Electronic structures and geometries of the XF3 (X = Cl, Br, I, At) fluorides

Abstract

The potential energy surfaces of the group 17 XF3 (X = Cl, Br, I, At) fluorides have been investigated for the first time with multiconfigurational wave function theory approaches. In agreement with experiment, bent T-shaped C(2v) structures are computed for ClF3, BrF3, and IF3, while we predict that an average D(3h) structure would be experimentally observed for AtF3. Electron correlation and scalar relativistic effects strongly reduce the energy difference between the D(3h) geometry and the C(2v) one, along the XF3 series, and in the X = At case, spin-orbit coupling also slightly reduces this energy difference. AtF3 is a borderline system where the D(3h) structure becomes a minimum, i.e., the pseudo-Jahn-Teller effect is inhibited since electron correlation and scalar-relativistic effects create small energy barriers leading to the global C(2v) minima, although both types of effects interfere.

The bonding picture in hypervalent XF3 (X = Cl, Br, I, At) fluorides revisited with quantum chemical topology

“…This is consistent with a stronger ionic character of the XF bonds while the electronegativity difference between the fluorine and the central atom increases. Moreover, we observe that the energy difference between the D 3h structure and the C 2v one, Δ E , is strongly reduced along the XF 3 series, from 91.5 kJ mol −1 in ClF 3 to 3.2 kJ mol −1 in AtF 3 , the T‐shaped and D 3h geometries becoming almost isoenergetic in the AtF 3 system, in agreement with previous studies …”

“…At first, we note that CCSD(T) equilibrium geometries of the T‐shaped structures are in very good agreement with the experimental ones for ClF 3 and BrF 3 . We recall here that the PBE0 functional is well suited to describe the geometries of the whole series, as the obtained results are in close agreement with the reference CCSD(T) values . For instance, the deviations for the computed XF bond lengths remains lower than 2%.…”

“…We have previously shown that such a structure is relatively stabilized because of both scalar‐relativistic effects and electron correlation that are missed at the non‐relativistic Hartree‐Fock level. Furthermore, we have confirmed that these two types of effects, when simultaneously taken into account, inhibit the effectiveness of the pseudo ‐Jahn‐Teller effect at the planar D 3h structure . The latter explanation, although rigorous, lacks connection with simple concepts that are familiar to chemists.…”

“…Thus, quantum mechanical computations are very useful to gain a better knowledge of such astatine systems. Interestingly, AtF 3 appears as a borderline system as a planar D 3h local‐minimum structure is also predicted, which is almost isoenergetic with the T‐shaped one . The trifling energy barriers to go back‐and‐forth from this structure to one of the C 2V equivalent ones, results in an overall predicted D 3h structure for this system in the gas phase …”

“…The T‐shaped C 2v geometries of X = Cl − I systems can be rationalized by the pseudo‐Jahn‐Teller effect that manifests at the planar D 3h ones, or more simply by the valence shell electron pair repulsion (VSEPR) model . However, the occurrence of a local minimum of D 3h symmetry for the AtF 3 system seems intriguing.…”