Article pubs.acs.org/JPCA
Nature of the Bonding in the AuNgX (Ng = Ar, Kr, Xe; X = F, Cl, Br, I) Molecules. Topological Study on Electron Density and the Electron Localization Function (ELF) Emilia Makarewicz, Agnieszka J. Gordon, and Slawomir Berski* Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie, 50-383 Wroclaw, Poland S Supporting Information *
ABSTRACT: Topological analysis of the electron localization function (ELF) has been carried out for the AuNgX (Ng = Ar, Kr, Xe; X = F, Cl, Br, I) molecules using the wave function approximated by the CCSD, MP2, and DFT(B3LYP, M062X) methods including zero-order regular approximation (ZORA). In the Ng−F bond, the bonding disynaptic attractor V(Ng,F) is missing; therefore, there are no signs of the covalent binding. The nature of the Au−Ng bond depends on the computational method used. Analysis of the ELF carried out for the AuArF and AuXeF molecules, with the wave function approximated by the CCSD and MP2 methods, shows the V(Au,Ng) attractor possibly corresponding to a partially covalent binding between the gold and noble gas atom. However, its very small basin population (0.06 e/bohr3). Positive values of the ∇2ρ(3,−1)(r) are between 0.094 (Xe) and 0.254 (Ar) e/bohr5 for the Au−Ng bond and 0.214 (Xe) and 0.310 (Ar) e/bohr5 for the Ng−F bond. Such results suggest that both bonds belong to the closed-shell type.44,45 It is worth noting that the ∇2ρ(3,−1)(r) value for the Au−Ng and Ng−F bonds decreases with the change from Ar to Xe. The electron density around the BCP is locally depleted. In contrast, results of H(r), the total electronic energy density, are negative. This indicates a major contribution of the potential energy density with a partially covalent contribution. The 2D distributions of the Laplacian of the electron density ∇2ρ(r) for the AuNgF molecules are shown in Figure 1. No concentration of the electron density is observed in the regions of the Au−Ng and Ng−F bonds. In context of the nature of the Au−Ng and Ng− X bonds, a combination of the values ∇2ρ(3,−1)(r) > 0 and H(r) < 0 can be interpreted as a “transient zone” between typical
Figure 1. 2D representations of the Laplacian of the electron density for AuArF, AuKrF, and AuXeF. All values for the (3,−1) CPs are in atomic units.
ionic and typical covalent bonds. Similar characterization has also been proposed for the metal−metal bond.46 3.3. Analysis of the η(r) Distribution. Topograhical analysis of the ELF has been performed on the basis of molecular orbitals generated at DFT(B3LYP)/TZ2P, CCSD/ Def2-TZVPPD, and DFT(B3LYP)/Def2-TZVPPD computational levels using geometrical structures optimized by Mou and Witek.7 For the DFT calculations with the TZ2P basis set, the relativistic effects were included through the ZORA (see the Computational Details section). For the calculations with the Def2-TZVPPD basis set, the relativistic effects were included through the pseudopotential approximation. The 2D distributions of the η(r) function for the AuNgF (Ng = Ar, Kr, Xe) molecules, obtained at the DFT(B3LYP)/TZ2P level, are presented in Figure 2a−c. The results for the CCSD/Def2TZVPPD level are shown in Figure S1 (Supporting Information). All methods used in this study yield very similar results for the valence regions; therefore, only DFT(B3LYP)/ TZ2P results will be discussed here. Topographical study shows three domains of high values of the ELF in valence regions that are associated with electron density of the gold (η(r) ≈ 0.7), noble gas atom (η(r) ≈ 0.9), and fluorine (η(r) ≈ 0.9). The probability of finding electron pairs with opposite spins (normalized with respect to the model of uniform electron gas) in those regions is high. The cloud of C
dx.doi.org/10.1021/jp508266k | J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
complete explanation for the strong stability of xenon difluoride must take into account the occurrence of the charge shift (CS) bonding.48 In the CS model, the fluctuation of electron density (an electron pair) in the bonding region A+B−, A−B+ is responsible for the stability of the chemical bond AB not the localization of the electron pair A−B. If the nature of the Au− Ng and Ng−F bonds has a CS character, no bonding basin should be found in the topological analysis of the ELF. Alternatively, a bonding basin with very small basin populations (
