Some New Applications of the FE Theory to the Spectra of Linear Unsaturated Compounds

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Abstract
A linear FE model with electron—electron and electron—core interaction appeared to be fairly accurate in reproducing the experimental data for (i) lowest excited singlet—triplet separation in polyenes, (ii) twisting frequencies of ethylene, and (iii) location of the absorption maxima in cumulenes. For calculations of (i) and (ii), electron—electron interaction has been assumed to play the dominant role. With the same assumption made, cis—trans isomerization of polyenes was found to be easier with long chains than with short. It also is easier about a central double bond in the chain than about an outer double bond. In the case of a single double bond, an important part of the energy path in isomerization is that for the triplet state; however, the role of triplet states in isomerization should diminish rapidly with an increase in the number of double bonds in the chain. With ethylene as the example, the effect of pi‐electron correlations in the FE model was briefly investigated, and the correlation for the perturbation calculus of an order higher than the first was found to be in the FE model much less important than in the LCAO theory. No empirical parameters other than the fundamental atomic constants and the length of the FE path of a given molecule were used in the calculations.