Fluorine, interatomic distances

The structure of LiTa02F2, as reported by Vlasse et al. [218], is similar to a ReC>3 type structure and consists of triple layers of octahedrons linked together through their vertexes. The layers are perpendicular to the c axis, and each layer is shifted, relative to the layer below, by half a cell in the direction (110). Lithium atoms are situated in the centers of the tetragonal pyramids (coordination number = 5). The other lithium atoms are statistically distributed along with tantalum atoms (coordination number = 6) at a ratio of 1 3. The sequence of the metal atoms in alternating layers is (Ta-Li) - Ta - (Ta-Li). Positions of oxygen and fluorine atoms were not determined. The main interatomic distances are (in A) Ta-(0, F) - 1.845-2.114 Li-(0, F) - 2.087-2.048 (O, F)-(0,F) - 2.717-2.844. 

For calculating the interatomic distance for a single bond, Ri, involving an atom of nitrogen, oxygen, or fluorine (or two such atoms), we use the equation of Schomaker and Stevenson namely... 

Hydrogen bonds between fluorinated substrates and biological macromolecules have been postulated in some enzyme-substrate complexes. However, it is rather difficult to determine if these hydrogen bonds really exist other factors may stabilize the conformation corresponding to the short H- F interatomic distance observed. Indeed, this conformation can be favored by other factors (e.g., other stronger hydrogen bonds, gauche effect), without participation of an H- F interaction to stabilize the supramolecular structure. The existence and possible... 

More recently, additional 2H "I J REDOR measurements on new PTX derivatives bound to MT provided three new interatomic distances that helped to define more precisely the bound conformation of PTX [87], The two PTX analogues labeled with fluorine and deuterium were designed to determine the three key distances based on the measurement of 2II— "I distances, as indicated in Fig. 9 (compounds 4 and 5). These interatomic distances rule out the polar and nonpolar conformers and further support the T-taxol conformation (Tablel). The polar conformation is rejected on the basis of distance IV, as the 4.5 A is much shorter than the > 8 A determined by REDOR. The nonpolar conformation can be ruled out on the basis of distances I and II, that are too short in the nonpolar conformer compared to the experimental ones (1.8 and 3.6 A short). Besides, bridged taxanes resembling the nonpolar conformation displayed no activity in the tubulin assembly assay [88], PTX-NY closely matches the REDOR distances. Nevertheless, it is discarded as the bioactive form of PTX due to its poor fit to the EC density [87],... 

Typical examples of compounds that form chain-type structures are simple pentafluorides of tantalum and niobium, TaFs and Nbfs, which were investigated by Edwards [199]. The cry stal structure of NbF< consists of rings made up of four NbF6 octahedrons that are linked by fluorine ligands that are each shared by two neighboring octahedrons, as shown in Pig. 29. The main interatomic distances (in A) are as follows Nb-Nb 5.80, 3.90 and 4.13 Nh-P,(Ki -. 1.75 and 1.78 Nb-Ftal% - 2.06 and 2.07 angle Nl>F[jf d,e-Nh - 182.5°. 

Potassium tetrafluoroargentate(III) was first described by Hoppe in 1957 [17]. It was synthesized by fluorination of mixtures of binary nitrates or halides. Hoppe and Homann [18] established from powder data that the interatomic distance in the square anion AgF4 is 1.90 A. At approximately the same time Edwards et al. [19] also reported a preparation of KAgF4 and their unit cell (also from the powder data), was in good agreement with that of Hoppe and Homann. 

PdF, is isostruotural with MnF, FeF, CoF, NiF, and ZnF,. Each palladium atom has six fluorine atoms forming an almost regular octahedron around it. The PdF, coordination octahedra are joined by sharing comers. The interatomic distances are given in Table 2. 

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