Bond values

The bond-clcctron matrix (BE-matrix) was introduced in the Dugundji-Ugi model [39], It can be considered as an extension of the bond matrix or as a mod-ific atinn of Spialter s atom connectivity matrix [38], The BE-inatrix gives, in addition to the entries of bond values in the off-diagonal elements, the number of free valence electrons on the corresponding atom in the diagonal elements (e.g., 03 = 4 in Figure 2-18). 

In addition to intermolecular interactions in the solid state, the relative importance of the two resonance structures A and B is an important issue in the structural determinations of selena- and tellura-diazoles. ° The Se-N bond lengths fall within the range 1.78-1.81 A and the Te-N bond lengths are 2.00-2.05 A compared to single bond values of 1.86 and 2.05 A, respectively. It can be concluded that resonance structure A is more important than B for the Se and, especially, the Te... 

There is some experimental data from nuclear magnetic resonance5 25 and sonic30 observations which indicate that a bond which would be expected to have a partial double bond character because of conjugation with an adjacent double bond will show a barrier intermediate between single and double bond values. 

It is, of course, debatable whether it might be more appropriate to choose other CarN single bond values from the tables of bond lenghts that were evaluated by the Cambridge Crystallographic Data Centre in collaboration with two other groups (Allen et al., 1987). 

It is seen that a small amount of double bond character causes a large decrease in interatomic distance below the single bond value, whereas only a small change from the double bond value is caused by even as much as fifty per cent, single bond character.8 In consequence, the interatomic distance criterion for resonance provides quantitative information only through about one-half of the bond character region. 

The observed decrease below the double-bond values may possibly be due to some triple bond formation. 

The most extensive application which was made of the resonance curve was to the carbon-chlorine bond in phosgene and the chloroethylenes. In the electron-diffraction study of these substances2 the carbon-carbon and carbon-oxygen doublebond values 1.38 and 1.28 A. were assumed the question accordingly arises as to what effect the new double bond values would have on the carbon-... 

The H and 13C NMR data for the three metal derivatives were similar. An X-ray crystallographic structure for the gallium derivative (Fig. 6) indicated a shortened Ga—C (carbene) distance of 1.935(6) A, whereas the Ga—C(Me) distances had normal single bond values, 1.994(8) and 1.988(8) A. Thus, the structure may be visualized as being composed of the two canonical forms... 

Coupling Constants. The usual three-bond H—H coupling constant in fluorobenzenes is about 8 Hz, whereas the four-bond coupling constant is between 1 and 3 Hz. Five-bond coupling is usually not observed. Likewise, the three-bond F—H coupling constant is about 8 Hz, the four-bond value 5-6 Hz, and the five-bond coupling constant about 1 Hz. 

The solid-state structures of (PhS)3N and (CgF5S)3N and the gas-phase structure of (CF3S)3N all show nearly planar S3N units with the S-N bond lengths longer than typical single-bond values. Tribenzenesulfenamide... 

TABLE 11.2. Bonding Energies, Mulliken Population Bond Values, and H- -H... 

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