Predicting UV-vis Transitions for Polyacetylene
Building, Optimization, Output Analysis
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
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
Benchmarks. Available band gap UV-visible spectroscopic
transitions for the test molecules are in the table below in the results
|Oligomer Size (n)
||Expt UV (cm-1)
||Computed UV (cm-1)|
Copyright 1995 by Paul M. Lahti. All Rights Reserved