Interatomic distance carbon compounds

The units by which crystallographers describe interatomic distances are Angstrom units (A = 10 8 cm.). Normal values for carbon-carbon interatomic distances are 1.34 A for a double bond (as in ethylene) and 1.54 A (as for-diamond) for a single bond. In a truly aromatic compound (such as benzene) the C-C bond length, as mentioned above, is 1.39 A. C-C-C angles are 109.5° for a tetrahedral carbon atom (sp3) and 120.0° for a trigonal carbon atom (sp2). 

Covalent radii are calculated from half the interatomic distance between two singly bonded like atoms. For diatomic molecules such as F2, this is no problem, but for other elements, such as carbon, which do not have a diatomic molecule, an average value is calculated from a range of compounds that contain a C-C single bond. 

In the Sm2Fei7Xy (X = C and/or N) materials, the Tc value and EMD are dominated by the content of X, which cause expansion of the Fe-Fe interatomic distance and enhance magnetic interactions. The carbide Sm2Fe17Cy consists of nonstoichiometric compounds with a wide range of carbon content and for y < 1 are stable at temperatures ( 1300 K) sufficient for sintering. On the other hand, Sm2Fe17N3, completely decomposes at such temperatures because no nitride with x < 1 is obtained. Similar effects on Tc and EMD are produced when Co substitutes for Fe, e.g. (Sm2Fe1 Co )l7C>,. 

Stereoisomerism refers to molecular species that have the same composition and bond sequence but a different arrangement in space of their atoms. Stereoisomers that are characterized by different interatomic distances between certain atoms that are not bound directly are called diastereoisomers. Examples of diastereoisomers are cis-trans isomers of compounds containing C = C bonds and syn-anti isomers of compounds containing C = N bonds. Other diastereoisomers are not based on the presence of a double bond. For example, a molecule with more than one tetrahedral carbon that has different substituents may form diastereoisomers. For example, for a head-to-tail polymerized monosubstituted vinyl monomer, there are three possible structures indicated as isotactic, syndiotactic and atactic, which are schematically shown below ... 

Acetylcholine and related esters can exist in two conformations (Fig. 3.5), the skewed and extended forms (200) (e.g., 44 and 45, respectively, for acetylcholine). The skewed form (44) of acetylcholine is closely related to the structure of muscarine (46) (200). Similarly the substituted aminoethyl esters, which are anticholinergics, may exist in two conformations. The skewed forms of acetylcholine (44), muscarine (46), the skewed form of aminoethyl esters (47), and the extended form of aminopropane derivatives (48) all interact at the same muscarinic receptors. In the former two compounds the interatomic distance between the quaternary nitrogen and the ether oxygen atom is nearly the same, and both of them are agonists. In (48) the interatomic distance between the quaternary nitrogen and the carbon atom to which the cyclic radicals are attached is the same, and both of them are... 

In compound 76 and in dimesitylmanganese, which crystallizes as the trimer [Mn(mesityl)2]3 (77) " the degree of association is limited by the bulk of the substituents. All of these systems show the characteristic features of 3c-2c Mn-C-Mn bridge bonding greater Mn-C interatomic distances to the bridging (hypercoordinated) carbon atoms than to their terminal counterparts sensitivity of the metal-carbon distance to the metal coordination number and acute Mn-C-Mn bond angles at the hypercoordinated carbon atoms. 

Wo might here deal briefly with the structure of the well-known interstitial compound iron carbide, or cementite. The largest interatomic distance in iron is 2-52A, which figure may be taken for the atomic diameter of iron, while that of carbon is 1-54A, so that the diameter ratio is 1 54/2 52 — (Mil i.e. the diameter ratio is greater than 0 59. The relatively small size of the iron atom therefore induces the formation of an interstitial structure which is more complex than the simple cubic or hexagonal types described above. ... 

We will show that the interatomic distances and their powers offer a better characterization of molecular shapes. The shape of an object is defined by its exterior, surface, or contour in the case of 2D objects. Hence, when characterizing molecular shape one should consider only the interatomic separations between atoms at the molecular periphery, excluding atoms in the interior part of the structure. In the case of cflto-condensed benzenoids, all carbon atoms are on the periphery, hence the shape profile and the molecular profile of such compounds are the same. In the case of peri-condensed benzenoids, only carbon atoms at the periphery will be taken into account. 

The lattices in which covalencies link atoms through many unit cells are themselves of various kinds. The simplest is perhaps that of diamond, in which each carbon is joined by tetrahedral oovalenoies to four others. The interatomic distances correspond to those between carbon atoms finked by single bonds in the molecules of organic compounds, and the total energy of the lattice is the sum of all the bond energies. 

For a compound with higher carbon content than this compound, no indication that the additional occupation of the C2 pairs in the metal octahedra occurs was found although the octahedron at z 0 exhibits a relatively large Sc-Sc distance. The shortest interatomic distances, 2.99, 3.11 and 3.18 A for Sc-Sc, as well as 2.24 and 2.26 A for Sc-C, are related to the noticeable ionic portion of the bond. The C-C distance in the C2 pair, 1.25 A, is in the range of that for the dicarbides of Ca, Y and the rare earths. All the interatomic distances have been reported by Jedlicka et al. (1971), showing that the ScijCig compound has relatively large values, 3.30 and 2.55 A, for the Sc-Sc and Sc-C distances. 

X-ray diffraction studies on single crystals of NdFe oSiCo 5 have established its structure with the space group I4i/amd and Z = 4 formula units per unit cell. The positions of the neodymium, iron and silicon atoms correspond to the BaCdn structure. The positional and thermal parameters for NdFejoSiCo s of BaCdj, type structure have been given by Le Roy et al. (1987), as well as the interatomic distances in this compound. This structure is a disordered version of LaMnnC, 52 (Ross et al. 1981). Each neodymium atom is surrounded by a polyhedron of 26 atoms, i.e. 18 iron atoms, four carbon atoms and four atoms which may be iron or silicon. Fe(l) is surrounded by 14 atoms, Fe(2) by 13 atoms, and the (Fe-Si) site by 12 atoms. As has been pointed out, the carbon atoms fill octahedral voids formed by four iron atoms and two neodymium atoms, with an occupancy of 25%. 

A third type of crown ether magnesium complex includes compounds containing an Mg-C bond to a carbon atom of the macrocycle, formed as internally com-plexed Grignard reagents 101 [375, 387, 388]. The crystal structure determination of 101 (X=Br) shows that the metal coordination is distorted pentagonal pyramidal, with bromine in apical position (Mg-Br 2.517 A) and two normal Mg-O distances (2.12 and 2.13 A) and two large interatomic distances (2.33 and 2.49 A) [388]. 

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