Predicting UV-vis Transitions for Polyacetylene

Methodology
MNDO/AM1/PM3 (SPARTAN)
ZINDO/S (HYPERCHEM)

Techniques Used
Building, Optimization, Output Analysis
Configuration Interaction


Abstract. You will optimize the structure of homologous, linear polyenes. You will then predict the longest wavelength UV-vis electronic transition of each, using a semiempirical molecular orbital plus configuration methodology. Finally, you will use these numbers to predict the band gap of polyacetylene.


Procedure. Optimize each of the following molecules, using a semiempirical method such as AM1 or PM3: 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, 1,3,5,7,9- decapentaene, 1,3,5,7,9,11-dodecahexaene.


Benchmarks. Available band gap UV-visible spectroscopic transitions for the test molecules are in the table below in the results section.


Results.

Oligomer Size (n) 1/n Expt UV (cm-1) Computed UV (cm-1)

2
0.50

45,980
 

3
0.33

34,760
 

4
0.25

33,050
 

5
0.20

30,120
 

6
0.167

27,750
 

infinite
 

14,500
 

Copyright 1995 by Paul M. Lahti. All Rights Reserved