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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: qw098 on April 01, 2011, 07:33:28 PM

Title: HOMO-LUMO Gap
Post by: qw098 on April 01, 2011, 07:33:28 PM

Hi,

I am reading a paper for my quantum seminar, and there is a table listing HOMO-LUMO gaps for a number of fullerenes with differing n (number of carbons).

I was wondering, what does the HOMO-LUMO gap signify? (ie: What can I say about one molecule having a larger HOMO-LUMO gap than another?) The values are given in units of eV.

Thank you.

Title: Re: HOMO-LUMO Gap
Post by: Polytriazole on April 01, 2011, 10:32:10 PM

The HOMO-LUMO gap is also called the "band gap".  It refers to the potential energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).  Basically, it's how much energy you have to feed into the molecule to kick it from the ground (most stable) state into an excited state.

For fullerenes, this may be related to their ability to generate photocurrent (like in a solar cell, light goes in and electricity comes out).  A large band gap means you have to use high-energy light, like UV wavelengths.  A small band gap means you don't need as much energy to excite the molecule, like green or even red wavelengths.

Aside from photocurrent, excitation can also drive other types of chemical reactions.

Title: Re: HOMO-LUMO Gap
Post by: qw098 on April 02, 2011, 03:36:56 PM

Thanks a lot Polytriazole!

You hit it right on the spot (!), this makes a lot of sense!

Title: Re: HOMO-LUMO Gap
Post by: Calafalas on August 22, 2011, 04:58:00 PM

Do you perhaps also know how this works for the reverse reaction of a Diels-Alder cyclo addition [4+2] product ?
What I think is that the energy that is needed for the reverse reaction, needs to be as high as the energy gap between the HOMO of the adduct and the LUMO of the adduct. Does anyone know more about this?