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Why polyacetylene (PA) is not a metal (Read 1604 times)
Gerrit-Jan Linker
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Why polyacetylene (PA) is not a metal
30.10.09 at 23:23:42
 
Why polyacetylene (PA) is not a metal
 
Polyacetylene (PA), can be written as (C2H2)n. In the unit cell there are two CH's. Polyacetylene can also be described as an undimerised structer where each bond is halfway between single and double. The unit cell contains one CH unit: (CH)n
 
In such a CH unit cell, one pz at C constructs the pi-band. Only 1 electron per unit cell is donated and the pi-system is half full. Thus PA is a metal.
 
However....
In PA the symmetry of the uniform CH chain is broken by dimerising such that the pi-electrons are concentrated between alternate pairs of carbon atoms. It is therefore more like the alternating single-double bond bond picture.
The proper chemical formula is no longer (CH)n but (CH=CH)n. Each unit cell contains 2 carbon atoms. There are 2 pi-sub-bands and 2 electrons are donated per unit cell. The lowest pi-sub-band is therefore fully occupied and the highest pi-sub-band, the pi*-sub-band is unoccupied. Since there is a pi-pi* band gap PA is a semi conductor and not a metal.
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« Last Edit: 10.12.09 at 11:23:43 by Gerrit-Jan Linker »  

Gerrit-Jan Linker
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