Selective Emergence of the Halogen Bond in Ground and Excited States of Noble‐Gas–Chlorine Systems

Abstract

Molecular‐beam scattering experiments and theoretical calculations prove the nature, strength, and selectivity of the halogen bonds (XB) in the interaction of halogen molecules with the series of noble gas (Ng) atoms. The XB, accompanied by charge transfer from the Ng to the halogen, is shown to take place in, and measurably stabilize, the collinear conformation of the adducts, which thus becomes (in contrast to what happens for other Ng‐molecule systems) approximately as bound as the T‐shaped form. It is also shown how and why XB is inhibited when the halogen molecule is in the 3Πu excited state. A general potential formulation fitting the experimental observables, based on few physically essential parameters, is proposed to describe the interaction accurately and is validated by ab initio computations.

Complexes of helium with neutral molecules: Progress toward a quantitative scale of bonding character

“…For example, as already noted previously, HeHF is definitely more stable than HeFH, but HeClH and HeBrH are more stable than the corresponding H‐coordinated isomers, the difference increasing on going from Cl to Br. We also note that, in line with the recent discussion of the isomeric He(Cl 2 ), the E ind of the linear HeClCl is more negative than the corresponding term of the He(Cl 2 )T [−9.23 vs. −5.24 cm −1 ], the difference of these terms mirroring, essentially, the difference of the corresponding interaction energies (−47.19 vs. −43.72 cm −1 ). But the most relevant results were obtained by evaluating the percentage contributions of E elst , E ind , and E disp to the total attractive part of the interaction ( E elst + E ind + E disp ).…”

“…When the CT takes place, it is convenient to come up with a definite numerical estimate, which can be done by taking the value of the CD curve at a specific point between the fragments. Based on previous studies, we choose to separate the fragments and to extract the CT at the so‐called isodensity boundary (IB), that is, at the point along z where the electron densities of the noninteracting fragments become equal.…”

“…The Ng(M) are, in particular, usually perceived as typical van der Waals molecules, held together by the favorable balance between dispersion and repulsion. The Ng, however, while generally resistant to share their electrons so to form true ionic or covalent bonds, sensibly respond to the polarization effects exerted by the ligands, and even the complexes with nonpolar molecules may include not only inductive, but also CT stabilizing components . In particular, searching for a mode to analyze this transition in the bonding character, we recently obtained evidence for a new quantitative index that is generally informative about the interactions occurring in noncovalent Ng complexes.…”

Noncovalent Complexes of the Noble‐Gas Atoms: Analyzing the Transition from Physical to Chemical Interactions

“…When the CT takes place, it is convenient to come up with a definite numerical estimate, which can be done by taking the value of the CD curve at a specific point between the fragments. According to our previous studies, we choose to separate the fragments and to extract the CT at the so‐called isodensity boundary, that is, at the point along z where the electron densities of the noninteracting fragments become equal.…”

“…The latter contribution is particularly relevant, as it marks the transition from a purely (or nearly purely) “physical” contact to a more specific “chemical” noncovalent bond. In particular, some of us recently ascertained that the CT plays a major role in the complexes of the Ng atoms, particularly He, Ne, and Ar, with H 2 O, NH 3 , CCl 4 , and Cl 2 . These systems could be, actually, assigned as remarkable examples of “nascent” hydrogen and halogen bonds.…”