Bond length alternation

The off-diagonal elements of P corresponding to directly bonded atoms are referred to as (7r-electron) bond orders. Hexatriene shows bond-length alternation there are distinct single and double bonds. Notice that the bond orders for the bonded atom pairs show a related alternation. 

A4,5,2,4-Benzotrithiadiazepineis a yellow air-stable solid.394 The X-ray structure384 shows that it is planar, but although it can be considered to be an aromatic 147t-systcm there is some bond-length alternation within the rings compared with the monocyclic trithiadiazepine. 

The SO bond length of 147.4 pm is similar to that in sulfurmonoxide (148 pm) and longer than that in SO2 (143 pm). The SS bonds vary in length between 195.7 and 228.3 pm. These bond lengths alternate around the ring... 

Another example is provided by [30] anmlene. Longuet-Higgins and Salem have shown that the observed visible and UV absorption spectrum and, in particular, the NMR proton chemical shifts of this molecule are very difficult to reconcile with the symmetrical nuclear configuration (Dg ) suggested by the superposition of the Kekule-type resonance structures. The hypothesis of a bond-length alternation of symmetry removes this difficulty. This indicates that the resonance between Kekule-type structures should be very much impeded also in this molecule. 

Meyers F, Marder SR, Pierce BM, Bredas JL (1994) Electric field modulated nonlinear optical properties of donor-acceptor polyenes sum-over-states investigation of the relationship between molecular polarizabilities (a, p, and y ) and bond length alternation. J Am Chem Soc 116 10703-10714... 

This kind of an rap2- -racr interaction may lead to a marked bond lengths alternation the extreme structural effect of such an interaction is certainly found for S7, but also other neutral and cationic polychalcogen compounds frequently exhibit this interaction. 

The structure of S7 is clearly the most intriguing59 61 of all known structures of neutral sulfur homocycles. Several modifications are known (sometimes only by powder or Raman data) however, the structural features are always the same and show a large bond length alternation and a range of S-S distances from 199 pm (bond order > > 1.0) to 218 pm (bond order < < 1.0 Figure 13). Other isomers were assessed by quantum chemistry.61... 

The only structural data available for S7+ comes from quantum chemical calculations43 and suggests that S7+ is a C2 symmetric S7 homocycle in the chair conformation and exhibits a much less pronounced bond lengths alternation... 

The presence of a very pronounced bond length alternation in both structures suggests that extensive charge delocalization occurs in these structures. This is likely achieved through a series of p2- cr interactions that is related to those shown above for S7 and Sei02+. 

In this chapter, we demonstrated that the restriction of building a compound with only one type of an element is not a restriction at all and a multitude of neutral, cationic as well as anionic polychalcogen structures is currently known. As expected for the more electronegative nonmetal (S) and meta metals (Se, Te), the bonding within these moieties is covalent and a small number of interactions, namely, p2-rap2 lone pair repulsion, n- and n -n bonding as well as p2- cr interactions, are sufficient to rationalize the structures and account for the bond lengths alternations or weak transannular interactions that are often found. 

The pronounced bond length alternation within the anion sheets of RbTeg is in stark contrast to the modest difference between the Te Te distances (d = 300.3(1), d2=307.7(l) pm) belonging to the planar thinned 44 nets... 

Kushmerick JG, Holt DB, Pollack SK, Ratner MA, Yang JC, Schull TL, Naciri J, Moore MH, Shashidhar R (2002) Effect of bond-length alternation in molecular wires. J Am Chem Soc 124 10654-10655... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

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