Van der Waals diameter

Figure 1.7 (a) Molecular model of coronene (b) hexagonal close packing at the van der Waals diameter parts (c) and (d) illustrate the packing advantage, which can be obtained by a concerted rotation, counterclockwise or clockwise, of all molecules to allow... 

Waals radii. In carbon tetrachloride the chlorine atoms are only 2.87 A apart, and yet the properties of the substance indicate that there is no great strain resulting from the repulsion that should correspond to the van der Waals diameter 3.6 A. Even in methylene chloride and chloroform, where the strain might be relieved bv increasing the bond angle, the chlorine-chlorine distance is only 2.92 A. We conclude that the nonbonded radius of an atom in directions close to the bond direction (within 35°) is about 0.5 A less than the van der Waals radius a unicovalent atom can be considered as a sphere that is whittled down on the side of the bond. 

In many layer structures, such as clay minerals, the extent of lattice adjustment on entry of guest molecules is intermediate between the behaviors of zeolites and of clathrates. The layers remain intact, but the distance between them changes substantially (8). For water-free smectite crystals the d(001) distance is 9.4 A. The van der Waals diameter of a water molecule is 2.8 A so that, in batavite, for example, the water layer in the Na form is about 14.8 — 9.4 = 5.4 A thick, corresponding with two monolayers. 

The cavity of the A-type zeolite is approximately spherical with diameter 11.4 A. If the occluded molecule is regarded as a hard sphere, the free diameter of the cavity (the distance through which the center of the occluded molecule can move) will be reduced by a distance a corresponding to the van der Waals diameter of the molecule. The free volume of the cavity, and hence the value of K0 may be estimated on this basis. This method of calculation is equally applicable to other zeolites which consist of discrete cavities interconnected through small (eight-membered) oxygen windows. 

Critical diameters are calculated from molecular dimensions taking the van der Waals diameter of an H atom as 2.3 A. 

In a review of the motions of guest molecules in hydrates, Davidson (1971) indicated that all molecules between the sizes of argon (3.8 A) and cyclobutanone (6.5 A) can form si and sll hydrates, if the above restrictions of chemical nature are obeyed. Ripmeester and coworkers note that the largest simple structure II former is tetrahydropyran (THP) (C5H10O) with a van der Waals diameter of 6.95 A (Udachin et al., 2002). Closely following THP are m- and p-dioxane and carbon tetrachloride, each with a molecular diameter of 6.8 A (Udachin et al., 2002). Molecules of size between around 7.1 and 9 A can occupy sH, provided that the below shape restrictions are obeyed and a help gas molecule such as methane is included. 

On the first stage of hydrogenation reaction the C/H composition of fulleranes can be approximately determined by XRD from increase of unit cell parameter. The crystal structure of fulleranes powder remains to be fee like in pristine C60 but the cell parameter increases proportionally to the number of attached hydrogen atoms due to increase of van der Waals diameter of molecules. Using chemical analysis on carbon/hydrogen ratio performed on samples hydrogenated at various conditions we constructed the dependence which could be used for approximate evaluation of hydrogen content from known cell parameter determined from powder XRD, see Fig. 5.3... 

One of the main interests in urea inclusion compounds is their potential application in the separation of linear and branched hydrocarbons in the petroleum industry. Because the channel diameter is only slightly larger than the van der Waals diameter of a linear hydrocarbon, only a small amount of branching can be tolerated. Whether a particular urea inclusion compound will form or not may be assessed simply by comparison of the channel size with the diameter of the guest. For example, benzene is... 

In the present paper, we report on the dynamics of He filling the fullerite C6o fee lattice octahedral and tetrahedral interstitial voids with the respective sizes of 4.12 and 2.26 A [2], both larger than the helium Van der Waals diameter of 2.14 A [3], We also present results of study of influence of He intercalation on of photoluminescence spectra of Ceo single crystal in the low temperature phase. The measurement technique as well as the experimental setup for structural [4-6] and luminescent [5, 6] studies have been reported elsewhere. 

---