Concept: Concept: Ground vs. Excited States4m
Hey guys, in this video weÕre going to discuss a very unique phenomenon that happens when a conjugated molecule is exposed to light, letÕs get started. So guys, conjugated molecules have the ability to absorb light energy and turn, convert that light energy into mechanical energy by kicking electrons up to a higher energy state, and when this happens, that means that your HOMO and LUMO orbitals are going to change, because now you have electrons resting in different orbitals that they initially were in the ground state, and this actually will have big implications on reactivity, but weÕre going to get that later, because right now you donÕt really know how HOMO and LUMO react, but later when we talk more about how HOMO and LUMO react to each other, the fact that you can excite electrons to a new HOMO and LUMO really will be important, okay? So, hereÕs an example: 1, 3- butadiene is irradiated with photons, exciting an electron up to a higher energy molecular orbital, so photons are light, youÕre shining the light and youÕre exciting an electron, okay? Predict the identity of the HOMO and LUMO orbitals after irradiation. So, what we would to start off with in the ground state guys, is just 4 pi electrons, right? Because you have 2 here, you have 2 here; 4 pi electrons and I will fill them in order of aufbau principle; one, two, three, four, and I already have the HOMO and LUMO written out for us. HOMO highest occupied is psi 2, LUMO lowest unoccupied is psi 3, and thatÕs what weÕre used to, this is what the molecule looks like in its ground state.
But if you shine enough light on this molecule, what is going to end up happening is that it absorbs one of the photons and is going to basically convert that energy into an electron being kicked up to a higher energy state, so itÕs going to look like after irradiation is this, so let me just get out the way here. These two electrons stay the same, but now, I only have one electron here and now I have one electron here, isnÕt that interesting? What basically we just did is we just took one of the electrons and moved it up here, now we know that before we had an up spin and down spin and now IÕm just making it two up spins, that doesnÕt matter, donÕt worry about whether is up spin or down spin, you donÕt need to worry about that for any of predicting HOMO and LUMO, all you need to worry about is which orbital itÕs in. So, that means that which orbital is now the HOMO? The HOMO is now psi 3, and which orbital is now the LUMO? The LUMO is now psi 4, so guys, this is very important because this means that we can use light to manipulate which orbitals react as a HOMO and which orbitals react as a LUMO, cool? Awesome, so thatÕs it for this video, letÕs move to the next one.
Example: Practice: Conjugated Properties of 4-MBC2m
For each of the following, give the letter for the best answer.
(i) Which one of the following has the longest wavelength λmax in its UV-VIS absorption spectrum?
(ii)The ground-state configuration for the π electrons of benzene can be given as π 12, π22, π32, π4* 0, π5* 0, π6* 0. Which one of the following is an excited-state configuration for the π electrons of the benzene anion radical?
(a) π12, π22, π32, π4* 1, π5* 0, π6* 0;
(b) π12, π22, π32, π4* 0, π5* 1, π6* 0;
(c) π12, π22, π31, π4* 2, π5* 0, π6* 0;
(d) π12, π22, π31, π4* 0, π5* 0, π6* 0;
(e) π12, π22, π30, π4* 1, π5* 0, π6* 0;
(f) π12, π21, π32, π4* 0, π5* 0, π6* 0