Periodic and Nonperiodic Alternating trans -Polyacetylene

In two subsequent papers the semiempirical all-valence electron MNDO (modified MINDO method ) was used to calculate clusters of CH2=CH-CH=CH-CH=CH2 (modeling periodic alternating PA), CH2=CH- CH - CH=CH2 (soliton in PA), and CH2=CH- CH - CH CH=CH2 (polaron in PA). The MNDO method, though approximate, takes into account electron-electron interaction in an SCF way and is known to give the correct trend for vibrational properties of organic molecules. In the case of the soliton cluster the unrestricted version of the MNDO method with an odd number of electrons (DODS) has been applied. 

In order to obtain a reasonable force field for trans- A., the force 

9 Vibrational Spectra and Transport Properties of Polymers 

Owing to the semiempirical nature of the MNDO method, the theoretical force constants are between 10 and 20% larger than the experimental values. Therefore, following Torok and Pulay empirical scale factors have been calculated in the case of the MNDO force field of butadiene for which molecule enough experimental information is available. The suggestion followed in the computation of these scale factors was that the geometrical mean value of the diagonal force constants be used.  

FIGURE 9.5. Bond lengths and angles used for fra 5-l,3,5-hexatriene. Bond lengths are given in A. 

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