Structures and stabilities of group 17 fluorides EF3 (E = I, At, and element 117) with spin–orbit coupling

Abstract

In this work, a recently developed CCSD(T) approach with spin–orbit coupling (SOC) as well as density functional theory (DFT) using various exchange–correlation (XC) functionals are employed to investigate structures and stabilities of group 17 fluorides EF₃ (E = I, At, and element 117). These molecules are predicted to have bent T-shaped C_2v structures according to the second-order Jahn–Teller (SOJT) effects or the valance shell electron pair repulsion (VSEPR) theory. For IF₃ and (117)F₃, our results are consistent with previous SOC-DFT calculations. However, different XC functionals provide different results for AtF₃ and our SOC-CCSD(T) calculations show that both the C_2v and D_3h structures are minima on the potential energy surface and the C_2v structure is the global minimum for AtF₃. The performance of XC functionals on structures and stabilities of IF₃ and AtF₃ is found to depend on the fraction of the Hartree–Fock exchange (HFX) included in the XC functionals and the M06-2X functional with 54% of HFX providing results that agree best with CCSD(T) results. In addition, although both the C_2v and D_3h structures are minima for AtF₃, the energy barrier between them is only 8 kJ mol⁻¹ for the C_2v structure and 0.05 kJ mol⁻¹ for the D_3h structure. This indicates that the D_3h structure could not possibly be observed experimentally and AtF₃ can convert easily between the three C_2v structures. The SOJT term is shown to be reduced by electron correlation for IF₃ and AtF₃. On the other hand, although SOC decreases the energy difference between the C_2v and D_3h structures and reduces the deviation of the C_2v structure from the D_3h structure, it decreases the frequency of the bond bending mode, which may indicate that SOC actually increases the SOJT term. This could be related to mixing of spin-singlet E′ states to low-energy spin-triplet states due to SOC.