Reference bond distances

The bond length variation in the substituted benzodicyclobutadiene shown in Fig. 14.3.6(d), as determined from X-ray analysis, can be fairly well accounted for by resonance between the canonical formulas la and lb. The central six-membered ring contains a pair of extremely long C(sp2)-C(sp2) bonds at 154.0(5) pm, which significantly exceed the reference bond distance of 149.0(3) pm observed for tris(benzocyclobutadieno)benzene [Fig. 14.3.6(e)], The bond length pattern in the latter compound indicates that its structure is better described by the radiallene formula Ha in preference to formula lib, which contains anti-aromatic cyclobutadiene moieties. 

Structural parameters [Reference], bond distances (A) CCDB code,... 

Table 6.2. Reference bond distances (in pm) for single, homo lytic bonds between the 17 elements, H, C, Si, Ge, Sn, N, P, As, Sb, O, S, Se, Te, F, Cl, Br and I. Bond distances are listed only once. Thus the prototype C-Si bond distance is listed under C and not repeated under Si. ...

Constrained optimization refers to optimizations in which one or more variables (usually some internal parameter such as a bond distance or angle) are kept fixed. The best way to deal with constraints is by elimination, i.e., simply remove the constrained variable from the optimization space. Internal constraints have typically been handled in quantum chemistry by using Z matrices if a Z matrix can be constructed which contains all the desired constraints as individual Z-matrix variables, then it is straightforward to carry out a constrained optimization by elunination. 

If we assume that there are certain "ideal" values for bond angles, bond distances, and so on, it follows that deviations from these ideal values will destabilize a particular structure and increase its potential energy. This increase in potential energy is referred to as the strain energy of the structure. Other terms for this increase include steric energy and steric strain. Arithmetically, the total strain energy (fj of an alkane or cycloalkane can be considered as... 

Draw a Lewis structure for cyclohexenone that involves charge separation for the most polar bond. Then, draw a Lewis structure that will delocalize one or both charges. Next, examine the actual geometry of cyclohexenone. Are the bond distances consistent with the Lewis structure shown above, or have they altered in accord with your alternative (charge separated) Lewis structure (Structures for cyclohexene and cyclohexanone are available for reference.)... 

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-... 

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. 

From the given Hamiltonian, adiabatic potential energy surfaces for the reaction can be calculated numerically [Santos and Schmickler 2007a, b, c Santos and Schmickler 2006] they depend on the solvent coordinate q and the bond distance r, measured with respect to its equilibrium value. A typical example is shown in Fig. 2.16a (Plate 2.4) it refers to a reduction reaction at the equilibrium potential in the absence of a J-band (A = 0). The stable molecule correspond to the valley centered at g = 0, r = 0, and the two separated ions correspond to the trough seen for larger r and centered at q = 2. The two regions are separated by an activation barrier, which the system has to overcome. 

Scheme 7.7, ORTEP adapted from reference 32), with [BCCgFj) or [CBnHgBrg] as the counterions. The X-ray structure clearly shows evidence of an sp hybridized center with a C -C -C angle of 178.8° and an abnormally short C -C double bond distance of 1.22 A (compared to 1.32 A, Table 7.1), and a nearly normal C -CA (sp -sp) distance of 1.45 A. The most striking feature of 8, however, is the very long C -Si distance of 1.97 A (compared to 1.87 A, Table 7.1), which is attributed to hyperconjugation, as shown in the scheme. Elongation of the bonds a- to the cation center is a characteristic of hyperconjugation, also observed in the structure of the adamantyl cation. ...

Taking the curves of oxygen, sulfur and fluorine in Fig. 2 as a reference it seems that a linear correlation can be established between the c.n. and the bond distances (c.n. running from 3 to 8). A rise of the c.n. by one unit increases the mean Sn-ligand distance by about 10 pm ... 

Complex Bond distance (A) Distance (A) of Re from the basal plane Re = O stretch (cm-1) Reference... 

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