Answer the following questions for the πmolecular orbitals of 1,3 -butadiene: a. Which are the b

Answer the following questions for the πmolecular orbitals...

Key Concepts

Molecular Orbital Theory

Molecular orbital theory is the framework used to describe the behavior of electrons in molecules. It explains that atomic orbitals combine to form molecular orbitals that extend over the entire molecule, and these molecular orbitals can be filled with electrons according to energy, leading to the electronic structure and properties of the molecule.

Bonding and Antibonding Orbitals

Bonding orbitals result from the constructive interference of atomic orbitals and are lower in energy because they increase electron density between nuclei, leading to a stabilizing effect. In contrast, antibonding orbitals are formed from destructive interference, exhibit a node between atomic centers, and are higher in energy; electrons in these orbitals can result in destabilization of the molecule.

Orbital Symmetry

Orbital symmetry refers to the behavior of molecular orbitals under symmetry operations, such as reflection, rotation, or inversion. Symmetric orbitals remain unchanged under a given symmetry operation, while asymmetric ones change sign. This property is important for understanding bonding interactions and selection rules in spectroscopy and reaction mechanisms.

Frontier Molecular Orbitals (HOMO and LUMO)

The Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) are referred to as the frontier molecular orbitals. These orbitals are crucial because they are responsible for a molecule’s chemical reactivity and its interaction with light. The HOMO contains the electrons that are most easily donated, while the LUMO can accept electrons, governing both ground state properties and excited state behaviors.

Electronic Excitation

Electronic excitation involves the promotion of an electron from a lower-energy orbital (typically the HOMO) to a higher-energy orbital (typically the LUMO). This process is fundamental in photochemistry and spectroscopy, influencing the electronic structure and reactivity of a molecule in its excited state, and is closely related to the orbital symmetry and energy ordering of the frontier molecular orbitals.

Transcript

00:01 This question is asking about the conjugated pi system in one three buta diene.

00:07 So if you remember a double bond is made when two pi orbitals interact constructively in a bonding interaction.

00:17 So we have the two pi orbitals here.

00:20 And when they're in the same phase, we can have overlap and have overlap.

00:25 We can form a double bond.

00:28 And that's a bonding interaction.

00:29 And then we have antibody interactions, antibonting, excuse me, where the orbitals are out of phase and we cannot get any bonding overlap between the p -orbital's, any sharing of electrons.

00:45 So when these interactions, when these double bonds are in the one -in -three position, here and here we get delocalization and conjugation of the pie system.

00:58 And i've gone ahead and just drawn all of the molecular orbitals here and i'll go ahead and give them names.

01:04 We call them sye 1, psi 2, psi 3, and sigh 4 and this is just a physical chemistry greek symbol there.

01:15 And they're increasing in energy.

01:17 So we're going to go through the questions and kind of explore why that might be.

01:22 So question they asked which are the bonding molecular orbitals, much of the antibody.

01:26 So i've already explained the difference between a bonding interaction and anti -bonding interaction with p orbitals.

01:32 So in this bottom one here, we have all bonding interactions.

01:38 We can have overlap between all the p -orbital.

01:42 And so therefore, this is a bonding.

01:45 This is bonding right here.

01:47 The side one is a bonding.

01:49 And this one, we've got a bonding here and a bonding here.

01:54 We've got overlap.

01:56 But we actually have one antibonding right here.

01:58 I'm just going to indicate that there.

02:01 We do not have overlap between those middle two.

02:04 But we still have more bonding interactions.

02:06 We've got two bonding interactions and one antibonding.

02:08 So net overall, we're still bonding.

02:11 This is also a bonding orbital.

02:18 But as we go up in energy, we get less and less bonding character and more and more anti -bonding character.

02:24 So you can see here we actually have antibonding here and here, and then one bonding.

02:29 So this actually is an antibonding, because we have two anti -bonding and one bonding interaction.

02:35 And then once our highest energy here, we have all anti -bonding, three anti -bonding and no bonding.

02:41 So this is an anti -bonding here.

02:43 So these are anti, and the bottom two are bonding.

02:49 And there are four electrons, one from hp orbital and the one three -rudeidine system.

02:54 So this would be our electronic configuration.

02:56 And notice how all the electrons are in bonding orbitals, which is why this is a low energy configuration.

03:03 So there, that's question a...

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