Bonding Motifs of Noble-Gas Compounds As Described by the Local Electron Energy Density

Abstract

The bonding situation of some exemplary noble-gas (Ng) compounds, including HNg(+), HNgF, FNgO(-), Ng-HF, and NgBeO (Ng = He-Xe) was assayed by examining their local electron energy density H(r). In general, this function partitions the space of atomic species (neutral and ionic) into inner regions of negative values and outer regions of positive values. In the formation of chemical bonds, these atomic regions combine so to form a molecular H(r), Hmol(r), whose plotted form naturally shows the "covalent" and "noncovalent" regions of the molecular species and allows also the recognition of different types of noncovalent interactions such van der Waals, hydrogen, and ionic or partially ionic bonds. The qualitative assignment of the various bonding motifs is corroborated by the topological analysis of Hmol(r), which typically includes several critical points of rank 3 and variable signature. These points are, in particular, characterized here in terms of their bond degree (BD). From a previous definition (Espinosa J. Chem. Phys. 2002, 117, 5529-5542), this quantity is taken as the ratio between the energy density calculated at the critical point of H(r), H(rc), and the corresponding electron density ρ(rc): BD = -H(rc)/ρ(rc). Thus, the BD is positive for covalent interactions (H(rc) < 0) and negative for noncovalent interactions (H(rc) > 0). For structurally related species, the BD result, in general, positively correlated with the binding energies and is, therefore, a semiquantitative index of stability. The present study suggests the general validity of the Hmol(r) to effectively assay the bonding motifs of noble-gas compounds.

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

“…As shown in the previous study, the analysis of the H ( r ) is informative about diverse types of bonding motifs, including those typically occurring in Ng compounds. Figures and show, in particular, the plotted H ( r ) of the Ng(HF) (Ng = He–Xe) and Ar(HX) (X = Cl, Br, I).…”

“…For R > R − , the H ( r ) is positive, and becomes vanishingly small at the largest distances, passing through a point of maximum. In essence, as best discussed in our previous study, the H ( r ) partitions the atomic space in two well‐recognizable regions, namely an inner one of negative values (that sometimes encloses tiny inner subregion[s] of positive values), indicated here as H − ( r ), and an outer one of positive values (that sometimes encloses tiny inner subregion[s] of negative values), indicated here as H + ( r ). The boundary of these two regions falls at a distance R − , that is typical of each atom; at this distance, H ( r = R − ) = 0.…”

“…This arises, essentially, from the bell shape of the H ( r ) in the outer H + ( r ) region (further details are given in Ref. ), that sometimes prevents the formation of a true HCP along the bond axis. In cases like these, we sign the interaction by including, in the plotted from of the H ( r ), the bond critical point (BCP) located from the AIM analysis.…”

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

“…At variance with the covalent bonds, these noncovalent contacts are not invariably signed by a HCP. This is due, essentially, to the bell shape of the H ( r ) in the outer H + ( r ) region (further details are given by Borocci et al, that sometimes prevents the formation of a true HCP along the bond axis. In cases like these, the interaction is conveniently signed by including, in the plotted H ( r ), the BCP located from the analysis of the ρ ( r ) (the classical AIM).…”

The interstellar formation and spectra of the noble gas, proton-bound HeHHe+, HeHNe+ and HeHAr+ complexes

“…However, a complete and reliable set of rovibrational spectroscopic data have yet to be produced for this simple system while other insights into its nature have been explored theoretically (Baccarelli, Gianturco & Schneider 1997;Panda & Sathyamurthy 2003;Bartl et al 2013). Other data for related noble gas molecules have been produced including those with helium (Fridgen & Parnis 1998;Lundell, Pettersson & Rasanen 1999;Koner, Vats & Panda 2012;Koner et al 2014;Borocci, Giordani & Grandinetti 2015;Grabowski et al 2016;Koner et al 2016), but full spectral charactization is still lacking for most of these structures.…”