The Xe-129 nuclear shielding surfaces for Xe interacting with linear molecules CO2, N-2, and CO

We have calculated the intermolecular nuclear magnetic shielding surfaces for Xe-129 in the systems Xe-CO2, Xe-N-2, and Xe-CO using a gauge-invariant ab initio method at the coupled Hartree-Fock level with gauge-including atomic orbitals (GIAO). Implementation of a large basis set (240 basis functions) on the Xe gives very small counterpoise corrections which indicates that the basis set superposition errors in the calculated shielding values are negligible. These are the first intermolecular shielding surfaces for Xe-molecule systems. The surfaces are highly anisotropic and can be described adequately by a sum of inverse even powers of the distance with explicit angle dependence in the coefficients expressed by Legendre polynomials P-2n(cos theta), n = 0-3, for Xe-CO2 and Xe-N-2. The Xe-CO shielding surface is well described by a similar functional form, except that P-n(cos theta), n = 0-4 were used. When averaged over the anisotropic potential function these shielding surfaces provide the second virial coefficient of the nuclear magnetic resonance (NMR) chemical shift observed in gas mixtures. The energies from the self-consistent field (SCF) calculations were used to construct potential surfaces, using a damped dispersion form. These potential functions are compared with existing potentials in their predictions of the second virial coefficients of NMR shielding, the pressure virial coefficients, the density coefficient of the mean-square torque from infrared absorption, and the rotational constants and other average properties of the van der Waals complexes. Average properties of the van der Waals complexes were obtained by quantum diffusion Monte Carlo solutions of the vibrational motion using the various potentials and compared with experiment. (C) 1997 American Institute of Physics.