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 D-3h structure of (117)F-3

Abstract

Spin-orbit and scalar relativistic effects on geometries, vibrational frequencies, and energies for group 17 fluorides EF3 (E = 1, 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 D-3h structure of (I 17)F-3 a stable local minimum, whereas IF3 and AtF3 retain C-2nu local minima even with relativistic effects. The valence shell electron pair repulsion model is not appropriate to explain the molecular structure of (I 17)F-3. The geometries of EF3 (E = 1, 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 Angstrom for r(e)(eq) and r(e)(ax), respectively. Spin-orbit effects widen the bond angle of C-2nu structures of re re IF3 and AtF3, i.e., spin-orbit effects diminish the second-order Jahn-Teller term. The bond angle alpha(e) of AtF3 increases by 3.9degrees due to spin-orbit interactions in addition to the increase of 4.8degrees by scalar relativistic effects. For (I 17)F-3, spin-orbit effects increase the bond length by 0.109 Angstrom. The spin-orbit interactions stabilize (I 17)F-3 by a significant margin (similar to1.2 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 7(P3/2) spinors of element 117 due to huge spin-orbit splitting of 7p.