The Halogen-Bond Nature in Noble Gas–Dihalogen Complexes from Scattering Experiments and Ab Initio Calculations

Abstract

In order to clarify the nature of the halogen bond (XB), we considered the prototype noble gas–dihalogen molecule (Ng–X2) systems, focusing on the nature, range, and strength of the interaction. We exploited data gained from molecular beam scattering experiments with the measure of interference effects to obtain a suitable formulation of the interaction potential, with the support of high-level ab initio calculations, and charge displacement analysis. The essential interaction components involved in the Ng–X2 adducts were characterized, pointing at their critical balance in the definition of the XB. Particular emphasis is devoted to the energy stability of the orientational Ng–X2 isomers, the barrier for the X2 hindered rotation, and the influence of the X2 electronic state. The present integrated study returns reliable force fields for molecular dynamic simulations in Ng–X2 complexes that can be extended to systems with increasing complexity and whose properties depend on the selective formation of XB.

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

“…It is worthwhile to remark that the employed CD analysis suggests the presence of a small CT component even in the helium cases, with a transferred charge of 0.39/0.47/0.55 me in He-Cl 2 /Br 2 /I 2 complexes, to be compared with 3.33/5.59/7.62 me computed for the correspondent Xe complexes [17]. Remarkably, the ab-initio CT magnitude values decay exponentially with the inter-fragments distance, matching the exponentially decay form adopted in the semi-empirical model potential.…”

“…In the Ng-X 2 complexes (Ng = He, Ne, Ar, Kr, Xe; X = Cl, Br, I) the halogen bond formation selectively stabilizes the collinear configuration of the adduct, with X 2 being in the X 1 Σ g ground state. On the other hand, for the adducts in the perpendicular configuration or with X 2 in the B 3 Π u excited state, this effect is missing (see Figure 2, top) [17,54]. The proposed analytical model PES is composed of three additive components-(1) the two-center vdW component, represented by an Improved Lennard-Jones (ILJ) formulation [55]; (2) an exponentially decaying "three-body" term, pointing at the interaction anisotropy due to the X 2 molecular orbitals-especially critical in the perpendicular and saddle configurations concerning the interaction of Ng with the X 2 outermost π g * density;…”

“…The nature, strength and selectivity of halogen bonds in prototype systems have been explored by applying an integrated theoretical-phenomenological approach, based on high level ab-initio calculations and gas phase molecular beam scattering experiments [17,18,54]. Starting from the experimental cross section data, which gives precious information on the interaction potential at both short and long range and applying a semi-empirical iterative procedure, we ended up with an analytical representation of the interaction potential energy surfaces (PESs) based on few physically meaningful parameters [54].…”

“…It is worthwhile to remark that the employed CD analysis suggests the presence of a small CT component even in the helium cases, with a transferred charge of 0.39/0.47/0.55 me in He-Cl2/Br2/I2 complexes, to be compared with 3.33/5.59/7.62 me computed for the correspondent Xe complexes. [17] Since it is well known that the ILJ function adequately reproduces the vdW interactions, any deviation of the interaction potential from the ILJ prediction suggests the occurrence of additional interaction components. Specifically, the presence of a CT component in the Ng-X 2 complexes bond has been ascertained by applying the CD analysis (see Figure 2, bottom).…”

“…Chemical insights will be drawn about the nature of halogen bond, its cooperativity and its influence on metal-ligand bond components.Molecules 2020, 25, 300 2 of 17 Some years ago, the Charge Displacement (CD) function was introduced to study the chemical bond between noble metals and noble gases [13] and then applied to various kind of chemical interactions. These include the quantitative study of charge transfer in hydrogen bonding [14][15][16] and halogen bonding [17,18] in weakly bound adducts, to characterize charge transfer (CT) effects on conduction band of dye sensitized solar cells [19] and also to describe the charge rearrangement in the electronic excited states [20]. Above all, it provided to be extremely versatile in coordination chemistry.…”