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 interaction between ground state Xe atoms and the monopositive metal cations Li+, Na+, K+, Cu+, and Ag+. Potential energy curves have been calculated at the SCF and CEPA (coupled electron pair approach) levels. The mechanism of the bonding between Xe and the cation has been analyzed. For the alkali ions Li+, Na+, and K+ the bonds are rather weak (0.367, 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 chemical effects and dispersion. Xe-129 NMR chemical shifts have been calculated at the SCF level by means of the IGLO method. Only the paramagnetic part of the magnetic shielding constant a of Xe is affected by the interaction with the cation, the diamagnetic part remains unchanged. For the alkali ions a small downfield shift is obtained which is attributed 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 shift is found which is caused by a mixing of the 5p, 4p, 3p orbitals at Xe with the 3d or 4d orbitals at the noble metals in the magnetic response function. This leads to an increased magnetic shielding. Comparison with Xe-129 NMR data in X and Y zeolites is made.