The use of explicitly correlated methods on XeF6 predicts a C3v minimum with a sterically active, free valence...

Explicitly correlated CCSD(T)-F12b calculations show that the lowest energy conformer of XeF(6) is the C(3v) structure with a stereoactive lone pair. The C(3v) structure is 1.08 kcal/mol below the C(2v) structure and 1.80 kcal/mol below the O(h) structure without vibrational corrections. The C(2v) conformer is a transition state connecting the different C(3v) minima on the pseudorotation potential energy surface, and the O(h) structure is a higher-order saddle point leading to the C(2v) transition states. The calculated vibrational frequencies for the C(3v) structure best fit the experimental frequencies. The calculated heats of formation for XeF(6), -62.1 ± 1.4 kcal/mol at 0 K and -64.0 ± 1.4 kcal/mol at 298 K, are the best available values and show that there are serious issues with the experimental values. The results show that the explicitly correlated CCSD(T)-F12b method can be used to address important electronic structure issues with smaller basis sets.