Bond alternation, and

In this study a C=C bond length difference of 0.010 A is determined, compared with the theoretically calculated difference of 0.006 A see Table 217. The single-bond double-bond alternation and the C=C—C valence angles are also quite similar in the two studies. 

The distances shown in the schematic drawing 20 display little bond alternation and support the notion of a delocalized carbanion. 

The aromaticity obtained by means of the DE values and their differences in the single pseudoazulene systems are also the result of calculations of the bond orders (Fig. 1). The calculated bond alternations46 57-77-79-82-117118-129 are in line with the heteroaromaticity of the systems. Similar relationships were calculated for azulene.219 The extent of the bond alternation, however, differs from one system to the other (see Fig. 1). In indeno[2.1-f>]-l-benzo-pyran (44a) the bond lengths of the central cyclo[fo]pyran system indicate pronounced bond alternation, and in the opinion of the authors, the systems have little aromatic character.129 Calculations with 1 //-pyrindines (26) and 2//-pyrindines (29) show that 26 is more stable than 29, but both are less aromatic than indole.63 A comparison of SCF calculations for pseudoazulenes 26 and 29 with their aza analogs 68 and 69-73, which contain a pyrrole-type nitrogen, shows only minor perturbations of carbon-carbon bond lengths on replacing a =CH— moiety by =N—.66... 

We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

Red shifts due to electron delocalization should be associated with a concommitant decrease in bond alternation and thus with a decreased frequency of the C=C stretching mode. As clearly exposed in Fig. 7, this has been observed both for model RSB and PRSB compounds (221,226,244) and for several pigments derived from opsin and bacterioopsin (221,222,225,226). 

Fig. 4.15 Chemical structures of trarcs-polyacetylene (a) with equal bond lengths, (b) bond alternation and (c) a bond alternation defect.

BAC vs Jfi correlates well, —0.94, due to almost the same mechanistic background. Since both kinds of deformations, bond alternation and bond elongation, cost energy (HOMA contains components from both), the correlation between REC and HOMA exists but is rather weak, whereas correlations of REC vs... 

The number of triple bonds in larger dehydroannulenes may also significantly affect the extent of delocalisation since, as discussed above, the introduction of a triple bond may induce bond alternation, and this becomes particularly significant where there is in any case a tendency, because of ring size, towards bond alternation. 

Nevertheless, even for polyacetylene, the electronic structure is not that of a simple metal in which the bond-alternation and the tc-tc gap have gone to zero there are infrared active vibrational modes (IRAV) and a pseudo-gap. This is indicated by the spectra in Figure 2 which demonstrate the remarkable similarity between the doping-induced absorption found with heavily doped trans-(CH)x, and the photoinduced absorption spectrum observed in the pristine semiconductor containing a very few photoexcitations. Not only are the same IRAV mode spectral features observed, they have almost identical frequencies. 

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