PNNL

Abstract

Atomization energies at 0ºK and heats of formation at 0ºK were obtained for XeF?, XeF?, XeF2, XeF4, XeF5?, and XeF6 from coupled cluster theory including noniterative, quasi-perturbative triple excitations (CCSD(T)) calculations with new correlation-consistent basis sets for Xe up through aug-cc-pV5Z in some cases. In order to achieve near chemical accuracy (± 1 kcal/mol) in the thermodynamic properties, up to four corrections were added to the estimated complete basis set binding energies based on frozen core coupled cluster theory energies: (1) a correction for core-valence effects, (2) a correction for scalar relativistic effects, (3) a correction for first order atomic spin-orbit effects, and (4), in some cases, a second order spin-orbit correction. Vibrational zero point energies were computed at the coupled cluster level of theory. The structure of XeF6 is difficult to obtain. The C3v and Oh structures for XeF6 are essentially degenerate with the Oh structure below the C3v structure by 0.19 kcal/mol at the CCSD(T)/CBS level based on an approximate geometry for the C3v structure. The two structures are probably isoenergetic with the C3v slightly lower. The calculated heats of formation are: ?Hf0 (XeF?) = 255.8, ?Hf0(XeF?) = -66.3, ?Hf0(XeF2) = -23.3, ?Hf0(XeF4) = -42.5, ?Hf0(XeF5?) = -160.6, and ?Hf0(XeF6) = -55.9 kcal/mol. The calculated heats of formation for the neutral XeFn fluorides are higher (less stable) than the experimental values from equilibrium measurements by 2.0, 7.7, and 12.2 kcal/mol for n =2, 4, and 6 respectively. The calculated values are even higher as compared to the heats of formation from photoionization measurements. The calculated results suggest that the experimental heats of formation need to be revised. The fluxional nature of the stereoactive lone pair in XeF6 is noted.