OF THE CHEMICAL-BOND AND THE XE-129 NMR CHEMICAL-SHIFTS IN M(-XE COMPOUNDS, M=LI, NA, K, CU, AG())

Quantum chemical ab initio calculations have been performed for the in teraction between ground state Xe atoms and the monopositive metal cat ions Li+, Na+, K+, Cu+, and Ag+. Potential energy curves have been cal culated at the SCF and CEPA (coupled electron pair approach) levels. T he mechanism of the bonding between Xe and the cation has been analyze d. For the alkali ions Li+, Na+, and K+ the bonds are rather weak (0.3 67, 0.210, 0.112 eV, respectively) and are caused mainly by inductive forces. In Cu+-Xe and Ag+-Xe the binding energies are slightly larger (0.632 and 0.380 eV) and contain small contributions of genuine chemic al effects and dispersion. Xe-129 NMR chemical shifts have been calcul ated at the SCF level by means of the IGLO method. Only the paramagnet ic part of the magnetic shielding constant a of Xe is affected by the interaction with the cation, the diamagnetic part remains unchanged. F or the alkali ions a small downfield shift is obtained which is attrib uted to the polarization of the Xe wave function by the charge of the cation. For the noble metal cations Cu+ and Ag+ an additional upheld s hift is found which is caused by a mixing of the 5p, 4p, 3p orbitals a t Xe with the 3d or 4d orbitals at the noble metals in the magnetic re sponse function. This leads to an increased magnetic shielding. Compar ison with Xe-129 NMR data in X and Y zeolites is made.