Effective bond orders from two-step spin–orbit coupling approaches: The I2, At2, IO+, and AtO+ case studies

Abstract

The nature of chemical bonds in heavy main-group diatomics is discussed from the viewpoint of effective bond orders, which are computed from spin-orbit wave functions resulting from contracted spin-orbit configuration interaction calculations. The reliability of the relativistic correlated wave functions obtained in such two-step spinorbit coupling frameworks is assessed by benchmark studies of the spectroscopic constants with respect to either experimental data, or state-of-the-art fully relativistic correlated calculations. The I 2 , At 2 , IO + , and AtO + species are considered, and differences and similarities between the astatine and iodine elements are highlighted.In particular, we demonstrate that spin-orbit coupling weakens the covalent character of the bond in At 2 even more than electron correlation, making the consideration of spin-orbit coupling compulsory for discussing chemical bonding in heavy (6p) main group element systems.

Relevance of effective bond orders in heterodiatomic molecules and role of the spin-orbit coupling in theAtX(X=At–F)series

“…In the present work, exact two-component (X2C) calculations will be performed for computing equilibrium distances, at which the EBOs will be computed at both the scalarrelativistic and SOCI levels. This way, we aim at solving for the issue of heterodiatomic systems in the simpler picture that is obtained while introducing the SOC a posteriori.Note that, in all the studied cases, the nature of the relativistic ground state is similar at both the SOCI and X2C levels, as was also observed in At 2 [13]. Therefore, we are confident in the conclusions that we will obtain at this level for the AtX (X = At -F) series, and consider the extension of the proposed methodology for heterodiatomic systems to the gEBO framework as a perspective.…”

“…(2) for defining the EBO at the SOCI level [12]. Alternatively, the interpretation of the wave function is possible without explicitly computing the occupation numbers [13], and strictly equivalent to the previous approach providing that the same set of MOs (in practice, the SA-SS-NOs) is used. One can also determine EBOs at the uncontracted SOCI level [36],asshownbysome of us [13].…”

“…Actually, although it is known that the spin-orbit coupling may affect chemical bonds [1], it is quite common in the quantum chemistry community to neglect it when discussing the binding in heavy-element systems [9][10][11]. Nevertheless, the inclusion of the spin-orbit coupling (SOC) in the computation of EBOs is quite straightforward and has been first reported by Gendron et al [12] and independently reported soon after by some of us [13]. Furthermore, the EBO concept has also been very recently generalized to the fully relativistic frameworks [14], and thus we consider that it should now be admitted that the SOC must not be neglected while computing EBOs in heavy-element systems.…”

On the Interplay between Charge‐Shift Bonding and Halogen Bonding

“…[42,44,[48][49][50][51][52][53] For instance, the interaction energy of At-mediated XBs can be affected by SOC effects up to 35 %. Nevertheless, many studies have demonstrated that the quantum calculations should also include the relativistic spin-orbit interaction for At-containing compounds.…”

“…Nevertheless, many studies have demonstrated that the quantum calculations should also include the relativistic spin-orbit interaction for At-containing compounds. [42,44,[48][49][50][51][52][53] For instance, the interaction energy of At-mediated XBs can be affected by SOC effects up to 35 %. [26,45,46] Hence, all these systems have been additionally investigated through twocomponent (2c) relativistic calculations.…”