Question

1. Cyclobutadiene is an anti-aromatic molecule. What was your calculated HOMO-LUMO gap for cyclobutadiene? Would you expect this molecule to be paramagnetic or diamagnetic based on the calculated gap?

The HOMO-LUMO gap I got using Webmo was 0.012 eV.

2. 11-cis-retinal plays a role in our vision. When an electron gets promoted it becomes easier to the cis-bond to rotate to the trans-isomer. What color light causes the promotion of an electron from the HOMO-LUMO for that molecule?

3. Pentacene has been tested for use in photovoltaic cells. How many electrons are in pentacenes pi-cloud? Is pentacene aromatic? What color light does pentacene absorb?

Answer 1

1) The energy of HOMO is (-5.375eV) and of LUMO is (-1.887eV). So the energy gap is E=E(LUMO)-E(HOMO)=(-1.887eV)-(-5.375eV)=3.488eV. From the energy gap it can be said that Cyclobutadiene is paramagnetic as the 2 electrons in the outermost orbital remain unpaired.

2)When an atom or molecule absorbs a photon, its electrons can move to higher-energy orbitals, and the atom or molecule makes a transition to a higher-energy state. In retinal, absorption of a photon promotes a p electron to a higher-energy orbital (a pi-pi* excitation). This excitation "breaks" the p component of the double bond, thus allowing free rotation about the bond between carbon atom 11 and carbon atom 12 .Thus, when 11-cis-retinal absorbs a photon in the visible range of the spectrum, free rotation about the bond between carbon atom 11 and carbon atom 12 can occur and the all-trans-retinal can form.

3) Pentacene is an aromatic compound. Since it has 22-pi-electrons in its electron cloud (It follows Huckel's rule which is 4n+2 pi electrons).Pentacene absorbs yellow light of wavelength 5800 $\AA$ and hence appears indigo.