Van der Waals radius of oxygen

Pauling gives the van der Waals radius of oxygen as 1.40 A and a methyl group as 2 A. Consequently, S will be OMe and M will be Et, so 24 should give the major product, particularly since it is favored by dipolar interactions. This prediction is exactly the opposite of the result obtained using Cornforth s model.54... 

Fig. 8.5 Sliorl, symmetrical hydrogen bond in the cation of bis(N-nitrosopyrrolidine)-hydrogen hexafluorophosphate which crystallize.s in the monoclinic space group P2,/c. The hydrogen bonded proton lies on a center of symmetry, with only one of the nilrosopyrrolidine molecules being crystullographically unique. Note that the 0—0 distance is only 247 pm whereas two limes the van der Waals radius of oxygen (Table 8.1) is 300 pm. [From Keefer, L. K. Hrabie. J. A. Ohannesian, L. Flippen-Anderson, J. L. George, C. J. Ain. Chem. Soc. 1988, no, 3701-3702. Reproduced with permission.]...

Covalently bound oxygen atoms would be less than 1.5 A apart, but no hydrogen bonds less than 2.4 A long are known. The upper limit for hydrogen bonding might be set at about 2.8 A, twice the van der Waals radius of oxygen. This limit must be raised a httle, however, since... 

Albertson and Elding (1977) estimated a value of 1.1 A as the best lit radius of the cavity for a hard-sphere model of the TCTP structure with equal prismatic and equatorial Ln O distances. In this model, lanthanides of smaller radii than Gd + (r = 1.107 A) for coordination number CN = 9 (Shannon 1976) are expected to destabilize the structure. These calculations were based on the assumption that the van der Waals radius of oxygen is 1.50 A. 

In the [2.2.2] complex with cesium, the Cs" ". .. O distances are somewhat smaller than the corresponding sum of the Van-der-Waals radius of oxygen and the ionic radius of Cs", as was similarly observed in the Cs" complex of nonactin (see Chap. 3). This indicates that the cation presses heavily on the ligand. In fact, it was concluded from Cs-NMR data that in solution, [2.2.2] forms an exclusive complex with Cs" ", i.e. the metal ion does not occupy the central ligand cavity but is only partly... 

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