Bond-length alteration

Js.JJa.s in these systems is 0.5, indicating appreciable bond length alterations. 74.5 is very similar to 7o of benzene (9 Hz), while 75.6 is only 4.0 Hz. In the pyrazolo[l,5-fl]pyrazine ring system, H-6 and H-7 appeared at low fields (> 8.1 ppm), and Jej is only 5 Hz, indicating that bond order in these systems is less than in benzene (7o = 9 Hz) (74CPB1814). 

Small bond-length alterations are not unknown however. In chrysene (10) (Bums and Iball, 1960), the bond AB has a length 1-468A. Bums and Iball point out that this is 0-03-0-04 A longer than theoretical predictions and attribute the extension to overcrowding of the hydrogen atoms at C and I. ... 

The types of stmcmral distortions observed in solids include bond length alterations and bond angle bending. Localized stmctural distortions occur around point defects such as vacancies and substimtional impurities owing to their differing mass and force constants. Such bond distortions occur when they can lower the overall energy of the system. For... 

For chemists, it is probably not a big surprise that a one-dimensional chain of hydrogen atoms does not exist and that it will immediately decompose into isolated H2 molecules. The Peierls distortion has important consequences for one-dimensional systems, such as polyacetylene with C-C bond-length alterations (instead of equal C-C distances) [74], infinite molecules with platinum-platinum bonding such as Krogmann s salts K2[Pt(CN)4]Xo,3 3H2O with X = Cl or Br [75], or other one-dimensional systems [76], and it also affects three-dimensional systems, in particular elemental structures (see Section 3.4). From a group-theoretical point of view, Peierls distortions are characterized by a loss of translational symmetry in the above example, the nonequidistant chain of H atoms is less symmetrical (in terms of translational symmetry) than the equidistant one. 

A substantial number of fulvene and fulvalene derivatives have been prepared. The chemical properties of these molecules are those of reactive polyenes. The molecular geometry of dimethylfulvene has been examined by electron diffraction methods. Strong bond-length alteration indicative of a localized structure is found. ... 

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