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.

For Hyperchem, carry out a single point ZINDO/S single-excitations configuration interaction (CI) computation with a configuration cutoff for the calculation on each molecule.
For Spartan, carry out a single point AM1 single-excitations CI computation on each molecule.
For each calculation, record the wavelength of the longest wavelength non-forbidden UV- vis transtion. Use the same level of CI for each molecule (why might this be a problem?).
Consider each polyene as an oligomer of ethene, e.g., 1,3,5,7,9,11-dodecahexaene is the n=6 oligomer. Plot the band gap (longest wavelength non-forbidden transition) as a function of 1/n. Polyacetylene may ideally be considered an infinitely long oligomer of ethene. How can you estimate the band gap of polyacetylene from your data? Do so, and compare to the literature data listed below.

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