Bonding Analysis in Homo‐ and Hetero‐Trihalide Species: A Charge Displacement Study

Abstract

The bonding in homo- (X3–) and hetero-trihalides (XYX–), with X = F, Cl, Br, and I; Y = I, has been analyzed through the charge displacement (CD) method, coupled with energy decomposition analysis (EDA). We focus our attention on how the different bond components vary with the distance between the terminal and the central halogen atoms. The results clearly show that all the homo-halide systems X3–feature the same trend of the charge transfer (CT) versus the asymmetry of the three-body system, defined as the ratio between the two bond lengths dX1–X2and dX2–X3, whereas in the case of the hetero-trihalides BrIBr–and ClICl–, the CT values are similar, but systematically lower with respect to the homo-trihalide case. Comparison with solid-state geometries also allows us to estimate the influence of the crystal lattice (the packing and the interactions with the surrounding fragments) on the geometry. Following the results reported here, the CD method is confirmed to be a useful alternative computational tool to analyze the nature and the origin of weak intermolecular interactions such as halogen and hydrogen bonding

Bond Analysis in Dihalogen–Halide and Dihalogen–Dimethylchalcogenide Systems

“…For the bonding analysis of the II‐I systems, the SAPT0 results are in agreement with the EDA ones, which have been exposed previously, even if at different values of distances . Here we just underline that the CT S obtained spans between –78.80 and –3.58 kcal/mol for II‐I(–2) and II‐I(2) , respectively.…”

“…It is important to remember here that, according to our previous paper, in the X 3 – systems (X = F, Cl, Br, I), the X – ··· X 2 interaction, in terms of CT CD , does not depend on the nature of X, but only on the geometric arrangement of the three atoms. Now we can generalize the message: even considering different fragments, charged or not, involving halogens or chalcogen atoms, the charge transfer mainly depends on the relative bond elongations.…”

“…Such a difference makes the ClI moiety less polar in ClI‐SeMe 2 (rel) , lowering the polarization effect on SeMe 2 , but it is also less prone to accepting electronic density from the selenium, as the iodine receives more electron density from the chlorine. As a result, CT CD is much lower than in the case of ClI‐SeMe 2 (0) , being 0.203 e instead of 0.261 e. But, as previously underlined, the fully relaxed structures in the gas phase does not reproduce the structural parameters observed in solid‐state structures, likely due to the polar microenvironment within the crystal lattice and its influence on the chemical bonding within the halogen/chalcogen three‐body system …”

“…Recently, to find a rationale for the experimental evidence that 22‐valence‐shell‐electron three‐body systems share the same nature in terms of chemical bonding, we analyzed X 3 – trihalide systems, by means of the charge‐displacement method (CD), energy decomposition analysis combined with natural orbital for chemical valence (EDA‐NOCV), and natural population analysis (NPA) methodologies, focusing attention on how the different bond components vary with the internuclear distance. Interestingly, different trihalide systems, showing the same values of δ AB and δ BC , also exhibit the same values of charge transfer.…”

Charge Displacement Analysis—A Tool to Theoretically Characterize the Charge Transfer Contribution of Halogen Bonds

“…Prompted by these results, we decomposed the symmetric I 3 − system in I 2 and I − , treating the system as an extreme case of XB. The 3D deformation maps of the electronic density showed that the iodide undergoes charge depletion and the iodine charge accumulation, with a severe polarization of the molecule (Figure 5a) [61]. A large amount of charge is displaced at the isoboundary (CT), 0.400 e, underlining the covalency of the 3c4e bond.…”