Bondi radius

Element Cavity radius (A) Bondi radius (A) Ratio vdWs coefficient (kJ/mol/nm2)... 

Allinger 7) has critized the Van der Waals radii of Pauling and Bondi. He argues that the Van der Waals radius must be larger than is expected from the distance between nonbonded atoms in a crystal as London forces will result in interpenetration of the Van der Waals radii. Allinger recommends a set of radii which are reported in Table 1. We find that the Van der Waals radii of Allinger, rVx, are highly linear in those of Bondi, rVB- The results of correlation with the equation... 

The S-S bond a appears to be almost normal, but there is also some shortening of the O-S distance b by comparison with the van der Waals contact distance. We have already accepted 1.70 A as the van der Waals radius of sulfur. According to Bondi,105 a mean value acceptable for oxygen seems to be 1.50 A. The S-0 contact distance would then be 3.20 A, which is much larger than the measured b distance, even if we make substantial allowance for the uncertainty in the determination of van der Waals radii. For instance, even if we accept errors of 0.2 A for both radii, we find a minimum S-0 contact distance of 2.80 A, still considerably larger than the measured value. 

Let us remember that for the corresponding trithiapentalene already discussed,112 we had the following distances a= 2.510 and b = 2.216 A. It is remarkable that the analogy with this trithiapentalene system is such that the Se-S distance b is shorter than the S-S distance a. According to Bondi,105 the van der Waals radius for selenium is approximately 1.90 A, but the data are scarce. So it seems safer to make a correction for sulfur data based on the comparison of H2S and H2Se,105 which shows, for the latter, an increment of 0.1 A for the van der Waals radius of the central atom. The contact distance evaluated as 3.4 A for S-S should be approximately 3.5 A for S-Se. At any rate, the values found are good evidence for abnormal S-S-Se bonding. 

Element Atomic mass Radius (nm) Pauling (1940) Bondi (1968) et al. (1970)... 

When a molecule is represented as an object with well-defined boundaries, geometric methods of analysis may be applied. Several steric parameters have been derived from the model in w hich atoms, hence molecules, are represented as geometric objects with surfaces and volumes. The boundary of an atom is usually determined by its van der Waals radius. Surface area and volume of groups and molecules can be estimated. Bondi,Hermann, and Pearlman are among those who have studied surface area and volume. Molar refractivity (MR), calculated from density and refractive index, has frequently been used to estimate the bulk or global volume of a molecule or group. A more recent trend is to use MR as a model for dispersion interactions. 

Bondi ° indicates that, for single-bonded sulfur. X-ray diffraction points to 1.83 A, whereas various physical properties suggest a radius of 1.80 A. The smaller values found for crystalline sulfur and polysulfides may be indicative of some double-bond character of the S-S bonds. 

By interlocking spheres centered on each nuclei and with the radius defined in terms of the respective van der Waals radii (Bondi [82]) multiplied by a factor 1.2, the van der Waals surface of the solute is constructed. The SE cavity is then defined as the contact surface of a probe sphere (with radius equal to the molecular radius of the solvent molecule) rolling on the solute van der Waals surface. 

The procedure we describe is essentially that of Bondi and Connor, except for minor differences in strategy (they evaluate asymptotic boundary conditions directly on the final hyperspherical radius). We begin by recalling the definition of the final hyperspherical R matrix,... 

In principle, the vdW radius of an atom must depend on its effective charge. However, Pauling [82] found that vdW radius of a nonmetal coincide with its anionic radius, inasmuch as the bonded atom presents the same face to the outside world in directions away from its bond as the ion does in aU direction and used this rule to suggest vdW radii for some elements. He also noticed that vdW radii exceed the covalent radii (r) of the same elements by S 0.8 A Bondi used this rule in constructing his system of radii (he assumed S = 0.76 A), as well as some other physical properties (e.g. critical volumes). This value can be deduced from covalent radii using the equations... 

Table 5 Atomic van der Waals radii (A) according to Bondi and Rowland and Taylor (in italics) (H radius in aromatic rings = 1.00 A).

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