Bond distance compounds

Bismuthides. Many intermetaUic compounds of bismuth with alkafl metals and alkaline earth metals have the expected formulas M Bi and M Bi, respectively. These compounds ate not saltlike but have high coordination numbers, interatomic distances similar to those found in metals, and metallic electrical conductivities. They dissolve to some extent in molten salts (eg, NaCl—Nal) to form solutions that have been interpreted from cryoscopic data as containing some Bi . Both the alkafl and alkaline earth metals form another series of alloylike bismuth compounds that become superconducting at low temperatures (Table 1). The MBi compounds are particularly noteworthy as having extremely short bond distances between the alkafl metal atoms. 

Bond distances in organic compounds are usually 1-2 A (1A = 10 °m). Because the angstrom (A) is not an SI unit, we will express bond distances in picometers (1 pm = 10 m). Thus,... 

Some typical errors in heat of formation for the MNDO, AMI and PM3 methods are given in Table 3.1. The exact numbers of course depend on which, and how many, compounds have been selected for comparison, thus the numbers should only be taken as a guideline for the accuracy expected. Some typical errors in bond distances are given in Table 3.2. 

Pandya et al. have used extended X-ray ascription fine structure (EXAFS) to study both cathodically deposited -Ni(OH)2 and chemically prepared / -Ni(OH)2 [44], Measurements were done at both 77 and 297 K. The results for / -Ni(OH)2 are in agreement with the neutron diffraction data [22]. In the case of -Ni(OH)2 they found a contraction in the first Ni-Ni bond distance in the basal plane. The value was 3.13A for / -Ni(OH)2 and 3.08A for a-Ni(OH)2. The fact that a similar significant contraction of 0.05A was seen at both 77 and 297K when using two reference compounds (NiO and / -Ni(OH)2) led them to conclude that the contraction was a real effect and not an artifact due to structural disorder. They speculate that the contraction may be due to hydrogen bonding of OH groups in the brucite planes with intercalated water molecules. These ex-situ results on a - Ni(OH)2 were compared with in-situ results in I mol L"1 KOH. In the ex-situ experiments the a - Ni(OH)2 was prepared electrochemi-cally, washed with water and dried in vac-... 

Structural Studies. X-ray powder diffraction patterns for I indicate that the crystal structure is isomorphous to Zr2(0H)2-(SOO3 (H20)>. Figure 1 depicts the structure of the zirconium compound (5). The structure of I is identical to that of the zirconium analog except for variations in bond distances and angles which do not affect the overall structure. We have as yet been unable to obtain single crystals of I which are suitable for X-ray diffraction studies. 

The quantitative comparison of measured. To values and 5 values calculated for models A for mesitylene and hexamethylbenzene is given in Tables XII and XIII. With omission of the innermost maximum and minimum, the unreliable third minimum, and the very weak fifth maximum for each substance, the average values sa/s0 = 1.000 and 1.002, respectively, are found. These correspond to the interatomic distances C — = 1.54 0.01 A. and Car-Car = 1.39 A. in both substances, the 0 —Qa distance being equal to the single-bond distance in aliphatic compounds and the Car—distance to that in benzene to within the probable error of the determination. 

We might expect that in compounds exhibiting delocalization the bond distances would lie between the values gives in Table 1.5. This is certainly the case for benzene, since the carbon-carbon bond distance is 1.40 A, which is between the 1.48 A for an sp -sp C—C single bond and the 1.32 A of the sp -sp C=C double bond. ... 

Thus, l,6-methano[10]annulene (77) and its oxygen and nitrogen analogs 78 and 79 have been prepared and are stable compounds that undergo aromatic substitution and are diatropic. For example, the perimeter protons of 77 are found at 6.9-7.3 5, while the bridge protons are at —0.5 5. The crystal structure of 77 shows that the perimeter is nonplanar, but the bond distances are in the range 1.37-1.42A. It has therefore been amply demonstrated that a closed loop of 10 electrons is an aromatic system, although some molecules that could conceivably have such a system are too distorted from planarity to be aromatic. A small distortion from planarity (as in 77) does not prevent aromaticity, at least in part because the s orbitals so distort themselves as to maximize the favorable (parallel) overlap of p... 

The order of aromaticity of these compounds is benzene > thiophene > pyrrole > fiiran, as calculated by an aromaticity index based on bond-distance measurements. This index has been calculated for five- and six-membered monocyclic and bicyclic heterocycles Bird, C.W. Tetrahedron, 1985, 41, 1409 1986, 42, 89 1987, 43, 4725. 

Thousands of dipole moments, with references, are collected in McClellan, Tables of Experimental Dipole Moments , vol. 1, W.H. Freeman, San Francisco, CA, 1963 vol. 2, Rahara Enterprises, El Cerrita, CA, 1974. Tables of Interatomic Distances and Configurations in Molecules and Ions , London Chemical Society Special publication no. 11, 1958, and its supplement. Special publication no. 18, 1965, include bond distances and angles for hundreds of compounds, along with references. 

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