A theoretical study on properties of heavy and superheavy noble gas fluorides (XF2 and XF4, X = Xe, Rn, Og)

XFn (X = Xe, Rn, Og; n = 2, 4) are important rare gas compounds, but they have received limited theoretical attention. There still exist controversies over the lowest energy structure of OgF4. In this work, the CCSD(T) method with spin-orbit coupling (SOC) is employed for determining geometries, harmonic frequencies, and dissociation energies of these heavy and superheavy rare gas fluorides. Our results show that the lowest-energy structure for XF2 is D∞h and that for XF4 is D4h. In addition, a Td structure emerges for OgF4 with SOC, lying about 2 kcal/mol above the D4h structure. Moreover, a C2v structure appears in scalar-relativistic calculations for XF2 and XeF4; however, the C2v structure for OgF2 vanishes when SOC is included. SOC increases both bond lengths and dissociation energies of these molecules and is essential for describing properties of RnFn and OgFn accurately. Furthermore, an atom-in-molecule (AIM) analysis program is developed to analyze electron densities of these systems with SOC. AIM and Mulliken population analyses reveal that the X-F bond exhibits a mixed ionic-covalent character. SOC diminishes the covalent character in these molecules, particularly for RnFn and OgFn, and relatively larger covalent character is observed in XF4 than that in XF2.