Anisotropy of van der Waals radii of atoms in molecules: alkali-metal and halogen atoms

Abstract

The van der Waals radii of the alkali-metal and halogen atoms in 28 diatomic molecules are estimated theoretically, where the radius is defined as the distance from the nucleus to the surface having a calculated electron density of 0.005a. u. (0.0337 e A-3). The anisotropy of their radii is studied. The electron-correlation effect on the electrondensity distribution in Na + and F-increases slightly (by 0.01-0.02/~) the radii calculated at the HartreeFock level of theory. A large 'polar flattening' is observed for the CI and Br atoms bonded to H or halogen atoms. This can arise from the fact that the Pz electrons of the C1 and Br atoms flow into the bonding region in forming the chemical bond. On the other hand, a small 'polar extension' is observed for the K and Na atoms bonded to halogen atoms. This phenomenon is described with a simple model calculation (Na + .... l e), where the F atom in an NaF molecule is replaced with a negative point charge. The larger repulsion energy of an (HF)2 dimer in a sideways-on contact than in a head-on contact reflects the anisotropy of the electron-density distribution around the F atom in the molecule.

A Continuum from Halogen Bonds to Covalent Bonds: Where Do λ3 Iodanes Fit?

“…The atoms that make up the "T" in λ 3 -iodanes form improper dihedrals and non-ideal bond angles. The causes of these angular and dihedral deviations are unknown, but have been related to the anisotropic nature of the electronic density distribution in iodine [21][22][23][24][25]. Bader et al showed that the vdW radius in iodine is larger at the equatorial position than at the axial position [26][27][28].…”

On the Anisotropy of van der Waals Atomic Radii of O, S, Se, F, Cl, Br, and I

“…In other cases, the agreement was poor: for F and O their anisotropy was much larger than the Nyburg–Faerman values; for S, it was much smaller. Similar trends were seen for selected molecules by Ikuta et al based on the analysis of HF densities. , Batsanov’s anisotropy values are significantly exaggerated for F, N, and O and to a lesser degree for Cl as compared to those of Nyburg and Faerman or the results of this work. A clear advantage of the probe methodology from the theoretical point of view is that it is not biased with respect to any models of halogen bonding , or other models of secondary bonding .…”

Charge density studies on 2,3,5,6-tetrafluoro- and pentafluoropyridine

“…Thus, as a converse argument one can conclude that these F … F contacts (3.2242(7) Å) are likely to be also attractive, although they are larger than the sum of two flattened van der Waals radii of fluorine in a lateral direction to the C-F bond, which is 2.78 Å. 26,27 PFP does not contain C-H functions and therefore its crystal structure cannot be dominated by hydrogen bonds. However, there are a number of F … F contacts, which are listed in Table 5.…”