Spin−Orbit and Relativistic Effects on Structures and Stabilities of Group 17 Fluorides EF3 (E = I, At, and Element 117): Relativity Induced Stability for the D3h Structure of (117)F3

Abstract

Spin−orbit and scalar relativistic effects on geometries, vibrational frequencies, and energies for group 17 fluorides EF3 (E = I, At, and element 117) are evaluated with two-component methods using relativistic pseudopotentials and effective one-electron spin−orbit operators. The inclusion of relativistic effects makes the D3h structure of (117)F3 a stable local minimum, whereas IF3 and AtF3 retain C2v local minima even with relativistic effects. The valence shell electron pair repulsion model is not appropriate to explain the molecular structure of (117)F3. The geometries of EF3 (E = I, At, and element 117) molecules are optimized at the HF level with and without spin−orbit effects. Spin−orbit interactions elongate the bond lengths and decrease the harmonic vibrational frequencies. In the case of AtF3, spin−orbit interactions increase the bond lengths by 0.044 and 0.023 Å for and , respectively. Spin−orbit effects widen the bond angle of C2v structures of IF3 and AtF3, i.e., spin−orbit effects diminish the second-order Jahn−Teller term. The bond angle αe of AtF3 increases by 3.9° due to spin−orbit interactions in addition to the increase of 4.8° by scalar relativistic effects. For (117)F3, spin−orbit effects increase the bond length by 0.109 Å. The spin−orbit interactions stabilize (117)F3 by a significant margin (∼1.2 eV). This stabilization of the molecule compared with open p-shell atoms is quite unusual. Enhanced ionic bonding may be responsible for this stabilization because the electronegative F atom can effectively polarize or attract electrons from the destabilized 7p3/2 spinors of element 117 due to huge spin−orbit splitting of 7p.