Molecular orbital calculations based on semi-empirical, ab initio, and DFT methods

Chapter 11, Problem 11.18

(choose chapter or problem)

Molecular orbital calculations based on semi-empirical, ab initio, and DFT methods describe the spectroscopic properties of conjugated molecules a bit better than simple Hückel theory.

(a) Using molecular modelling software \({ }^2\) and the computational method of your choice (semi-empirical, ab initio, or density functioncal methods), calculate the energy separation between the HOMO and LUMO of ethene, butadiene, hexatriene, and octatetraene.

(b) Plot the HOMO-LUMO energy separations against the experimental frequencies for \(\pi^* \leftarrow \pi\) ultraviolet absorptions for these molecules (Problem 11.15). Use mathematical software to find the polynomial equation that best fits the data.

(c) Use your polynomial fit from part (b) to estimate the frequency of the \(\pi^* \leftarrow \pi\) ultraviolet absorption of decapentaene from the calculated HOMO-LUMO energy separation.

(d) Discuss why the calibration procedure of part (b) is necessary.

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